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  Earth Science:
Evolution and Structure of Planet Earth
  Earth is the third planet from the sun. It was formed with the rest of the solar system by consolidation of interstellar dust approximately 4.6 billion years ago. Earth is the only planet on which life is known to exist. Its life-supporting atmosphere is composed of nitrogen (78.09%), oxygen (20.95%), argon (0.93%), carbon dioxide (0.03%), and less than 0.0001% neon, helium, krypton, hydrogen, xenon, ozone, and radon. Life here began an estimated 3.5–4.0 billion years ago.  
  About 70% of earth's surface, including the north and south polar ice caps, is covered with water, totaling 361,000,000 sq km/139,400,000 sq mi. The greatest depth of earth's hydrosphere is the Mariana Trench in the Pacific Ocean, at a depth of 11,034 m/36,201 ft. Earth's land surface covers 150,000,000 sq km/57,500,000 sq mi. The greatest height above sea level is Mount Everest at 8,872 m/29,118 ft.  
  Earth science is the scientific study of the planet earth as a whole. The mining and extraction of minerals, weather prediction, earthquakes, the pollution of the atmosphere, and the forces that shape the physical world all fall within its scope of study. The term "earth science" reflects a broadening of the traditional field of geology to include any physical or life sciences applied to the study of the earth. The broadening of the discipline reflects scientists' concern that an understanding of the global aspects of the earth's structure and its past will hold the key to how humans affect its future, ensuring that its resources are used in a sustainable way. Similarly, we now know that our understanding of earth is linked to our knowledge of the other planets in our solar system and the evolution of the universe.  
  Earth science includes geology, mineralogy, and paleontology, as well as geophysics, geochemistry, mineral physics, paleomagnetics, meteorology, and oceanography. Meteorology, oceanography, and the study of earth's hydrosphere and atmosphere will be discussed in the following chapter.  
  Structure and Composition of the Earth  
  Earth is almost spherical in shape, flattened slightly at the poles, and is composed roughly of three concentric layers: the crust, the mantle, and the core.  
The outermost part of the structure of earth is the crust. There are two distinct types of crust, oceanic crust and continental crust. The oceanic crust is on average about 10 km/6.2 mi thick and consists mostly of basaltic types of rock. By contrast, the continental crust is largely made of granite and is more complex in its structure. Because of the movements of plate tectonics (see below), the oceanic crust is in no place older than about 200 million years. However, parts of the continental crust are over 3 billion years old.
  Beneath a layer of surface sediment, the oceanic crust is made up of a layer of basalt, followed by a layer of gabbro. The composition of the oceanic crust overall shows a high proportion of silicon and magnesium oxides, hence named sima by geologists. The continental crust varies in thickness from about 40 km/25 mi to 70 km/45 mi, being deeper beneath mountain ranges. The surface layer consists of many  




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  earth Inside the earth. The surface of the earth is a thin crust about 6 km/4 mi
thick under the sea and 40 km/25 mi thick under the continents. Under the
crust lies the mantle about 2,900 km/1,800 mi thick and with a temperature of
1,500–3,000°C/2,700–5,400°F. The outer core is about 2,250 km/1,400 mi thick,
of molten iron and nickel. The inner core is probably solid iron and nickel at
about 5,000°C/9,000°F.
  kinds of sedimentary and igneous rocks. Beneath lies a zone of metamorphic rocks built on a thick layer of granodiorite. Silicon and aluminum oxides dominate the composition and the name sial is given to continental crustal material.  
  To remember the most common elements of the planet's crust, in descending order:  
  Only silly asses in college study past midnight  
  (Oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium)  


  The crust is sometimes confused with the lithosphere, earth's brittle outer skin forming the jigsaw of plates that take part in the movements of plate tectonics. The crust is only the topmost layer of the lithosphere. The lithosphere is comprised of crust underlain by a rigid portion of the upper mantle. The lithosphere is about 100 km/63 mi thick and moves about on the more elastic and less rigid asthenosphere. The movements of the lithospheric plates are responsible for earth's major physical features, such as mountains, islands, and deep-sea trenches.  
  The seven large land masses of earth's crust, as distinct from the oceans, are called continents. They are Asia, Africa, North America, South America, Europe, Australia, and Antarctica. Because continents are part of lithospheric plates, they are constantly moving and evolving due to plate tectonics. A continent does not end at the coastline; its boundary is the edge of the shallow continental shelf, which may extend several hundred miles out to sea.  
  At the center of each continental mass lies a shield or craton, a deformed mass of old metamorphic rocks  
  To remember the seven continents:  
  Eat an aspirin after a night-time snack  
  (Europe, Antarctica, Asia, Africa, Australia, North America, South America)  
  The second letter in the first three "A" words helps to remember the "A" continents.  





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  crust The crust of the earth is the top layer of the tectonic plates. These plates
are bounded by different kinds of margins. In midocean, there are constructive
plate margins, where magma wells up from the earth's interior, forming new crust.
On continent–continent destructive margins, mountain ranges are flung up by
the collision of two continents. At an ocean–continent destructive margin, the
plate containing denser oceanic crust is forced beneath the plate with less dense
continental crust, forming an area of volcanic instability.
  dating from Precambrian times. The shield is thick, compact, and solid (the Canadian Shield is an example), having undergone all the mountain-building activity it is ever likely to, and is usually worn flat. Around the shield is a concentric pattern of mountains comprised of folded rock, with older ranges, such as the Rockies, closest to the shield, and younger ranges, such as the coastal ranges of North America, farther away. This general concentric pattern is modified when two continental masses have moved together and they become welded with a great mountain range along the join, the way Europe and northern Asia are joined along the Urals. If a continent is torn apart, the new continental edges have no mountains formed by wrinkling and folding of the crust; for instance, the western coast of Africa, which rifted apart from South America 200 million years ago.  
  The theoretical balance in buoyancy of all parts of the earth's crust, as though they were floating on a denser layer beneath, is called isostasy. There are two  
  continent The North American continent is growing in the west as a result of
collision with the Pacific plate. On the east of the wide area of the Ozark Plateau
shield lie the Appalachian Mountains, showing where the continent once collided
with another continent. The eastern coastal rifting formed when the continents
broke apart. On the western edge, new impact mountains have formed.




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  theories of the mechanism of isostasy, the Airy hypothesis and the Pratt hypothesis, both of which have validity. In the Airy hypothesis crustal blocks have the same density but different depths: like ice cubes floating in water, higher mountains have deeper roots. In the Pratt hypothesis, crustal blocks have different densities allowing the depth of crustal material to be the same.  
  There appears to be more geological evidence to support the Airy hypothesis of isostasy. During an ice age the weight of the ice sheet pushes that continent into the earth's mantle; once the ice has melted, the continent rises again. This accounts for shoreline features being found some way inland in regions that were heavily glaciated during the Pleistocene period.  
  The mantle is the intermediate zone of the earth between the crust and the core, accounting for 82% of earth's volume. The boundary between the mantle and the crust above is the Mohorovicic discontinuity, or Moho, located at an average depth of 32 km/20 mi. The lower boundary with the core is the Gutenburg discontinuity at an average depth of 2,900 km/ 1,813 mi.  
  The mantle is subdivided into upper mantle, transition zone, and lower mantle, based upon the different velocities with which seismic waves travel through these regions. The upper mantle includes a zone characterized by low velocities of seismic waves, called the  
  isostasy Isostasy explains the vertical distribution of earth's crust. George Bedell
Airy proposed that the density of the crust is everywhere the same and the thick-
of crustal material varies. Higher mountains are compensated by deeper  roots.
This explains the high elevations of most major mountain chains, such as the Himalayas.
G. H. Pratt hypothesized that the density of the crust varies, allowing the base of the
crust to be the same everywhere. Sections of crust with high mountains, therefore, would
be less dense than sections of crust where there are lowlands. This applies to instances
where density varies, such as the difference between continental and oceanic crust.




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Highest and Lowest Elevation by Continent
Continent Highest elevation
Deepest elevation Depth below sea level
Africa Kilimanjaro, Tanzania
Lake Assal, Djibouti
Antarctica Vinson Massif
Hollick–Keyon plateau1
Asia Everest, China–Nepal
Dead Sea, Israel/Jordan
Europe Elbrus, Russia
Caspian Sea, Azerbaijan/Russia/ Kazakhstan/Turkmenistan/Iran
North America McKinley (AK), U.S.A.
Death Valley (CA), U.S.A.
Oceania Jaya, New Guinea
Lake Eyre, South Australia
South America Cerro Aconcagua, Argentina
Valdés Peninsula, Argentina
  1 This is the deepest estimated depression, beneath the Marie Byrd Land ice cap. Vast areas of western Antarctica would be below sea level if stripped of their ice sheet; lower points beneath the ice may exist.  


  low velocity zone, at 72 km/45 mi to 250 km/155 mi depth. This zone corresponds to the aesthenosphere (see below) upon which earth's tectonic plates of lithosphere glide. Seismic velocities in the upper mantle are overall less than those in the transition zone and those of the transition zone are in turn less than those of the lower mantle. Faster propagation of seismic waves in the lower mantle implies that the lower mantle is more dense than the upper mantle.  
  The mantle is composed primarily of magnesium, silicon, and oxygen in the form of silicate minerals. In the upper mantle, the silicon in silicate minerals, such as olivine, is surrounded by four oxygen atoms. Deeper in the transition zone, greater pressures promote denser packing of oxygen such that some silicon is surrounded by six oxygen atoms, resulting in magnesium silicates with garnet and pyroxene structures. Deeper still in the lower mantle, all silicon is surrounded by six oxygen atoms so that the new mineral MgSiO3-perovskite predominates.  
  The asthenosphere is the rigid layer of the upper mantle that underlies the lithosphere, typically beginning at a depth of approximately 100 km/63 mi and extending to depths of approximately 260 km/160 mi. Sometimes referred to as the "weak sphere," the asthenosphere is characterized by being weaker and more elastic than the surrounding mantle. Its lack of shear strength results from the high temperature of the rocks approaching the melting point. Since seismic waves travel more slowly in the asthenosphere; it is also referred to as the "low velocity zone."  
  The asthenosphere's elastic behavior and low  
Deepest Geographical Depressions in the World
Depression Location Maximum depth below sea level
Dead Sea Israel/Jordan
Turfan Depression Xinjiang, China
Lake Assal Djibouti
Qattâra Depression Egypt
Poloustrov Mangyshlak Kazakhstan
Danakil Depression Ethiopia
Death Valley California, U.S.A.
Salton Sink California, U.S.A.
Zapadnyy Chink Ustyurta Kazakhstan
Priaspiyskaya Nizmennost Russia/Kazakhstan
Ozera Sarykamysh Uzbekistan/Kazakhstan
El Faiyûm Egypt
Valdés Peninsula Argentina





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  viscosity allow the overlying plates to move laterally and also allow overlying crust and mantle to move vertically in response to gravity to achieve isostatic equilibrium.  
  The innermost part of earth is the core. It is divided into an outer core, which begins at a depth of 2,898 km/1,800 mi, and an inner core, which begins at a depth of 4,982 km/3,095 mi. Both parts are thought to consist of iron-nickel alloy. The outer core is liquid and the inner core is solid.  
  The fact that seismic shear waves (see seismic wave below) disappear at the mantle—outer core boundary indicates that the outer core is molten, since shear waves cannot travel through fluid. Scientists infer the iron-nickel rich composition of the core from earth's density and its moment of inertia. The temperature of the core, as estimated from the melting point of iron at high pressure, is thought to be at least 4,000°C/7,232°F, but remains controversial. Earth's magnetic field is believed to be the result of the motions involving the inner and outer cores.  
  Plate Tectonics  
  Plate tectonics is the theory formulated in the 1960s to explain the phenomena of c0016-01.gifcontinental drift and seafloor spreading, and the formation of the major physical features of the earth's surface. The earth's outermost layer, the lithosphere, is regarded as a jigsaw puzzle of rigid plates that move relative to each other, probably under the influence of convection currents in the mantle beneath. At the margins of the plates, where they collide or move apart, major landforms such as mountains, volcanoes, ocean trenches, and mid-ocean ridges are created. The rate of plate movement is at most 15 cm/6 in per year.  
  New plate material is generated between two plates along the mid-ocean ridges, where basaltic lava is poured out by submarine volcanoes. Basaltic lava spreads outward away from the ridge crest at 1–6 cm/0.5–2.5 in per year. Plate material is consumed at a rate of 5–15 cm/2–6 in per year at the site of the deep ocean trenches, for example, along the Pacific coast of South America. These trenches are sites, called subduction zones, where two plates of lithosphere meet; the one bearing denser ocean-floor basalts plunges beneath the adjacent continental mass at an angle of 45°, giving rise to shallow earthquakes near the coast and progressively deeper earthquakes inland. In places the sinking plate may descend beneath an island arc of offshore islands, as in the Aleutian Islands and Japan, and in this case the shallow earthquakes will occur beneath the island arc. The destruction of ocean crust in this way accounts for another well-known geological fact—that there are no old rocks found in the ocean basins. The oldest sediments found are 150 million years old, but the vast majority are less than 80 million years old. This suggests that plate tectonics has been operating for at least the last 200 million years. In other areas plates slide past each other along transform faults, giving rise to shallow earthquakes. Sites where three plates meet are known as triple junctions.  
  Plate Tectonics
  Well-illustrated site on this geological phenomenon. As well as the plentiful illustrations, this site also has a good clear manner of explaining the way the plates of the earth's crust interact to produce seismic activity.  
  A plate (or tectonic plate) is one of several sections of lithosphere approximately 100 km/60 mi thick and at least 200 km/120 mi across, which together comprise the outermost layer of the earth like the pieces of the cracked surface of a hard-boiled egg.  
  The plates are made up of two types of crustal material: oceanic crust (sima) and continental crust (sial), both of which are underlain by a solid layer of mantle. Dense oceanic crust lies beneath the earth's oceans and consists largely of basalt. Continental crust, which underlies the continents and their continental shelves, is thicker, less dense, and consists of rocks rich in silica and aluminum.  
  Due to convection in the earth's mantle these pieces of lithosphere are in motion, riding on a more plastic layer of the mantle, called the aesthenosphere. Mountains, volcanoes, earthquakes, and other geological features and phenomena all come about as a result of interaction between the plates.  
  There are three types of plate margins: constructive, destructive, and conservative.  
  constructive margins Where two plates are moving apart from each other, molten rock from the mantle wells up in the space between the plates and hardens to form new crust, usually in the form of a mid-ocean ridge (such as the Mid-Atlantic Ridge). This is called a constructive margin because the newly formed crust accumulates on either side of the ridge, causing the seafloor to spread; the floor of the Atlantic Ocean is growing by 5 cm/2 in each year because of the wellingup of new material at the Mid-Atlantic Ridge.  
  destructive margins Destructive margins occur where two plates are moving toward each other. When plates  




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  plate tectonics The three main types of action in plate tectonics. (top) Seafloor
spreading. The upwelling of magma forces apart the plates, producing new crust
at the joint. Rapid extrusion of magma produces a domed ridge; more gentle
spreading produces a central valley. (middle) The drawing downward of an
oceanic plate beneath a continent produces a range of volcanic mountains
parallel to the plate edge. (bottom) Collision of continental plates produces
immense collisional mountains, such as the Himalayas. Younger mountains
are found near the coast with older ranges inland. The plates of the earth's
lithosphere are always changing in size and shape of each plate as material is
added at constructive margins and removed at destructive margins. The
process is extremely slow, but it means that the tectonic history of the earth
cannot be traced back further than about 200 million years.
  with denser oceanic crust meet plates with less dense continental crust, the denser of the two plates may be forced under the other into a region called the subduction zone. The descending plate melts to form a body of c0016-01.gifmagma, which may then rise to the surface through cracks and faults to form volcanoes. If the two plates consist of more buoyant continental crust, subduction does not occur. Instead, the crust crumples gradually to form ranges of young mountains, such as the Himalayas in Asia, the Andes in South America, and the Rockies  




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  constructive margin A rift in the lithospheric plates where new material is being
formed causes the plates to be pushed apart. This usually occurs as a result of
volcanic action.
  in North America. This process of mountain building is termed orogeny, or orogenesis.  
  conservative margins Plate boundaries in which two plates slide past each other are called conservative margins because crust is neither created nor destroyed. An example is the c0016-01.gifSan Andreas Fault, California, where the movement of the plates sometimes takes the form of sudden jerks, causing the earthquakes common in the San Francisco–Los Angeles area. Most of the earthquake and volcanic zones of the world are found in regions where two plates meet or are moving apart.  
  causes of plate movement The causes of plate movement are poorly understood. It has been known for some time that heat flow from the interior of the earth is high over the ocean ridges, and so thermal convection in the mantle has been proposed as a driving mechanism for the plates. The rising limbs of the convective mantle cells may be the plumes of hot, molten material that rise beneath the ocean ridges to be extruded as basaltic lava. The descending limbs of the convective cells may be linked to subduction zones. Oceanic crust is continually produced by, and returned to, the mantle, but the continental crust rocks, because of their buoyancy, remain on the surface.  
  development of plate tectonics theory The concept of continental drift was first put forward in 1915 in a book entitled The Origin of Continents and Oceans by the German meteorologist Alfred Wegener, who recognized that continental plates rupture, drift apart, and eventually collide with one another. Wegener's theory explained why the shape of the east coast of the Americas and that of the west coast of Africa seem to fit together like pieces of a jigsaw puzzle; evidence for the drift came from the presence of certain rock deposits which indicated that continents have changed position over time. In the early 1960s scientists discovered that most earthquakes occur along lines parallel to ocean trenches and ridges, and in 1965 the theory of plate tectonics was formulated by Canadian geophysicist John Tuzo Wilson; it has now gained widespread acceptance among earth scientists who have traced the movements of tectonic plates millions of years into the past. The widely accepted belief is that all the continents originally formed part of an enormous single land mass, known as Pangaea. This land was surrounded by a giant ocean known as Panthalassa. About 200 million years ago, Pangaea began to break up into two large masses called Gondwanaland and Laurasia, which in turn separated into the continents as they are today, and which have drifted to their present locations. In 1995 U.S. and French geophysicists produced the first direct evidence that the Indo-Australian plate has split in two in the middle of the Indian Ocean, just south of the Equator. They believe the split began about 8 million years ago.  
  Major Physical Features of the Earth's Surface  
  fault A fracture in the earth either side of which rocks have moved past one another is called a fault. Faults involve displacements, or offsets, ranging from the microscopic scale to hundreds of miles. Large offsets along a fault are the result of the accumulation of smaller movements (yards or less) over long  




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  fault Faults are caused by the movement of rock layers, producing such features as
block mountains and rift valleys. A normal fault is caused by a tension or stretching
force acting in the rock layers. A reverse fault is caused by compression forces. Faults
can continue to move for thousands or millions of years.
  periods of time. Large motions cause detectable earthquakes.  
  Faults are planar features. Fault orientation is described by the inclination of the fault plane with respect to the horizontal (see c0016-01.gifdip) and its direction in the horizontal plane (see c0016-01.gifstrike). Faults at a high angle with respect to the horizontal (in which the fault plane is steep) are classified as either normal faults, where one block has apparently moved downhill along the inclined fault plane, or reverse faults, where one block appears to have moved uphill along the fault plane. Normal faults occur where rocks on either side have moved apart. Reverse faults occur where rocks on either side have been forced together. A reverse fault that forms a low angle with the horizontal plane is called a thrust fault.  
  A lateral fault, or tear fault, occurs where the relative movement along the fault plane is sideways. A particular kind of fault found only in ocean ridges is the transform fault. On a map, an ocean ridge has a stepped appearance. The ridge crest is broken into sections, each section offset from the next. Between each section of the ridge crest the newly generated plates are moving past one another at different rates. Transform faults form to accommodate these variations in spreading rates.  
  Faults produce lines of weakness on the earth's surface (along their strike) that are often exploited by processes of weathering and erosion. Coastal caves and geos (narrow inlets) often form along faults and, on a larger scale, rivers may follow the line of a fault.  
  San Andreas Fault
  Detailed tour of the San Andreas Fault and the San Francisco Bay area, with information on the origination of the fault.  
  mid-ocean ridge A mid-ocean ridge is a mountain range on the seabed indicating the presence of a constructive plate margin (where tectonic plates are moving apart and magma rises to the surface; see plate tectonics above). Ocean ridges, such as the MidAtlantic Ridge, consist of many segments offset along transform faults, and can rise thousands of yards above the surrounding seabed.  
  Ocean ridges usually have a rift valley along their crests, indicating where the flanks are being pulled apart by the growth of the plates of the lithosphere beneath. The crests are generally free of sediment; increasing depths of sediment are found with increasing distance down the flanks.  
  ocean trench Ocean trenches are deep trenches in the seabed indicating the presence of a destructive margin (produced by the movements of plate tectonics). The subduction, or dragging downward of one plate of the lithosphere beneath another, means that the ocean floor is pulled down. Ocean trenches are found around the edge of the Pacific Ocean and the northeastern Indian Ocean; minor ones occur in the Caribbean and near the Falkland Islands.  
  Ocean trenches represent the deepest parts of the ocean floor, the deepest being the Mariana Trench which has a depth of 11,034 m/36,201 ft. At depths of below 6 km/3.6 mi there is no light and very high pressure; ocean trenches are inhabited by crustaceans, coelenterates (for example, sea anemones), polychaetes (a type of worm), mollusks, and echinoderms.  
  mountain Mountains are natural upward projections of the earth's surface that are higher and steeper than hills. Mountains are at least 330 m/1,000 ft above the surrounding topography. The process of mountain building (c0016-01.giforogeny) consists of volcanism, folding, faulting, and thrusting, resulting from the collision of two tectonic plates at a convergent margin. The existing rock is also subjected to high temperatures and  




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  fold The folding of rock strata occurs where compression causes them to buckle.
Over time, folding can assume highly complicated forms, as can sometimes be seen
in the rock layers of cliff faces or deep cuttings in the rock. Folding contributed to
the formation of great mountain chains such as the Himalayas.
  pressures causing metamorphism. Plutonic activity also can accompany mountain building.  
  island An island is an area of land surrounded entirely by water. Australia is classed as a continent rather than an island, because of its size.  
  Islands can be formed in many ways. Continental islands were once part of the mainland, but became isolated (by tectonic movement, erosion, or a rise in sea level, for example). Volcanic islands, such as Japan, were formed by the explosion of underwater volcanoes. Coral islands consist mainly of coral, built up over many years. An atoll is a circular coral reef surrounding a lagoon; atolls were formed when a coral reef grew up around a volcanic island that subsequently sank or was submerged by a rise in sea level. Barrier islands are found by the shore in shallow water, and are formed by the deposition of sediment eroded from the shoreline.  
  Earth Phenomena:
Earthquakes, Volcanism, and Magnetism
  An abrupt motion that propagates through the earth and along its surfaces is called an earthquake. Earthquakes are caused by the sudden release in rocks  
  coral atoll The formation of a coral atoll by the gradual sinking of a volcanic
island. The reefs fringing the island build up as the island sinks, eventually
producing a ring of coral around the spot where the island sank.




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Highest Mountains in the World by Region
Tanzania 5,895
  Kenya (Batian)  
Kenya 5,199
  Ngaliema (formerly Mt Stanley and Margherita Peak)  
Democratic Republic of Congo/Uganda 5,110
  Duwoni (formerly Umberto Peak)  
Uganda 4,896
  Baker (Edward Peak)  
Uganda 4,843
Alpine Europe      
  Mont Blanc  
France/Italy 4,807
  Monte Rosa  
Switzerland 4,634
Switzerland 4,545
Switzerland/Italy 4,527
Switzerland 4,505
  Vinson Massif  
China/Nepal 8,848
Kashmir/Jammu 8,611
India/Nepal 8,598
China/Nepal 8,511
  Yalung Kang  
India/Nepal 8,502
Snowy Mountains, New South Wales 2,230
Slovak Republic 2,655
Romania 2,544
Romania 2,535
Romania 2,518
Romania 2,509
  Elbrus, West Peak  
Russia 5,642
  Dykh Tau  
Russia/Georgia 5,203
Russia/Georgia 5,201
  Kashtan Tau  
Russia/Georgia 5,144
  Dzanghi Tau  
Russia 5,049
New Zealand      
  Cook (called Aorongi in Maori)  
west coast, South Island 3,754
North and Central America      
Alaska, U.S.A. 6,194
  Logan, Yukon  
Canada 6,050
  Citlaltépetl (Orizaba)  
Mexico 5,610
  St. Elias  
Alaska, U.S.A./Yukon, Canada 5,489
Mexico 5,452


  (table continued on next page)  




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  (table continued from previous page)  
West Irian, Papua New Guinea 5,030
West Irian, Papua New Guinea 4,922
  Oost Carstensz (also known as Jayakusumu Timur)  
West Irian, Papua New Guinea 4,840
West Irian, Papua New Guinea 4,730
West Irian, Papua New Guinea 4,717
  Mauna Kea  
Hawaü, U.S.A. 4,205
  Mauna Loa  
Hawaü, U.S.A. 4,170
  Pico de Aneto  
Spain 3,404
  Pico de Posets  
Spain 3,371
  Monte Perdido  
Spain 3,348
  Pico de la Maladeta  
Spain 3,312
  Pic de Vignemale  
France/Spain 3,298
Norway 2,472
Norway 2,469
Norway 2,405
Norway 2,286
South America      
  Cerro Aconcagua  
Argentina 6,960
  Ojos del Salado  
Argentina/Chile 6,908
Argentina 6,872
  Nevado de Pissis  
Argentina/Chile 6,779
  Huascarán Sur  
Peru 6,768
1 Including all of Papua New Guinea.      


Largest Islands in the World
Island Location
  sq km  
  sq mi  
Greenland northern Atlantic
New Guinea southwestern Pacific
Borneo southwestern Pacific
Madagascar Indian Ocean
Baffin Canadian Arctic
Sumatra Indian Ocean
Honshu northwestern Pacific
Great Britain northern Atlantic
Victoria Canadian Arctic
Ellesmere Canadian Arctic
Sulawesi Indian Ocean
South Island, New Zealand southwestern Pacific
Java Indian Ocean
North Island, New Zealand southwestern Pacific
Cuba Caribbean Sea
Newfoundland northwestern Atlantic
Luzon western Pacific


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Island Location
  sq km  
  sq mi  
Iceland northern Atlantic
Mindanao western Pacific
Ireland (Northern Ireland and the Republic of Ireland) northern Atlantic
Hokkaido northwestern Pacific
Sakhalin northwestern Pacific
Hispaniola–Dominican Republic and Haiti Caribbean Sea
Banks Canadian Arctic
Tasmania southwestern Pacific
Sri Lanka Indian Ocean
Devon Canadian Arctic


  of strain accumulated over time as a result of tectonics. The study of earthquakes is called seismology. Most earthquakes occur along faults (fractures or breaks) and c0016-01.gifBenioff zones. Plate tectonic movements generate the major proportion: as two plates move past each other they can become jammed. When sufficient strain has accumulated, the rock breaks, releasing a series of elastic waves (seismic waves) as the plates spring free. The force of earthquakes (magnitude) is measured on the Richter scale, and their effect (intensity) on the Mercalli scale. The point at which an earthquake originates is the seismic focus or hypocenter; the point on the earth's surface directly above this is the epicenter.  
  earthquake Image Information System
  EqIIS–earthquake Image Information System–is a fully searchable library of almost 8,000 images from more than 80 earthquakes.  
  Most earthquakes happen at sea and cause little damage. However, when severe earthquakes occur in highly populated areas they can cause great destruction and loss of life. The Alaskan earthquake of March 27, 1964 ranks as one of the greatest ever recorded. The c0016-01.gifSan Andreas fault in California, where the North American and Pacific plates move past each other, is a notorious site of many large earthquakes. The 1906 San Francisco earthquake is among the most  
Richter Scale
The Richter scale is based on measurement of seismic waves, used to determine the magnitude of an earthquake at its epicenter. The magnitude of an earthquake differs from its intensity, measured by the Mercalli scale, which is subjective and varies from place to place for the same earthquake. The Richter scale was named for the U.S. seismologist Charles Richter.
Magnitude Relative amount of energy released Examples Year
1 1    
2 31    
3 960    
4 30,000 Carlisle, England (4.7) 1979
5 920,000 Wrexham, Wales (5.1) 1990
6 29,000,000 San Fernando, California, U.S.A. (6.5) 1971
    northern Armenia (6.8) 1988
7 890,000,000 Loma Prieta, California, U.S.A. (7.1) 1989
    Kobe, Japan (7.2) 1995
    Rasht, Iran (7.7) 1990
    San Francisco, California, U.S.A. (7.7–7.9)1 1906
8 28,000,000,000 Tangshan, China (8.0) 1976
    Gansu, China (8.6) 1920
    Lisbon, Portugal (8.7) 1755
9 850,000,000,000 Prince William Sound, Alaska, U.S.A. (9.2) 1964
1 Richter's original estimate of a magnitude of 8.3 has been revised by two recent studies carried out by the California Institute of Technology and the U.S. Geological Survey.





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  Earthquake Locator Site
  Edinburgh University, Scotland, runs this worldwide earthquake locator site that allows visitors to search for the world's latest earthquakes. The locator works on a global map on which perspective can be zoomed in or out. There are normally around five or six earthquakes a day; you'll find it surprising how few make the news. The site also has some general information on earthquakes.  
  famous in history. The deadliest, most destructive earthquake in historical times is thought to have been in China in 1556.  
  A reliable form of earthquake prediction has yet to be developed, although the seismic gap theory has had some success in identifying likely locations. In 1987 a California earthquake was successfully predicted by measurement of underground pressure waves; prediction attempts have also involved the study of such phenomena as the change in gases issuing from the crust, the level of water in wells, slight deformation of the rock surface, a sequence of minor tremors, and the behavior of animals.  
  The possibility of earthquake prevention is remote. However, rock slippage might be slowed at movement points, or promoted at stoppage points, by the extraction or injection of large quantities of water underground, since water serves as a lubricant. This would ease overall pressure.  
  seismic wave A seismic wave is an energy wave generated by an earthquake or an artificial explosion. There are two types of seismic wave: body waves that travel through the earth's interior, and surface waves that travel through the surface layers of the crust and can be felt as the shaking of the ground, as in an earthquake.  
  body wave There are two types of body wave: P-waves and S-waves, so-named because they are the primary and secondary waves detected by a seismograph. P-waves are longitudinal waves (wave motion in the direction the wave is traveling), whose compressions and rarefactions resemble those of a sound wave. S-waves are transverse waves or shear waves, involving a back-and-forth shearing motion at right angles to the direction the wave is traveling.  
Most Destructive Earthquakes in the World
Source: U.S. Geological Survey National Earthquake Information Center
(N/A = not available.)
Date Location Estimated number of deaths Magnitude (Richter scale)
January 23, 1556 Shaanxi, China
October 11, 1737 Calcutta, India
July 27, 1976 Tangshan, China
  255,000 1  
August 9, 1138 Aleppo, Syria
May 22, 1927 near Xining, China
December 22, 856 Damghan, Iran
December 16, 1920 Gansu, China
March 23, 893 Ardabil, Iran
September 1, 1923 Kwanto, Japan
December 30, 1730 Hokkaido, Japan
September 1290 Chihli, China
November 1667 Caucasia, Russia
November 18, 1727 Tabriz, Iran
December 28, 1908 Messina, Italy
November 1, 1755 Lisbon, Portugal
December 25, 1932 Gansu, China
May 31, 1970 northern Peru
1268 Cilicia, Asia Minor
January 11, 1693 Sicily, Italy
February 4, 1783 Calabria, Italy
June 20, 1990 Iran
May 30, 1935 Quetta, India
1 This is the official casualty figure; the estimated death toll is as high as 750,000.





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  Two kinds of seismograph  
  seismograph A seismogram, or recording made by a seismograph.
Such recordings are used to study earthquakes and in prospecting.
  Because liquids have no resistance to shear and cannot sustain a shear wave, S-waves cannot travel through liquid material. The earth's outer core is believed to be liquid because S-waves disappear at the mantle-core boundary, while P-waves do not.  
  surface wave Surface waves travel in the surface and subsurface layers of the crust. Rayleigh waves travel along the free surface (the uppermost layer) of a solid material. The motion of particles is elliptical, like a water wave, creating the rolling motion often felt during an earthquake. Love waves are transverse waves trapped in a subsurface layer due to different densities in the rock layers above and below. They have a horizontal side-to-side shaking motion transverse (at right angles) to the direction the wave is traveling.  
  earth's volcanism A volcano is a crack in the earth's crust through which hot magma (molten rock) and gases well up. The magma is termed lava when it reaches the surface. A volcanic mountain, usually cone shaped with a crater on top, is formed around the opening, or vent, by the build-up of solidified lava and ashes (rock fragments). Most volcanoes arise on plate margins (see plate tectonics above), where the movements of plates generate magma or allow it to rise from the mantle beneath. However, a number are found far from plate-margin activity, on ''hot spots" where the earth's crust is thin.  
  volcano There are two main types of volcano,
but three distinctive cone shapes. Composite
volcanoes emit a stiff, rapidly solidifying lava
which forms high, steepsided cones. Volcanoes
that regularly throw out ash build up flatter
domes known as cinder cones. The lava from a
shield volcano is not ejected violently, flowing
over the crater rim forming a broad low profile.




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Major Volcanoes Active in the 20th Century, by Region
Date of last
eruption or activity
isolated mountain, Cameroon
Virungu, Democratic Republic of Congo
Democratic Republic of Congo
  OI Doinyo Lengai  
  Lake Nyos  
Ross Island, McMurdo Sound
  Deception Island  
South Shetland Island
Sumatra, Indonesia
Lombok, Indonesia
Java, Indonesia
Java, Indonesia
Java, Indonesia
Bali, Indonesia
Honshu, Japan
Java, Indonesia
Sumatra, Indonesia
Honshu, Japan
  Nigata Yake-yama  
Honshu, Japan
Luzon, Philippines
Negros, Philippines
Honshu, Japan
Java, Indonesia
Honshu, Japan
  Sangeang Api  
Lesser sunda Island, Indonesia
Luzon, Philippines
Java, Indonesia
Sumatra, Indonesia
Atlantic Ocean        
  Pico de Teide  
Tenerife, Canary Islands, Spain
Cape Verde Islands
Jan Mayen Island, Norway
  La Grande Soufrière  
Basse-Terre, Guadeloupe
  La Soufriére St Vincent  
St Vincent and the Grenadines
  Soufriére Hills/Chances Peak  
Central America        
Sierra Madre, Guatemala
Sierra Madre, Guatemala
Sierra Madre, Guatemala
  Santa Maria  
Sierra Madre, Guatemala
Cordillera Central, Costa Rica
Cordillera Central, Costa Rica
Cordillera Central, Costa Rica
Sierra Madre, Guatemala
  San Miguel  
El Salvador
Costa Rica


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Date of last
eruption or activity
Kamchatka Peninsula, Russia
Kamchatka Peninsula, Russia
Kamchatka Peninsula, Russia
Sicily, Italy
Kamchatka Peninsula, Russia
Kurile Islands, Russia
Kurile Islands, Russia
  Sarychev Peak  
Kurile Islands, Russia
Lipari Islands, Italy
  Santorini (Thera)  
Cyclades, Greece
Indian Ocean        
  Piton de la Fournaise (Le Volcan)  
Réunion Island, France
  Mauna Loa  
Hawaü, U.S.A.
Hawaü, U.S.A.
North America        
Altiplano de México, Mexico
Altiplano de México, Mexico
Alaska Range (AK) U.S.A.
  Lassen Peak  
California, U.S.A.
Alaska Range (AK) U.S.A.
Alaska Range (AK) U.S.A.
Aleutian Islands, Alaska, U.S.A.
  St. Helens  
Washington, U.S.A.
Alaska Range (AK) U.S.A.
Alaska Range (AK) U.S.A.
  Novarupta (Katmai)  
Alaska Range (AK) U.S.A.
  El Chichon  
Altiplano de México, Mexico
Aleutian Islands, Alaska, U.S.A.
New Zealand
Papua New Guinea
New Zealand
Papua New Guinea
Papua New Guinea
Papua New Guinea
Papua New Guinea
New Zealand
Papua New Guinea
Papua New Guinea
Mariana Islands
  White Island  
New Zealand
South America        
  San Pedro  
Andes, Chile
Andes, Chile
Andes, Chile
  San José  
Andes, Chile
Andes, Ecuador
Andes, Ecuador
Andes, Peru
Andes, Chile
Andes, Chile
  Nevado del Ruiz  
Andes, Colombia
Andes, Colombia
Andes, Ecuador





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  Composite volcanoes, such as Stromboli and Vesuvius in Italy, are found at destructive plate margins (areas where plates are being pushed together), usually in association with island arcs and coastal mountain chains. The magma is mostly derived from plate material and is rich in silica. This makes a very stiff lava such as andesite, which solidifies rapidly to form a high, steep-sided volcanic mountain. The magma often clogs the volcanic vent, causing violent eruptions as the blockage is blasted free, as in the eruption of Mount St. Helens, United States, in 1980. The crater may collapse to form a caldera.  
  Shield volcanoes, such as Mauna Loa in Hawaü, United States, are found along the rift valleys and ocean ridges of constructive plate margins (areas where plates are moving apart), and also over hot spots. The magma is derived from the earth's mantle and is quite free-flowing. The lava formed from this magma—usually basalt—flows for some distance over the surface before it sets and so forms broad low volcanoes. The lava of a shield volcano is not ejected violently but simply flows over the crater rim.  
  Site examining the complex geology of Vesuvius and its famous eruption of A.D. 79. There are images of the volcano and historical drawings. There is also a link to a local site campaigning for an improved civil defense plan as the volcano prepares once more to explode.  
  The type of volcanic activity is also governed by the age of the volcano. The first stages of an eruption are usually vigorous as the magma forces its way to the surface. As the pressure drops and the vents become established, the main phase of activity begins, composite volcanoes giving pyroclastic debris and shield volcanoes giving lava flows. When the pressure from below ceases, due to exhaustion of the magma chamber, activity wanes and is confined to the emission of gases and in time this also ceases. The volcano then enters a period of quiescence, after which activity may resume after a period of days, years, or even thousands of years. Only when the root zones of a volcano have been exposed by erosion can a volcano be said to be truly extinct.  
  Provided by Michigan Technological University, this site includes a world map of volcanic activity with information on recent eruptions, the latest research in remote sensing of volcanoes, and many spectacular photographs.  
  Many volcanoes are submarine and occur along mid-ocean ridges. The chief terrestrial volcanic regions are around the Pacific rim (Cape Horn to Alaska); the central Andes of Chile (with the world's highest volcano, Guallatiri, 6,060 m/19,900 ft); North Island, New Zealand; Hawaü; Japan; and Antarctica. There are more than 1,300 potentially active volcanoes on earth. Volcanism has helped shape other members of the solar system, including the moon, Mars, Venus, and Jupiter's moon Io.  
  There are several methods of monitoring volcanic activity. They include seismographic instruments on the ground, aircraft monitoring, and space monitoring using remote sensing satellites.  
  earth's magnetism The change in polarity of earth's magnetic field is called a polar reversal. Like all magnets, earth's magnetic field has two opposing regions, or poles, one of attraction and one of repulsion,  
Major Pre-20th Century Volcanic Eruptions
Volcano Location Year
Estimated number of deaths
Santorini (Thera) Greece c. 1470 B.C.
Vesuvius Italy A.D. 79
Kelut Java, Indonesia 1586
Etna Sicily, Italy 1669
Vesuvius Italy 1631
Papandayan Java, Indonesia 1772
Laki Iceland 1783
Unzen Japan 1792
Tambora Sumbawa, Indonesia 1815
Krakatoa Indonesia 1883
1 Number of deaths unknown; the explosion was four times more powerful than Krakatoa.
2 Estimates vary greatly, from 2,000 (lowest) to 20,000 (highest).
3 A further 82,000 deaths were caused by starvation and disease brought on by the eruption.





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Major Volcanic Eruptions in the 20th Century
Volcano Location Year
Estimated number of deaths
Santa María Guatemala 1902
Pelée Martinique 1902
Taal Philippines 1911
Kelut Java, Indonesia 1919
Vulcan Papua New Guinea 1937
Lamington Papua New Guinea 1951
St Helens U.S.A. 1980
El Chichon Mexico 1982
Nevado del Ruiz Colombia 1985
Lake Nyos Cameroon 1986
Pinatubo Luzon, Philippines 1991
Unzen Japan 1991
Mayon Philippines 1993
Loki Iceland 1996
Soufriere Montserrat 1997
Merapi Java, Indonesia 1998


  positioned approximately near the geographical North and South Poles. During a period of normal polarity the region of attraction corresponds with the North Pole. Today, a compass needle, like other magnetic materials, aligns itself parallel to the magnetizing force and points to the North Pole. During a period of reversed polarity, the region of attraction would change to the South Pole and the needle of a compass would point south.  
  magnetic field The earth's magnetic field
is similar to that of a bar magnet with poles
near, but not exactly at, the geographic poles.
Compass needles align themselves with the
magnetic field, which is horizontal near the
equator and vertical at the magnetic poles.
  Studies of the magnetism retained in rocks at the time of their formation (like little compasses frozen in time) have shown that the polarity of the magnetic field has reversed repeatedly throughout geological time.  
  Polar reversals are a random process. Although the average time between reversals over the last 10 million years has been 250,000 years, the rate of reversal has changed continuously over geological time. The most recent reversal was 700,000 years ago; scientists have no way of predicting when the next reversal will occur. The reversal process takes about a thousand years. Movements of the earth's molten core are thought to be responsible for the magnetic field and its polar reversals. Dating rocks using distinctive sequences of magnetic reversals is called paleomagnetic stratig-raphy.  
  secular variation The changes in the position of earth's magnetic poles measured with respect to geographical positions, such as the North pole, throughout geological time is called secular variation.  
  Earth's crust is composed of rocks, aggregates of minerals or materials of organic origin that have consolidated into hard masses. There are three types of rocks: igneous, sedimentary, and metamorphic. The property of a rock will depend on its components and the conditions of its formation. Igneous rocks are formed by the cooling and solidification of magma, the molten rock material that originates in the lower part of the earth's crust, or mantle, where it reaches temperatures  




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  oldest known rocks  
  The oldest known rocks are found near the Great Slave and Great Bear lakes in Canada. The rocks, which include granite, are 4.03 billion years old.  


  as high as 1,000°C. The rock may form on or below the earth's surface and is usually crystalline in texture. Larger crystals are more common in rocks such as granite which have cooled slowly within the earth's crust; smaller crystals form in rocks such as basalt which have cooled more rapidly on the surface.  
  Sedimentary rocks are formed by the compression of particles deposited by water, wind, or ice. They may be created by the erosion of older rocks, the deposition of organic materials, or they may be formed from chemical precipitates. For example, sandstone is derived from sand particles, limestone from the remains of sea creatures, and gypsum is precipitated from evaporating salt water. Sedimentary rocks are typically deposited in distinct layers or strata and many contain fossils.  
  Metamorphic rocks are formed through the action of high pressure or heat on existing igneous or sedimentary rocks, causing changes to the composition, structure, and texture of the rocks. For example, marble is formed by the effects of heat and pressure on limestone, while granite may be metamorphosed into gneiss, a coarse-grained foliated rock.  
  rock identification  
  Rocks can often be identified by their location and appearance. For example, sedimentary rocks lie in stratified, or layered, formations and may contain fossils; many have markings such as old mud cracks or ripple marks caused by waves. Except for volcanic glass, all igneous rocks are solid and crystalline. Some appear dense, with microscopic crystals, and others have larger, easily seen crystals. They occur in volcanic areas, and in intrusive formations that geologists call batholiths, laccoliths, sills, dikes, and stocks. Many metamorphic rocks have characteristic bands, and are easily split into sheets or slabs. Rock formations and strata are often apparent in the cliffs that line a seashore, or where rivers have gouged out deep channels to form gorges and canyons. They are also revealed when roads are cut through hillsides, or by excavations for quarrying and mining. Rock and fossil collecting has been a popular hobby since the 19th century and such sites can provide a treasure trove of finds for the collector.  
  Where deposits of economically valuable minerals occur rocks are termed ores. Rocks break down as a result of weathering into very small particles that combine with organic materials from plants and animals to form soil. In geology the term "rock" can also include unconsolidated materials such as sand, mud, clay, and peat.  
  rock studies  
  The study of the earth's crust and its composition fall under a number of interrelated sciences, each with its own specialists. Among these are geologists, who identify and survey rock formations and determine when and how they were formed, petrologists, who identify and classify the rocks themselves, and mineralogists, who study the mineral contents of the rocks. Paleontologists study the fossil remains of plants and animals found in rocks.  
  applications of rock studies Data from rock studies and surveys enable scientists to trace the history of the earth and learn about the kind of life that existed here millions of years ago. The data are also used in locating and mapping deposits of fossil fuels such as coal, oil, and natural gas, and valuable mineral-containing ores providing metals such as aluminum, iron, lead, and tin, and radioactive elements such as radium and uranium. These deposits may lie close to the earth's surface or deep underground, often under oceans. In some regions, entire mountains are composed of deposits of iron or copper ores, while in other regions rocks may contain valuable nonmetallic minerals such as borax and graphite, or precious gems such as diamonds and emeralds.  
  rocks as construction materials In addition to the mining and extraction of fuels, metals, minerals, and gems, rocks provide useful building and construction materials. Rock is mined through quarrying, and cut into blocks or slabs as building stone, or crushed or broken for other uses in construction work. For instance, cement is made from limestone and, in addition to its use as a bonding material, it can be added to crushed stone, sand, and water to produce strong, durable concrete, which has many applications, such as the construction of roads, runways, and dams.  
  Among the most widely used building stones are granite, limestone, sandstone, marble, and slate. Granite provides one of the strongest building stones and is resistant to weather, but its hardness makes it difficult to cut and handle. Limestone is a hard and lasting stone that is easily cut and shaped and is widely used for public buildings. The color and texture of the stone can vary with location. Sandstone varies in color and texture; like limestone, it is relatively easy to quarry and work and is used for similar purposes. Marble is a classic stone, worked by both builders and sculptors. Pure marble is white, streaked with veins of black, gray, green, pink, red, and yellow. Slate is  




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  intrusion Igneous intrusions can be a variety of shapes and sizes. Laccoliths are
domed circular shapes, and can be many miles across. Sills are intrusions that flow between
rock layers. Pipes or necks connect the underlying magma chamber to surface volcanoes.
  fine-grained rock that can be split easily into thin slabs and used as tiles for roofing and flooring. Its color varies from black to green and red.  
  igneous rocks  
  Igneous rocks are formed from cooling magma or lava, solidifying from a molten state. Igneous rocks are largely composed of silica (SiO2) and they are classified according to their crystal size, texture, method of formation, or chemical composition, for example by the proportions of light and dark minerals.  
  plutonic or intrusive igneous rocks Igneous rocks that crystallize from magma below the earth's surface are called plutonic or intrusive, depending on the depth of formation. They have large crystals produced by slow cooling; examples include gabbro and granite.  
  gabbro A gabbro is a mafic (consisting primarily of dark-colored crystals) igneous rock formed deep in the earth's crust. It contains pyroxene and calcium-rich feldspar, and may contain small amounts of olivine and amphibole. Its coarse crystals of dull minerals give it a speckled appearance.  
  Gabbro is the plutonic version of basalt (that is, derived from magma that has solidified below the earth's surface), and forms in large, slow-cooling intrusions.  
  granite Granite is a coarse-grained intrusive igneous rock, typically consisting of the minerals quartz, feldspar, and biotite mica. It may be pink or gray, depending on the composition of the feldspar. Granites are chiefly used as building materials.  
  Granites often form large intrusions in the core of mountain ranges, and they are usually surrounded by zones of metamorphic rock (rock that has been altered by heat or pressure). Granite areas have characteristic moorland scenery. In exposed areas the bedrock may be weathered along joints and cracks to produce a tor, consisting of rounded blocks that appear to have been stacked upon one another.  
  extrusive or volcanic igneous rocks Igneous rocks extruded at the surface from lava are called extrusive or volcanic. Rapid cooling results in small crystals; basalt is an example.  
  basalt Basalt is the commonest extrusive igneous rock in the solar system. Much of the surfaces of the terrestrial planets Mercury, Venus, earth, and Mars, as well as the moon, are composed of basalt. Earth's ocean floor is virtually entirely made of basalt. Basalt is mafic, that is, it contains relatively little c0016-01.gifsilica: about 50% by weight. It is usually dark gray but can also be green, brown, or black. Its essential constituent minerals are calcium-rich feldspar and calcium and magnesium-rich pyroxene.  
  The groundmass may be glassy or finely crystalline, sometimes with large crystals embedded. Basaltic lava tends to be runny and flows for great distances before solidifying. Successive eruptions of basalt have formed the great plateaus of Colorado and the Deccan plateau region of southwest India. In some places, such as Fingal's Cave in the Inner Hebrides of Scotland and the Giant's Causeway in Antrim, Northern Ireland, shrinkage during the solidification of the molten lava caused the formation of hexagonal columns.  
  The dark-colored lowland maria regions of the  




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  moon are underlain by basalt. Lunar mare basalts have higher concentrations of titanium and zirconium and lower concentrations of volatile elements like potassium and sodium relative to terrestrial basalts. Martian basalts are characterized by low ratios of iron to manganese relative to terrestrial basalts, as judged from some martian meteorites and spacecraft analyses of rocks and soils on the Martian surface.  
  sedimentary rocks  
  Sedimentary rocks are formed by the accumulation and cementation of deposits that have been laid down by water, wind, ice, or gravity. They cover more than two-thirds of the earth's surface and comprise three major categories: clastic, chemically precipitated, and organic (or biogenic). Clastic sediments are the largest group and are composed of fragments of preexisting rocks; they include clays, sands, and gravels. Chemical precipitates include some limestones and evaporated deposits such as gypsum and halite (rock salt). Coal, oil shale, and limestone made of fossil material are examples of organic sedimentary rocks.  
  Most sedimentary rocks show distinct layering (stratification), caused by alterations in composition or by changes in rock type. These strata may become folded or fractured by the movement of the earth's crust, a process known as deformation.  
  chalk Chalk is a soft, fine-grained, whitish sedimentary rock composed of calcium carbonate (CaCO3), extensively quarried for use in cement, lime, and mortar, and in the manufacture of cosmetics and toothpaste. Blackboard chalk in fact consists of gypsum (calcium sulfate, CaSO4.2H2O).  
  Chalk was once thought to derive from the remains of microscopic animals or foraminifera. In 1953, however, it was seen under the electron microscope to be composed chiefly of coccolithophores, unicellular lime-secreting algae, and hence primarily of plant origin. It is formed from deposits of deep-sea sediments called oozes.  
  Chalk was laid down in the later Cretaceous period (see below) and covers a wide area in Europe. In England it stretches in a belt from Wiltshire and Dorset continuously across Buckinghamshire and Cambridgeshire to Lincolnshire and Yorkshire, and also forms the North and South Downs, and the cliffs of southern and southeast England.  
  limestone Limestone is a sedimentary rock composed chiefly of calcium carbonate (CaCO3), either derived from the shells of marine organisms or precipitated from solution, mostly in the ocean. Various types of limestone are used as building stone.  
  Marble is metamorphosed limestone. Certain so-called marbles are not in fact marbles but fine-grained fossiliferous limestones that take an attractive polish. Caves commonly occur in limestone. Karst is a type of limestone landscape.  
  sandstone Sandstones are sedimentary rocks formed from the consolidation of sand, with sand-sized grains (0.0625–2 mm/0.0025–0.08 in) in a matrix or cement. Their principal component is quartz. Sandstones are commonly permeable and porous, and may form freshwater aquifers. They are mainly used as building materials.  
  Sandstones are classified according to the matrix or cement material (whether derived from clay or silt; for example, as calcareous sandstone, ferruginous sandstone, siliceous sandstone).  
  shale A fine-grained and finely layered sedimentary rock composed of silt and clay is called shale. It is a weak rock, splitting easily along bedding planes to form thin, even slabs (by contrast, mudstone splits into irregular flakes). Oil shale contains kerogen, a solid bituminous material that yields petroleum when heated.  
  fossils A fossil (from Latin fossilis, "dug up") is a cast, impression, or the actual remains of an animal or plant preserved in rock. Fossils were created during periods of rock formation, caused by the gradual accumulation of sediment over millions of years at the bottom of the sea bed or an inland lake. Fossils may include footprints, an internal cast, or external impression. A few fossils are preserved intact, as with mammoths fossilized in Siberian ice, or insects trapped in tree resin that is today amber. The study of fossils is called paleontology. Paleontologists are able to deduce much of the geological history of a region from fossil remains.  
  About 250,000 fossil species have been discovered—a figure that is believed to represent less than 1 in 20,000 of the species that ever lived. Microfossils are so small they can only be seen with a microscope. They include the fossils of pollen, bone fragments, bacteria, and the remains of microscopic marine animals and plants, such as foraminifera and diatoms.  
  Trace Fossils Left By Dinosaurs
  All about the trace fossils left by dinosaurs. The site is divided into several categories of dinosaur fossils: tracks, eggs and nests, tooth marks, gastroliths, and coprolites and also includes images and descriptions of each one.  




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  metamorphic rocks  
  Metamorphic rocks are those formed when either igneous or sedimentary rocks are altered in structure and composition by pressure, heat, or chemically active fluids after original formation. (If heat is sufficient to melt the original rock, technically it becomes an igneous rock upon cooling.) The term was coined in 1833 by the Scottish geologist Charles Lyell (1797–1875).  
  The mineral assemblage present in a metamorphic rock depends on the composition of the starting material (which may be sedimentary or igneous) and the temperature and pressure conditions to which it is subjected. There are two main types of metamorphism. Thermal metamorphism, or contact metamorphism, is brought about by the baking of solid rocks in the vicinity of an igneous intrusion (molten rock, or magma, in a crack in the earth's crust). It is responsible, for example, for the conversion of limestone to marble. Regional metamorphism results from the heat and intense pressures associated with the movements and collision of tectonic plates (see plate tectonics above). It brings about the conversion of shale to slate, for example.  
  gneiss A gneiss is a coarse-grained metamorphic rock, formed under conditions of high temperature and pressure, and often occurring in association with schists and granites. It has a foliated, or layered, structure consisting of thin bands of micas and/or amphiboles, dark in color, alternating with bands of granular quartz and feldspar that are light in color. Gneisses are formed during regional metamorphism; paragneisses are derived from metamorphism of sedimentary rocks and orthogneisses from metamorphism of granite or similar igneous rocks.  
  marble Marble is formed by metamorphosis of sedimentary limestone. It takes and retains a good polish, and is used in building and sculpture. In its pure form it is white and consists almost entirely of the mineral calcite (CaCO3). Mineral impurities give it various colors and patterns. Carrara, Italy, is known for white marble.  
Metamorphic Rocks
Main primary material (before metamorphism)
Typical depth
and temperature
Shale with
several minerals
Sandstone with
only quartz
Limestone with
only calcite
50,000 ft/570°F slate quartzite marble
65,000 ft/750°F schist    
82,000 ft/930°F gneiss    
98,500 ft/1,100°F hornfels quartzite marble


Mohs' scale of hardness
  To remember the order of hardness:  
  Tall gyroscopes can fly apart, orbiting quickly to complete disintegration.  
  (talc, gypsum or rock salt, calcite, fluorite, apatite, orthaclase, quartz, topaz, coorundum, diamond)  


  A mineral is any naturally formed inorganic substance with a particular chemical composition and a regularly repeating internal structure. Either in their perfect crystalline form or otherwise, minerals are the constituents of rocks. In more general usage, a mineral is any substance economically valuable for mining (including coal and oil, despite their organic origins).  
  Mineral forming processes include: melting of preexisting rock and subsequent crystallization of a mineral to form magmatic or volcanic rocks; weathering  
Mohs Scale
Number Defining mineral Other substances compared
1 talc  
2 gypsum 2c0035-03.gif fingernail
3 calcite 3c0035-03.gif copper coin
4 fluorite  
5 apatite 5c0035-03.gif steel blade
6 orthoclase 5c0035-04.gif glass
7 quartz 7 steel file
8 topaz  
9 corundum  
10 diamond  
Note that the scale is not regular; diamond, at number 10 the hardest natural substance, is 90 times harder in absolute terms than corundum, number 9





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Crystal System
Crystal system Minimum symmetry Possible shape Mineral examples
cubic 4 threefold axes cube, octahedron, dodecahedron diamond, garnet, pyrite
tetragonal 1 fourfold axis square-based prism zircon
orthorhombic 3 twofold axes or mirror planes matchbox shape barite
monoclinic 1 twofold axis or mirror plane matchbox distorted in one plane gypsum
triclinic no axes or mirror planes matchbox distorted in three planes plagioclase feldspar
trigonal 1 threefold axis triangular prism, rhombohedron calcite, quartz
hexagonal 1 sixfold axis hexagonal prism beryl


  of rocks exposed at the land surface, with subsequent transport and grading by surface waters, ice, or wind to form sediments; and recrystallization through increasing temperature and pressure with depth to form metamorphic rocks.  
  Minerals are usually classified as magmatic, sedimentary, or metamorphic. The magmatic minerals include the feldspars, quartz, pyroxenes, amphiboles, micas, and olivines that crystallize from silica-rich rock melts within the crust or from extruded lavas. The most commonly occurring sedimentary minerals are either pure concentrates or mixtures of sand, clay minerals, and carbonates (chiefly calcite, aragonite, and dolomite). Minerals typical of metamorphism include andalusite, cordierite, garnet, tremolite, lawsonite, pumpellyite, glaucophane, wollastonite, chlorite, micas, hornblende, staurolite, kyanite, and diopside.  
  crystal The sodium chloride, or common
salt, crystal is a regular cubic array of
charged atoms (ions)–positive sodium
atoms and negative chlorine atoms.
Repetition of this structure builds up
into cubic salt crystals.
  One of the ways to distinguish one mineral from another is by hardness. Mohs' scale of hardness is an established scale of hardness for minerals. The scale is useful in mineral identification because any mineral will scratch any other mineral lower on the scale than itself, and similarly it will be scratched by any other mineral higher on the scale.  
  crystals and crystallography  
  A mineral can often be identified by the shape of its crystals and the system of crystallization determined. A crystal is any substance with an orderly three-dimensional arrangement of its atoms or molecules, thereby creating an external surface of clearly defined smooth faces having characteristic angles between them. Examples are table salt and quartz.  
  Each geometrical form, many of which may be combined in one crystal, consists of two or more faces—for example, dome, prism, and pyramid. A single crystal can vary in size from a submicroscopic particle to a mass some 30 m/100 ft in length. Crystals fall into seven crystal systems or groups, classified on the basis of the relationship of three or four imaginary axes that intersect at the center of any perfect, undistorted crystal.  
  Three common crystalline forms are: (1) the simple cubic structure of ionic crystals, such as those of sodium chloride (NaCl); (2) the face-centered cubic structure of metals such as aluminum, copper, gold, silver, and lead; and (3) the hexagonal close-packed structure of metals such as cadmium and zinc.  
  Understand the shapes and symmetries of crystallography, with these interactive drawings of cubic, tetrahedral, octahedral, and dodecahedral solids.  
  The scientific study of crystals is called crystallography. In 1912 it was found that the shape and size of the repeating atomic patterns (unit cells) in a crystal could be determined by passing X-rays through a sample. This method, known as X-ray diffraction,  




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  opened up an entirely new way of ''seeing" atoms. It has been found that many substances have a unit cell that exhibits all the symmetry of the whole crystal; in table salt (sodium chloride, NaCl), for instance, the unit cell is an exact cube.  
  Many materials were not even suspected of being crystals until they were examined by X-ray crystallography. It has been shown that purified biomolecules, such as proteins and DNA, can form crystals, and such compounds may now be studied by this method. Other applications include the study of metals and their alloys, and of rocks and soils.  
  calcite Calcite is a colorless, white, or light-colored common rock-forming mineral, calcium carbonate (CaCO3). It is the main constituent of limestone and marble and forms many types of invertebrate shell. Calcite often forms stalactites and stalagmites in caves and is also found deposited in veins through many rocks because of the ease with which it is dissolved and transported by groundwater; oolite is a rock consisting of spheroidal calcite grains. It rates 3 on the Mohs' scale of hardness. Large crystals up to 1 m/3 ft have been found in Oklahoma and Missouri, United States. Iceland spar is a transparent form of calcite used in the optical industry; as limestone it is used in the building industry.  
  diamond A diamond is a generally colorless, transparent mineral, an allotrope of carbon. It is regarded as a precious gemstone, and is the hardest substance known (10 on the Mohs scale). Industrial diamonds, which may be natural or synthetic, are used for cutting, grinding, and polishing.  
  Diamond crystallizes in the cubic system as octahedral crystals, some with curved faces and striations. The high refractive index of 2.42 and the high dispersion of light, or "fire," account for the spectral displays seen in polished diamonds.  
  Diamonds may be found as alluvial diamonds on or close to the earth's surface in riverbeds or dried watercourses; on the sea bottom (off southwest Africa); or, more commonly, in diamond-bearing volcanic pipes composed of "blue ground," kimberlite or lamproite, where the original matrix has penetrated the earth's crust from great depths. They are sorted from the residue of crushed ground by X-ray and other recovery methods.  
  feldspar Feldspar is actually a group of silicate minerals. Feldspars are the most abundant mineral type in the earth's crust. They are the chief constituents of igneous rock and are present in most metamorphic and sedimentary rocks. All feldspars contain silicon, aluminum, and oxygen, linked together to form a framework. Spaces within this framework structure are occupied by sodium, potassium, calcium, or occasionally barium, in various proportions. Feldspars form white, gray, or pink crystals and rank 6 on the Mohs scale of hardness.  
  The four extreme compositions of feldspar are represented by the minerals orthoclase (KAlSi3O8); albite (NaAlSi3O8); anorthite (CaAl2Si2O8); and celsian (BaAl2Si2O8). Plagioclase feldspars contain variable amounts of sodium (as in albite) and calcium (as in anorthite) with a negligible potassium content. Alkali feldspars (including orthoclase) have a high potassium content, less sodium, and little calcium.  
  The type known as moonstone has a pearl-like effect and is used in jewelry. Approximately 4,000 metric tons of feldspar are used in the ceramics industry annually.  
  quartz Quartz is the crystalline form of c0016-01.gifsilica (SiO2), one of the most abundant minerals of the earth's crust (12% by volume). Quartz occurs in many different kinds of rock, including sandstone and granite. It ranks 7 on the Mohs' scale of hardness and is resistant to chemical or mechanical breakdown. Quartzes vary according to the size and purity of their crystals. Crystals of pure quartz are coarse, colorless, transparent, show no cleavage, and fracture unevenly; this form is usually called rock crystal. Impure colored varieties, often used as gemstones, include c0016-01.gifagate, citrine quartz, and c0016-01.gifamethyst. Quartz is also used as a general name for the cryptocrystalline and noncrystalline varieties of silica, such as chalcedony, chert, and opal.  
  Quartz is used in ornamental work and industry, where its reaction to electricity makes it valuable in electronic instruments. Quartz can also be made synthetically.  
  Crystals that would take millions of years to form naturally can now be "grown" in pressure vessels to a standard that allows them to be used in optical and scientific instruments and in electronics, such as quartz wristwatches.  
  talc Talc (Mg3Si4O10(OH)2) is a hydrous magnesium silicate mineral. It occurs in tabular crystals, but the massive impure form, known as steatite or soapstone, is more common. It is formed by the alteration of magnesium compounds and is usually found in metamorphic rocks. Talc is very soft, ranked 1 on the Mohs scale of hardness. It is used in powdered form in cosmetics, lubricants, and as an additive in paper manufacture.  
  French chalk and potstone are varieties of talc. Soapstone has a greasy feel to it, and is used for carvings such as Inuit sculptures.  




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  Mineral Gallery
  This Mineral Gallery is a collection of descriptions and images of minerals, organized by mineral name, class (sulfides, oxides, carbonates, and so on), and grouping (such as gemstones, birth stones, and fluorescent minerals).  
  Geological Record:
Divisions of Geological Time and Dating
  The geological time scale embraces the history of the earth from its physical origin to the present day. Geological time is traditionally divided into eons (Precambrian and Phanerozoic, in ascending chronological order), which in turn are subdivided into eras, periods, epochs, ages, and finally chrons.  
  The terms eon, era, period, epoch, age and chron are geochronological units representing intervals of geological time. Rocks representing an interval of geological time comprise a chronostratigraphic unit. Each of the hierarchical geochronological terms has a chronostratigraphic equivalent. Thus, rocks formed during an eon (a geochronological unit) are members of an eonothem (the chronostratigraphic unit equivalent of eon). Rocks of an era belong to an erathem. The chronostratigraphic equivalents of period, epoch, age, and chron are system, series, stage, and chronozone, repectively.  
  eon Eons are the largest units of geological time and include the Precambrian eon, spanning from 4.6 billion years to 570 million years, and the Phanerozoic eon, lasting from 570 million years to the present. Rocks representing an eon of geological time comprise an eonothem.  
  era Eras are subdivisions of eons. The currently recognized eras all fall within the Phanerozoic eon—or the vast span of time, starting about 570 million years ago, when fossils are found to become abundant. The eras in ascending order are the Paleozoic, Mesozoic, and Cenozoic. We are living in the Recent epoch of the Quaternary period of the Cenozoic era. Rocks representing an era of geological time comprise an erathem. Eras are further subdivided into periods.  
geological periods
  To remember the geological periods:  
  Camels often sit down carefully. Perhaps their joints creak? Early oiling might prevent permanent rheumatism.  
  (Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Eocene, Oligocene, Miocene, Pliocene, Pleistocene [Recent])  


  period Periods are subdivisions of eras and are themselves subdivided into epochs. They are the basic time units of the geological timescale. However, the periods of the Mesozoic era—the Triassic, Jurassic, and Cretaceous periods—are not divided further into epochs.  
  epoch Epochs are subdivisions of geological periods in the geological time scale. Epochs are sometimes given their own names (such as the Paleocene, Eocene, Oligocene, Miocene, and Pliocene epochs comprising the Tertiary period), or they are referred to as the late, early, or middle portions of a given period (such as the Late Cretaceous or the Middle Triassic epoch). Rocks representing an epoch of geological time comprise a series. The subdivisions of epochs are ages.  
  Dating is the process of determining the age of geological structures, rocks, and fossils, and placing them in the context of geological time. The techniques are of two types: relative dating and absolute dating. Relative dating can be carried out by identifying fossils of creatures that lived only at certain times (marker fossils), and by looking at the physical relationships of rocks to other rocks of a known age.  
  Absolute dating is achieved by measuring how much of a rock's radioactive elements have changed since the rock was formed, and is called radiometric dating. Radiometric dating was discovered in the first part of the 20th century by the U.S. radiochemist Bertram Boltwood, who used the ratio of the radioactive element uranium to the product of its decay, the element lead, to date rocks. Using his uranium—lead technique he determined that the earth was at least 2.2 billion years old. Many radiometric dating methods have been developed since Boltwood's time, such as rubidium—strontium (Rb–Sr) and potassium—argon (K–Ar) methods, enabling earth scientists to establish the age of the earth as 4.5 billion years.  
  radiocarbon dating Radiocarbon dating, or carbon dating, is a method of dating more recent organic materials (for example, bone or wood), which used in geology and archeology. Plants take up carbon dioxide gas from the atmosphere and incorporate it into their tissues, and some of that carbon dioxide contains the radioactive isotope of carbon, 14C or carbon-14. As this decays at a known rate (half of it decays every 5,730 years), the time elapsed since the plant died can  




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  be measured in a laboratory. Animals take carbon-14 into their bodies from eating plant tissues and their remains can be similarly dated. After 120,000 years so little carbon-14 is left that no measure is possible.  
  Radiocarbon dating was first developed in 1949 by the U.S. chemist Willard Libby. The method yields reliable ages back to about 50,000 years, but its results require correction since Libby's assumption that the concentration of carbon-14 in the atmosphere was constant through time has subsequently been proved wrong. Discrepancies were noted between carbon-14 dates for Egyptian tomb artifacts and construction dates recorded in early local texts. Radiocarbon dates from tree rings showed that material before 1000 B.C. had been exposed to greater concentrations of carbon-14. Now radiocarbon dates are calibrated against calendar dates obtained from tree rings, or, for earlier periods, against uranium/thorium dates obtained from coral. The carbon-14 content is determined by counting beta particles with either a proportional gas or a liquid scintillation counter for a period of time. A new advance, accelerator mass spectrometry, requires only tiny samples and counts the atoms of carbon-14 directly, disregarding their decay.  
  paleomagnetic stratigraphy The use of distinctive sequences of magnetic polarity reversals to date rocks is called paleomagnetic stratigraphy. Magnetism retained in rocks at the time of their formation are matched with known dated sequences of polar reversals or with known patterns of secular variation.  
  Geological Timescale  
  Precambrian Eon representing the time from the formation of earth (4.6 billion years ago) up to 570 million years ago. Its boundary with the succeeding Cambrian period marks the time when animals first developed hard outer parts (exoskeletons) and so left abundant fossil remains. The Precambrian eon comprises about 85% of geological time and is divided into two eras: the Archean, in which no life existed, and the Proterozoic, in which there was life in some form.  
  Archean, or Archeozoic The widely used term for the earliest era of geological time is the Archean; the first part of the Precambrian eon, spanning the interval from the formation of earth to about 3.5 billion years ago.  
  Proterozoic The Proterozoic era of geological time is the second division of the Precambrian eon from 3.5 billion to 570 million years ago. It is defined as the time of simple life, since many rocks dating from this era show traces of biological activity, and some contain the fossils of bacteria and algae.  
  Phanerozoic Eon consisting of the most recent 570 million years. It comprises the Paleozoic, Mesozoic, and Cenozoic eras. The vast majority of fossils come from this eon, owing to the evolution of hard shells and internal skeletons. The name means "interval of well-displayed life."  
  Paleozoic The era of geological time 570–245 million years ago is called the Paleozoic. It is comprised of the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. The Cambrian, Ordovician, and Silurian constitute the Lower or Early Paleozoic; the Devonian, Carboniferous, and Permian make up the Upper or Late Paleozoic. The era includes the evolution of hard-shelled multicellular life forms in the sea; the invasion of land by plants and animals; and the evolution of fish, amphibians, and early reptiles. The earliest identifiable fossils date from this era.  
  The climate at this time was mostly warm with short ice ages. The continents were very different from the present ones but, toward the end of the era, all were joined together as a single world continent called Pangea.  
  Cambrian The Cambrian period of geological time lasted from 570–510 million years ago; it is the first period of the Paleozoic era. All invertebrate animal life appeared, and marine algae were widespread. The Cambrian Explosion 530–520 million years ago saw the first appearance in the fossil record of all modern animal phyla; the earliest fossils with hard shells, such as trilobites, date from this period. The name comes from Cambria, the medieval Latin name for Wales, where Cambrian rocks are typically exposed and were first described.  
  Kirschvink Cambrian Explosion Research
  Part of a larger site maintained by Scientific American, this page reports on the research of Joseph Kirschvink of the California Institute of Technology that suggests the so-called "Cambrian Explosion" resulted from a sudden shifting of the earth's crust.  
  Ordovician The Ordovician period of geological time is the second period of the Paleozoic era, from 510–439 million years ago. Animal life was confined to the sea: reef-building algae and the first jawless fish  




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  are characteristic. The period is named for the Ordovices, an ancient Welsh people, because the system of rocks formed in the Ordovician period was first studied in Wales.  
  Silurian The third period of the Paleozoic era is the Silurian, lasting from 439–409 million years ago. Silurian sediments are mostly marine and consist of shales and limestone. Luxuriant reefs were built by coral-like organisms. The first land plants began to evolve during this period, and there were many ostracoderms (armored jawless fishes). The first jawed fishes (called acanthodians) also appeared.  
  Devonian The Devonian is the fourth period of the Paleozoic era representing 408–360 million years ago. Many desert sandstones from North America and Europe date from this time. The first land plants flourished in the Devonian period, corals were abundant in the seas, amphibians evolved from air-breathing fish, and insects developed on land. The name comes from the county of Devon in southwest England, where Devonian rocks were first studied.  
  Carboniferous The fifth period of the Paleozoic era—362.5 to 290 million years ago—is the Carboniferous. In the United States it is represented by two periods: the Mississippian (lower Carboniferous) and the Pennsylvanian (upper Carboniferous). Typical of the lower-Carboniferous rocks are shallow-water limestones, while upper-Carboniferous rocks have delta deposits with coal (hence the name). Amphibians were abundant, and reptiles evolved during this period.  
  Permian The sixth and last period of the Paleozoic era, from 290–245 million years ago, is the Permian. Its end was marked by a significant change in marine life, including the extinction of many corals and trilobites. Deserts were widespread, terrestrial amphibians and mammal-like reptiles flourished, and cone-bearing plants (gymnosperms) came to prominence. In the oceans, 49% of families and 72% of genera vanished in the late Permian. On land, 78% of reptile families and 67% of amphibian families disappeared.  
  The transition from the Permian period to the Triassic period of the Mesozoic era is referred to as the Permo-Triassic boundary. This boundary represents the largest mass extinction in geological history; 90% of all species became extinct.  
  Mesozoic Following the Paleozoic era is the Mesozoic era of geological time, spanning 245–65 million years ago. The Mesozoic era consists of the Triassic, Jurassic, and Cretaceous periods. At the beginning of the era, the continents were joined together as Pangea; dinosaurs and other giant reptiles dominated the sea and air; and ferns, horsetails, and cycads thrived in a warm climate worldwide. By the end of the Mesozoic era, the continents had begun to assume their present positions, flowering plants were dominant, and many of the large reptiles and marine fauna were becoming extinct.  
  Triassic The period of geological time 245–208 million years ago is the Triassic, the first period of the Mesozoic era. The continents were fused together to form the world continent Pangea. Triassic sediments contain remains of early dinosaurs and other reptiles now extinct. By late Triassic times, the first mammals had evolved. The climate was generally dry; desert sandstones are typical Triassic rocks.  
  Jurassic The middle period of the Mesozoic era, 208–146 million years ago, is the Jurassic. Climates worldwide were equable, creating forests of conifers and ferns; dinosaurs were abundant, birds evolved, and limestones and iron ores were deposited. The name comes from the Jura Mountains in France and Switzerland, where the rocks formed during this period were first studied.  
  Cretaceous The Cretaceous (Latin creta "chalk") is the period of geological time approximately 144.2–65 million years ago. It is the last period of the Mesozoic era, during which angiosperm (seed-bearing) plants evolved, and dinosaurs reached a peak before their extinction at the end of the period. The northern European chalk, which forms the white cliffs of Dover, was deposited during the latter half of the Cretaceous.  
  K-T boundary The geologists' shorthand for the boundary between the rocks of the Cretaceous and the Tertiary periods 65 million years ago is the K-T boundary. It coincides with the end of the extinction of the dinosaurs and in many places is marked by a layer of clay or rock enriched in the element iridium. Extinction of the dinosaurs at the K-T boundary and deposition of the iridium layer are thought to be the result of either the impact of a meteorite (or comet) that crashed into the Yucatán Peninsula (forming the Chicxulub crater) or the result of intense volcanism on the continent of India.  
  Cenozoic, or Caenozoic The Cenozoic is the era of geological time that began 65 million years ago and continues to the present day. It is divided into the Tertiary and Quaternary periods. The Cenozoic marks the emergence of mammals as a dominant group, including humans, and the formation of the mountain chains of the Himalayas and the Alps.  
  Tertiary The Tertiary period of geological time, spanning 65–1.64 million years ago, is divided into five  




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What Killed the Dinosaurs?
  By Edward Young  
Sixty-five million years ago dinosaurs disappeared. With them went 70% of all species of the time. This "mass extinction," one of many such events throughout earth's history, marks the end of the Cretaceous period and the beginning of the Tertiary period, an interval of geological time known as the K-T boundary (K for Kreide, German for Cretaceous). What killed the dinosaurs? Two rival hypotheses dominate. One is that a huge asteroid or comet collided with earth during K-T time, the other is that voluminous volcanism in what is now western India was responsible. Distinguishing between these two hypotheses has proven difficult. In either case, the possible link between these events and the demise of the dinosaurs is forcing scientists to reexamine the role of catastrophe in earth's history.
from catastrophism to uniformitarianism
Geology arose as a modern scientific discipline in the early 19th century with the fall of catastrophism, the notion that the history of our planet was shaped by successive catastrophic events, and the rise of uniformitarianism, which holds that everyday processes, operating however slowly, are sufficient to explain the principle features of earth's history.
Naturalists of the late 18th century understood that fossil marine animals and the sedimentary rock in which they were found represented ancient sea floors. But the cast accumulations of fossil-bearing rock revealed in the sides of mountains posed a time problem. The earth is just 6,000 years old, it was believed, and any casual observer of the oceans could see that sediment did not accumulate fast enough to yield kilometer thicknesses of marine sediment (later turned to rock) unless normal processes were somehow accelerated by cataclysmic events. The great flood described in the Old Testament of the Bible was one catastrophe commonly invoked. Catastrophic upheavals were also called upon to explain mountains.
Near the end of the 18th century a Scottish scientist named James Hutton reasoned that all of the earth's geological features could be explained by the slow and unchanging forces operating all around us if the earth was very much older than had been previously imagined. This new view, uniformitarianism, proved immensely successful in explaining geological observations, though not at first. Several decades later another Scot, Charles Lyell, refined and popularized Hutton's uniformitarianism. Lyell emphasized that humankind had existed for sufficient time as to bear witness to all kinds of processes that affect earth history. Hutton and Lyell imagined earth where the past looked much like the present, with no beginning nor an end. But it was their principle of uniformitarianism that allowed Charles Darwin to conceive of the evolution of species by natural selection and show that earth's inhabitants, at least, had changed irrevocably over geological time.
Without uniformitarianism there could have been no modern geology and no Darwinian theory of evolution and geologists have been understandably reluctant to dismiss the premise that "the present is the key to the past.
the new catastrophism
Was Lyell right? Have human beings actually witnessed all of the important agents of earth change? Since the early years of uniformitarianism we have learned that earth is 4.5 billion years old and that our solar system formed from the collapse of a fragment of molecular interstellar cloud approximately 4.6 billion years ago. Human history spans a mere one-tenth of one-thousandth of this interval and we would have been fortunate indeed if our existence had coincided with all of the forces that periodically effect our planet over millions and even billions of years. Therefore the most likely answer to the question: was Lyell right? must be no. Meteorite impact or volcanism on a massive scale may have killed off the dinosaurs. But both of these events, as it turns out, are normal in the course of the evolution of our planet. Should they be considered catastrophes in the context of earth history?
In July 1994 comet Shoemaker-Levy 9 collided with Jupiter. The impact was a catastrophe (for Jupiter) the like of which we have not seen before. Despite the fact that this event was unique in terms of human experiences, such colossal collisions are business-as-usual in astronomical terms. Numerous impact craters scar the rocky surfaces of most bodies in our solar system. These craters reveal that prior to 3.8 billion years ago, planets were routinely bombarded by meter to kilometer-sized objects. The collisions constituted the final stages of rocky accretion as the planets swept up the debris from which they were made. After 3.8 billion years accretion was essentially complete and impacts upon the planets became less frequent, but as Shoemaker-Levy 9 reminded us, they have not ceased entirely.
Impact structures dating back to the time of frequent bombardment have been destroyed on earth by erosion and tectonic processes that constantly deform and make over the crust. Nonetheless, scientists have thus far managed to identify more than 100 younger impact craters on earth. It is estimated that, on average, asteroids or comets measuring 5–10 km/3–6 mi in diameter impact our planet every 50 to 100 million years. Kilometer-sized bodies are thought to hit roughly every million years. Objects in the order of 50 m/165 ft in diameter strike about once every 1,000 years. For comparison, Shoemaker-Levy 9 was composed of several objects ranging from one to several kilometers in size, their collision with Jupiter released more energy than all of the nuclear weapons on earth combined.
At present 150 asteroids are known to pass within one earth orbit of the sun. They range in size from a few meters to 8 km/5 mi. A working group under the auspices of the United States National Aeronautics and Space Administration (NASA) suggests that there may be 2,100 such bodies in all. The impact of a single asteroid of 2 or more kilometers/1.3 or more miles would have enough energy to profoundly affect earth's biosphere, hydrosphere, and atmosphere. Fortunately, the likelihood of a collision of severe consequence in the next few hundred years is remote.
Not all catastrophes arrive from space. In 1783, just as catastrophism was about to give way to uniformitarianism, the Laki volcano erupted on Iceland. Unusually harsh winter conditions ensued in North America and Europe. Sulfuric acid rain destroyed Iceland's crops and livestock. Several events of comparable magnitude have confirmed that volcanic eruptions can bring about changes in climate. Have humans experienced the full extent of the influence of volcanism on earth history? The geological record suggests probably not the impact hypothesis.
In 1980, physicist and Nobel Prize laureate Luis Alvarez and his colleagues Walter Alvarez, Frank Asaro, and Helen Michel





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suggested that the K-T mass extinction resulted from the impact of an asteroid or comet (the term bolide describes an impactor of any sort). A bolide greater than 10 km/6 mi in diameter and traveling 10 km/6 mi per second, they submitted, would liberate enough energy to trigger environmental disaster across the globe. The earth would be plunged into months of darkness as the dust propelled into the air by the collision blocked the sun's rays. Photosynthesis would cease and plant-eating animals would be deprived of food, touching off a breakdown in the food chain. Temperatures would fall and as the shroud of dust prevented the sun from warming earth's surface, a phenomenon referred to as "impact winter" would occur. While land-dwelling animals were fighting starvation and the cold, marine creatures would have to battle acidification of the oceans; aerosols of debris and chemical reactions triggered by an atmospheric shock wave would result in nitric and sulfuric acid rain on a grand scale.
Alvarez and co-workers made their proposal on the basis of an unusual enrichment of the rare element iridium in a layer of clay marking the K-T boundary in Italy. Iridium, chemically similar to platinum and gold, is rare on earth but much more abundant in primitive rocky meteorites. Since their original report, the "iridium anomaly," as it has come to be known, has since been found in rocks and sediments deposited at the K-T boundary around the world. There is other evidence supportive of an impact. Ratios of the isotope of the element osmium, a platinum-group element like iridium, from some K-T samples are similar to primitive meteorites. Large amounts of soot in K-T boundary clay is thought to have come from large-scale burning of vegetation ignited by debris ejected from the atmosphere that fell back to earth like a hail of red-hot meteors. Quartz grains shocked by pressures exceeding 100,000 atmospheres have been found in K-T deposits. Previously, shocked quartz had only been found at impact craters (Arizona's Meteor Crater for example) or sites where underground nuclear weapons were tested.
In 1991 a circular impact structure of K-T age was found buried beneath 1 km/0.6 mi of Tertiary carbonate rock in Mexico's Yucatan peninsula. The structure is referred to as the Chicxulub crater after the nearby town of Chicxulub Puerto. Experts agree that Chicxulub is the most spectacular crater on earth and is the best candidate for the K-T impact site envisioned by Alvarez and co-workers. There are only two other impact structures of comparably large size and they are about 2 billion years old and poorly preserved. The precise size of Chicxulub has been debated. Estimates range from 180 to 300 km/112 to 186 mi in diameter. Most recent studies indicate that the hole upon impact was approximately 120 km/75 mi wide.
Despite its obvious attraction, the impact hypothesis is not without problems. Paleontologists find evidence that the K-T extinction was not instantaneous and may have begun up to a million years before K-T time and continued for tens of thousands of years after. Shocked quartz and even the iridium anomaly are found to occur just above and below the K-T sediment. Disastrous environmental conditions that would have besieged the earth after impact of a large bolide should have lasted about a decade and so none of these observations is easily explained by the crash of a single asteroid or comet. To redress this apparent shortcoming of the hypothesis, some proponents have argued that more than one bolide struck earth at about K-T time.
the volcanism hypothesis
Another competing catastrophe theory has been put forward to explain not only the chemical and physical features of the K-T boundary but are also the protracted nature of the mass extinction. Beginning in the 1970s it was observed that the largest episode of volcanism in the past 200 million years coincided with the K-T mass extinction. Remains of this volcanism are exposed today in the Deccan plateau region of western India. Here, vast quantities of basalt (a type of dark volcanic rock) known as the Deccan Traps are exposed. Combined, the ancient flows are more than 2 km/1.2 mi thick and comprise roughly 3% of the entire Indian continent. Deccan volcanism lasted several hundreds of thousands of years and was most active at the time of the K-T mass extinction. American geologist Dewey McLean and French geophysicist Vincent Courtillot, among others, argue that it was the voluminous Deccan volcanism that was responsible for the K-T mass extinction. Recall the climatic effects of the Laki eruption? Imagine the effects of continuous large-scale volcanism for hundreds of thousands of years.
The chemical composition of earth's mantle is more like that of primitive meteorites than the crust. Deccan lavas came from the mantle. Thus instead of being derived from dust thrown up by an asteroid, the iridium anomaly and other chemical signatures of the K-T boundary could be the result of a deluge of volcanic dust. Similarly, shocked quartz, cited as critical evidence for bolide impact, can apparently be formed by eruption of some types of explosive volcanoes.
The environmental effects of large-scale volcanism and bolide impact may be similar. Volcanism, like impact, would loft large amounts of dust high into the atmosphere where it would be transported around the world. As with the impact hypothesis, the earth might cool as the dust blocked the sun's rays. Sulfur released by the volcanoes would cause rain to be acidic. Alternatively, McLean has argued that it was carbon dioxide, a greenhouse gas released during volcanism, that was the killer. Large amounts of carbon dioxide released in to the atmosphere would have changed the chemistry of the oceans and caused global warming, both of which would be harmful to life.
an unlucky coincidence
New theoretical studies suggest that without a vulnerability to extinction, catastrophe may have little influence on the diversity of living organisms. Mass extinctions occur with a crude 30 million year periodicity. The largest, in which 90% of species disappeared, was at the end of the Paleozoic era approximately 250 million years ago. Smaller extinctions are more common. Theoreticians have shown recently that a pattern of frequent smaller extinctions and less frequent larger extinctions is the inevitable consequence of the dependence of living organisms on one another. The disappearance of an animal's prey, for example, may make the animal less fit for survival. The predator species might then vacate its ecological niche and in turn affect fitness of another species and so it goes until eventually an "avalanche" of linked extinctions occurs. During an extinction "avalanche" large numbers of species enter new habitats for which they are not well adapted. Mathematical simulations of such processes show that mass extinction requires the coincidence of both large numbers of vulnerable species due to an extinction avalanche, the result of normal evolutionary change, and an unusual amount of stress caused by an environmental catastrophe. Thus it seems the dinosaurs that lived during the Cretaceous period must have been ripe for expiration. Unfortunately for them, their vulnerability coincided with a particularly unlucky time in earth's history when volcanism was rampant and a collision of the sort that occurs just once every 50 million years actually happened.





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  epochs: Paleocene, Eocene, Oligocene, Miocene, and Pliocene. During the Tertiary period mammals took over all the ecological niches left vacant by the extinction of the dinosaurs, and became the prevalent land animals. The continents took on their present positions, and climatic and vegetation zones as we know them became established.  
  To remember the different epochs:  
  Please eliminate old men playing poker honestly  
  (Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, Holocene)  


  Paleocene The first epoch of the Tertiary period of geological time, 65–56.5 million years ago, is the Paleocene (Greek "old" + "recent"). Many types of mammals spread rapidly after the disappearance of the great reptiles of the Mesozoic. Flying mammals replaced the flying reptiles, swimming mammals replaced the swimming reptiles, and all the ecological niches vacated by the reptiles were adopted by mammals.  
  At the end of the Paleocene there was a mass extinction that caused more than half of all bottom-dwelling organisms in the oceans to disappear worldwide, over a period of around one thousand years. Surface dwelling organisms remained unaffected, as did those on land. The cause of this extinction remains unknown, though U.S. paleontologists have found evidence (released in 1998) that it may have been caused by the earth releasing tons of methane into the oceans causing increased water temperatures.  
  Eocene The second epoch of the Tertiary period, 56.5–35.5 million years ago, is the Eocene. Originally considered the earliest division of the Tertiary, the name means "early recent," referring to the early forms of mammals evolving at the time, following the extinction of the dinosaurs.  
  Oligocene The Oligocene is the third epoch of the Tertiary period, 35.5–23.5 million years ago. The name, from Greek, means "a little recent," referring to the presence of the remains of some modern types of animals existing at that time.  
  Miocene The Miocene ("middle recent") is the fourth epoch of the Tertiary period, 23.5–5.2 million years ago. At this time grasslands spread over the interior of continents, and hoofed mammals rapidly evolved.  
  Pliocene The fifth and last epoch of the Tertiary period, 5.2–1.64 million years ago, is the Pliocene ("almost recent"). The earliest hominid, the humanlike ape Australopithecines, evolved in Africa.  
  Quaternary The Quaternary period of geological time began 1.64 million years ago and is still in process. It is divided into the Pleistocene and Holocene epochs.  
  Pleistocene The Pleistocene is the first epoch of the Quaternary period, beginning 1.64 million years ago and ending 10,000 years ago. The polar ice caps were extensive and glaciers were abundant during the ice age of this period, and humans evolved into modern Homo sapiens sapiens about 100,000 years ago.  
  Holocene The Holocene epoch began 10,000 years ago and is the second and current epoch of the Quaternary period. During this epoch the glaciers retreated, the climate became warmer, and humans developed significantly.  
  Exploration of possible causes of the Late Pleistocene extinction of most large mammals in North America.  




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  Earth Science:
Evolution and Structure Chronology
Earth Science: Evolution and Structure Chronology
c. 1470 B.C. The island of Thera is destroyed in a volcanic eruption. It causes a tidal wave and subsequent famine in Egypt, and destroys the Minoan civilization on the island of Crete over 120 km/75 mi away. It may be the source of the Atlantis myth.
c. 1226 B.C. The first record of Mount Etna in Italy erupting is made. It has erupted 190 times since.
c. 780 B.C. Chinese scholars make the first record of an earthquake.
c. 560 B.C. The Greek philosopher Xenophanes correctly recognizes the nature of fossils when he suggests that fossil seashells are the result of a great flood that buried them in the mud.
c. 530 B.C. Pythagoras of Samos proposes the notion of a spherical earth.
c. 350 B.C. Aristotle defends the doctrine that the earth is a sphere, in De caelo/Concerning the Heavens, and estimates its circumference to be about 400,000 stadia (one stadium varied from 154 m/505 ft to 215 m/705 ft). It is the first scientific attempt to estimate the circumference of the earth.
c. 295 B.C. Greek philosopher Theophrastus writes De lapidibus/On Stones, a classification of 70 different minerals. It is the oldest known work on rocks and minerals and is the best treatise on the subject for nearly 2,000 years.
c. 100 The geographer Marinus of Tyre develops a system of equal spacing for lines of latitude and longitude. Maps by Marinus are the first in the Roman Empire to show China.
114 A Chinese sculpture of this date shows an early form of compass–a polished magnetite spoon that spins to align with the earth's magnetic field when placed on a smooth surface.
132 Chinese engineer Zhang Heng develops the first seismograph for determining the position of an earthquake's epicenter. It uses a series of balls suspended in the mouths of eight carved dragons. The balls that are dislodged indicate the direction of the earthquake center.
1154 The Egyptian Muslim scholar al-Tifashi writes his pioneering work on mineralogy Flowers of Knowledge of Precious Stones.
1282 The Italian scholar Ristoro d'Arezzo writes his encyclopedia Della composizione del mondo/On the Composition of the World. It discusses the structure of the world and develops some sound geological theories despite its links with astrology.
1492 German navigator Martin Behaim, with painter Goerg Glockendon, constructs a terrestrial globe at Nuremberg, the earliest still in existence.
1492 The explorer Christopher Columbus, crossing the Atlantic, notes that the deviation of his compass from true north has changed from the east to the west, an early discovery of the variation of the earth's magnetism.
1513 A new edition of Ptolemy's Geography shows the New World as two separate continents.
Jan 26, 1531 An earthquake destroys the Portuguese capital of Lisbon, killing 50,000 people.
1536 Flemish cartographer Gerardus Mercator collaborates with geographer and mathematician Gemma Frisius to construct a terrestrial globe.
1544 German mineralogist Agricola (Georg Bauer) writes De ortu et causis subterraneis/On Subterranean Origin and Causes, a founding work in geology, identifying the erosive power of water, and the origin of mineral veins as depositions from solution.
1546 Flemish cartographer Gerardus Mercator states that the earth must have a magnetic pole separate from its "true" pole, in order to explain the deviation of a compass needle from true North.
1576 English scientist Robert Norman discovers the magnetic "dip" or inclination in a compass needle that is caused by the earth's magnetic field not running exactly parallel to the surface.
1580 An earthquake shakes London, England.
1622 British scientist Edward Gunter discovers that the magnetic needle does not retain the same declination in the same place all the time—the first evidence for variation in earth's magnetic field.
1650 German geographer Bernhardus Varenius publishes his Georgraphia generalis/General Geography, establishing scientific principles for general and regional geography.
1691 German scientist Gottfried von Leibniz publishes Protogaea/The Primordial Earth, a study of geology containing many new insight, including a belief that the earth was formed from a molten state.
1736 Swedish scientist Anders Celsius leads a French-sponsored expedition to Lapland. By measuring the length of a degree of meridian at high latitude, they prove the earth is flattened at the poles.
1745 Russian mineralogist Mikhail Vasileyevich Lomonosov publishes a catalog of over 3,000 minerals.
1746 French naturalist Jean-Etienne Guettard makes the first mineralogical map (of France). Among his discoveries, he identifies extinct volcanoes in the Auvergne region, and volcanic rocks in large deposits across France.





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1749 French naturalist Georges-Louis de Buffon publishes the first book of of his 36-volume Histoire naturelle, genérale et particulière/Natural Histroy, General and Particular, the first attempt to bring together the various fields of natural history.
1774 French naturalist Nicolas Desmarest's essays on extinct volcanoes demonstrate the volcanic origin of basalt, disproving the theory that all rocks are formed by sedimentation from primeval seas.
1776 English chemist James Keir suggests that some rocks may have formed as molten material that cooled and crystallized.
1778 In Epoques de la nature/Epochs of Nature, French scientist George-Louis Leclerc, comte de Buffon, reconstructs geological history as a series of stages—the first to recognize such stages. It contradicts the doctrine that the earth is only 6,000 years old.
1779 Swiss geologist Horace Bénédict de Saussure coins the word "geology."
1783–84 Mount Skaptar in Iceland erupts, killing 9,000 people, about one-fifth of the population.
1788 Scottish geologist James Hutton's paper Theory of the earth expounds his uniformitarian theory of continual change in the earth's geological features and marks a turning point in geology.
1798 English natural philosopher Henry Cavendish determines the mean density of the earth; it is 5.5 times as dense as water.
1801 French mineralogist René-Just Häuy publishes Traité de minéralogie/Treatise on Mineralogy, a theory of the crystal structure of minerals that establishes him as one of the founders of crystallography.
1809 René-Just Häuy publishes Tableau comparatif/Comparative Tables, one of the first classifications of minerals.
Dec 16, 1811 The first, and largest, earthquake recorded in the United States destroys the city of New Madrid, Missouri. Two other earthquakes hit the town on January 23 and February 7, 1812.
1817–59 German geographer Carl Ritter publishes Earth Science in its Relation to Nature and the History of Man. The first of 19 volumes surveying world geography, it establishes him as cofounder, along with Alexander von Humboldt, of modern geography.
1822 Mary Anning discovers the first fossil to be recognized as that of a dinosaur—an Iguanodon—in Devon, England.
July 1830–April 1833 Scottish geologist Charles Lyell publishes his three-volume work Principles of Geology in which he argues that geological formations are the result of presently observable processes acting over millions of years. It creates a new time frame for other sciences such as biology and paleontology.
1837 U.S. geologist James Dana publishes System of Mineralogy, which is still the standard text on mineralogy.
1839 Scottish geologist Robert Murchison publishes The Silurian System, a geological treatise which establishes the geological sequence of the Early Paleozoic rocks.
1841 German astronomer Friedrich Bessel deduces the elliptical distortion of the earth—the amount it departs from a perfect sphere—to be 1/299.
c. 1850 French naturalist Antonio Snider-Pellegrini suggests that the similarities between European and North American plant fossils could be explained if the two continents were once in contact.
1854 English astronomer George Biddell Airy calculates the mass of the earth by swinging a pendulum at the top and bottom of a deep coal mine and measuring the different gravitational effects on it.
1862 Scottish physicist William Thomson (Lord Kelvin), using the earth's temperature, estimates the earth to be between 20 and 400 million years old.
1862 Scottish-born German astronomer Johann von Lamont discovers the electrical current within the earth's crust.
1877 Cotopaxi volcano in Ecuador erupts; over 1,000 people are killed by mudflows.
1877 The Como Bluff paleontological site is discovered in Wyoming. It contains a large number and variety of dinosaur remains, including the first specimens of Stegosaurus, Brontosaurus (now known as Apatosaurus), and Allosaurus.
1880 British geologist John Milne invents the modern seismograph for measuring the strength of earthquakes.
1883–88 Austrian geologist Eduard Suess publishes Das Antlitz der Erde/Face of the Earth, in which he postulates the existence of an ancient supercontinent in the southern hemisphere called Gondwanaland, and discusses how various processes are responsible for the present features of the earth's surface.
1888 Swedish geologist Alfred Elis Törnebohm presents the theory that mountain ranges are the result of overthrusting, in which the upper surface of a fault plane moves over the rocks of the lower surface.
1889 U.S. geologist Clarence Edward Dutton discovers a method of determining the epicenter of earthquakes and accurately measuring the speed of seismic waves.
1892 U.S. geologist Clarence Edward Dutton publishes the paper "On Some of the Greater Problems of Physical Geology," in which he advances the idea of isostasy, whereby lighter material in the earth's crust rises to form continents and mountains, while heavier material sinks, to form basins and oceans.





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1902 New Zealand-born British physicist Ernest Rutherford and British physicist Frederick Soddy discover thorium X and publish The Cause and Nature of Radioactivity, which outlines the theory that radioactivity involves the disintegration of atoms of one element into atoms of another, laying the foundation for radiometric dating of natural materials.
1905 U.S. chemist Bertram Boltwood suggests that lead is the final decay product of uranium. His work will eventually lead to the uranium–lead dating method.
1905 The Carnegie Institution of Washington establishes the Geophysical Laboratory in Washington, D.C.
1906 Scientists discover two distinct forms of rock magnetism: some rocks magnetized with their north ''poles" parallel to earth's present magnetic field and others magnetized with their poles reversed with respect to earth's magnetic field.
1906 Irish geologist Richard Oldham proves that the earth has a molten core, by studying seismic waves.
1907 French physicist Pierre-Ernest Weiss develops the domain theory of ferromagnetism, which suggests that in a ferromagnet, such as loadstone, there are regions, or domains, where the molecules are all magnetized in the same direction. His theory leads to a greater understanding of rock magnetism.
1907 U.S. chemist Bertram Boltwood uses the ratio of lead and uranium in some rocks to determine their age. He estimates his samples to be 410 million to 2.2. billion years old.
1909 Croatian physicist Andrija Mohorovicic discovers the Mohorovicic discontinuity in the earth's crust. Located about 30 km/18 mi below the surface, it forms the boundary between the crust and the mantle.
1909 U.S. geologist and secretary of the Smithsonian Institution Charles D. Walcott discovers fossils of soft parts of organisms in the Cambrian Burgess Shale of the Canadian Rockies. The discovery provides unprecedented evidence pertaining to the rapid evolution of life that started in the Cambrian period.
1910 U.S. physicist Percy Bridgman invents a device, called the "collar," that allows him to squeeze all kinds of materials to pressures comparable to the base of earth's crust, giving rise to the new fields of high-pressure physics and mineral physics.
1910 Swedish archeologist Gerhard de Geer publishes A Geochronology of the Last 12,000 Years, setting out his influential system for dating rock strata.
1912 German meteorologist Alfred Wegener suggests the idea of continental drift and proposes the existence of a supercontinent (Pangea) in the distant past.
1912 German physicist Max von Laue demonstrates that crystals are composed of regular, repeated arrays of atoms by studying the patterns in which they diffract X-rays. It is the beginning of X-ray crystallography.
1913 English geologist Arthur Holmes uses radioactivity to date rocks, establishing that the earth is 4.6 billion years old.
1914 German-born U.S. geologist Beno Gutenberg discovers the discontinuity that marks the boundary between the earth's lower mantle and outer core, about 2,800 km/1,750 mi below the surface.
1920 English physicist Frederick Soddy suggests that isotopes can be used to determine geological age.
1926 The Scott Polar Research Institute is opened in Cambridge, England, to conduct Antarctic research.
1928 U.S. geochemist Norman L. Bowen publishes The Evolution of the Igneous Rocks in which he suggests that earth's crust is the product of melting of parts of the mantle, a process known as differentiation. His work firmly establishes the potential of the physical chemical approach to geology.
1929 By studying the magnetism of rocks, the Japanese geologist Motonori Matuyama shows that the earth's magnetic field periodically reverses direction.
1929 Norwegian chemist Victor Moritz Goldschmidt produces the first table of ionic radü useful for predicting crystal structures.
1930 The Indian physicist Chandrasekhara Venkata Raman receives the Nobel Prize for Physics for his work on the scatering of light and for the Raman effect. Raman spectroscopy later becomes an important tool in mineral physics.
1931 U.S. chemist Harold C. Urey discovers deuterium, a heavy isotope of hydrogen, ushering in the modern field of stable isotope geochemistry.
1936 U.S. archeologist Andrew E. Douglass develops dendrochronology; a dating system based on the measurement of growth rings in trees.
1937 Finnish chemist and geologist Victor Moritz Goldschmidt tabulates the absolute abundances of chemical elements in earth from solar and meteorite chemical data.
1939 U.S. chemist A. O. Nier with E. A. Gulbransen report natural variations in the isotopes of carbon. Geochemists will later use the stable isotopes of carbon to study fossil bones, teeth, and rocks.
1939 U.S. chemist Linus Pauling consolidates his theory of the chemical bond in The Nature of the Chemical Bond, and the Structure of Molecules and Crystals in which he sets out





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  rules for understanding mineral structures. "Pauling's rules" revolutionize the understanding of the structure and chemistry of minerals.
1939 U.S. geophysicist Walter Maurice Elsasser formulates the "dynamo model" of the earth, which proposes that eddy currents in the earth's molten iron core cause its magnetism.
1946 Carbon-13, a stable isotope of carbon, is discovered. Its abundance will ultimately be used for deciphering the geochemistry of carbon.
1947 U.S. physicist Willard Libby develops carbon-14 dating.
1948 U.S. chemists L. T. Aldrich and A. O. Nier find argon from decay of potassium in four geologically old minerals, confirming predictions by German physicist C. F. von Weizsacker made in 1937. The basis for potassium-argon dating is established.
1949 U.S. seismologist Hugo Benioff identifies planes of earthquake foci extending from deep ocean trenches to beneath adjacent continents at approximately 45° as faults. These faults, later called Benioff zones or Benioff–Wadati zones, will later be identified as the tops of plates of lithosphere.
1953 U.S. chemists Stanley Miller and Harold Urey demonstrate experimentally how organic compounds might have begun on earth, by exposing a warm, gaseous mixture of inorganic compounds to electrical discharges. The gases are meant to represent the pre-biotic atmosphere, the "primordial soup" out of which life evolved.
1953 U.S. geophysicist William Ewing announces that there is a crack, or rift, running along the middle of the Mid-Atlantic Ridge.
1954 Researchers at General Electric produce the first synthetic diamonds.
1954 The existence of pre-metazoan life becomes widely accepted when diverse fossil microscopic organisms are discovered in the Gunflint rocks along the north shore of Lake Superior, northern USA/southern Canada. The Gunflint biota prove that there had been significant evolutionary activity by at least about 2 billion years ago.
1956 U.S. geologists Bruce Heezen and William Ewing discover a global network of oceanic ridges and rifts 60,000 km/37,000 mi long that divide the earth's surface into "plates."
28 Feb 1959 The U.S. Air Force launches Discover 1 into a low polar orbit where it photographs the entire surface of the earth every 24 hours.
1960 U.S. geophysicist Harry Hess develops the theory of seafloor spreading (the term is coined by R. S. Dietz in 1961), in which molten material wells up along the mid-oceanic ridges forcing the seafloor to spread out from the ridges. The flow is thought to be the cause of continental drift.
1961 L. O. Nicolaysen invents the "isochron" method of rubidium–strontium and uranium–lead dating of geological materials.
1961 U.S. researchers establish Arctic Research Lab Ice Station II (Arliss II), a drifting sea ice station.
1962 U.S. geologist Harry Hess publishes History of Ocean Basin, in which he formally proposes seafloor spreading, the idea that the ocean crust is like a giant conveyor belt produced by volcanism at the mid-ocean ridges, pushed or pulled away from the ridge axis, and eventually destroyed by plunging down into the deep-sea trenches.
1962 Part of the north summit of Mount Huascaran, Peru's highest mountain, breaks off during a thaw; 3,500 people are killed.
1963 British geophysicists Fred Vine and Drummond Matthews analyse the magnetism of rocks in the Atlantic Ocean floor, which assume a magnetization aligned with the earth's magnetic field at the time of their creation. It provides concrete evidence of seafloor spreading.
1965 Canadian geologist John Tuzo Wilson publishes A New Class of Faults and Their Bearing on Continental Drift, in which he formulates the theory of plate tectonics to explain continental drift and seafloor spreading. He suggests that the mid-ocean ridges, deep-sea trenches, and the faults that connect them combine to divide the earth's outer layer into rigid independent plates. The connecting faults are shown by Wilson to have a unique geometry, and are called "transform faults."
1965 The Large Aperture Seismic Array is established in Montana, U.S.A. The signals from 525 seismometers, dispersed over an area of 30,000 sq km/11,600 sq mi, are combined to record seismic events with a high degree of sensitivity.
1966 U.S. geologists Allan Cox, Richard Doell, and Brent Dalrymple publish a chronology of magnetic polarity reversals going back 3 million years. It is useful in dating rocks and fossils.
1967 Geophysicist Lynn Sykes uses first-motion seismic studies to establish that mid-ocean ridges form with offsets rather than being offset later, a major advance in understanding the formation of ocean basins.
1967 British geophysicist D. McKenzie and U.S. geophysicist Jason Morgan describe the motions of plates across earth's surface. Morgan calls the plates "tectosphere." They are later referred to as plates of lithosphere.
1968 French geophysicist Xavier Le Pichon, working at the Lamont Observatory in New York, describes the motions of earth's six largest plates using poles of rotation derived from the patterns of magnetic anomalies and fracture zones about mid-ocean ridges.
1968 The U.S. survey ship Glomar Challenger starts drilling cores in the sea bed as part of the Deep Sea Drilling Project. Capable of drilling in water up to 6,000 m/20,000 ft deep, it can return core samples from 750 m/2,500 ft below the sea floor.





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1969 The Joint Oceanographic Institutions Deep Earth Sampling (JOIDES) project begins. It makes boreholes in the ocean floor and confirms the theory of seafloor spreading and that the ocean crust everywhere is less than 200 million years old.
1970 British geologist John F. Dewey with J. M. Bird relate the positions of earth's mountain belts to the motions of lithospheric plates.
1972 U.S. paleontologists Stephen Jay Gould and Nils Eldridge propose the punctuated equilibrium model—the idea that evolution progresses in fits and starts rather than at a uniform rate.
July 23, 1972 The U.S. launches Landsat 1, the first of a series of satellites for surveying the earth's resources from space.
1974 Earth scientist John Liu discovers that the lower mantle, comprising most of the earth, is likely composed of silicate perovskite, a mineral with a structure wildly different from the minerals found in earth's upper mantle and crust.
1975 David Mao and Peter Bell of the Geophysical Laboratory use a diamond-anvil cell to produce pressures exceeding a million atmospheres.
1975 The United States launches Landsat 2; located 180° away from Landsat 1, the two together provide a view of the same geographic area with the same sun angle every nine days.
Oct 1975 The United States launches the first Geostationary Operational Environmental Satellite (GOES); it provides 24-hour coverage of U.S. weather.
1976 The U.S. Lageos (Laser Geodynamic Satellite) is launched; it uses laser beams to make precise measurements of the earth's movements in an attempts to improve the prediction of earthquakes. Placed in an orbit 9,321 km/5,793 mi high, it is expected to remain in orbit for 8 million years.
1978 Core samples from the seabed are collected by the U.S. research vessel Glomar Challenger from a record depth of 7,042 m/23,104 ft.
1980 A thin layer of iridium-rich clay, about 65 million years old, is found around the world. U.S. physicist Luis Alvarez suggests that it was caused by the impact of a large asteroid or comet which threw enough dust into the sky to obscure the sun and cause the extinction of the dinosaurs.
1980 M. Ikeya and T. Liki of Yamaguchi University, Japan, announce a new method of dating fossil remains: electron spin resonance spectroscopy, which measures the amount of natural radiation received by such remains.
1980 The U.S. Magsat satellite completes its mapping of the earth's magnetic field.
1983 Studies from the Lageos satellite (launched in 1976) indicate that the earth's gravitational field is changing.
1984 Australian geologists Bob Pidgeon and Simon Wilde discover zircon crystals in the Jack Hills north of Perth, Australia, that are estimated to be 4.2 billion years old—the oldest minerals ever discovered.
1990 Canadian scientists discover fossils of the oldest known multicellular animals, dating from 600 million years ago.
1991 A circular impact structure of Cretaceous–Tertiary (K–T) age is found buried beneath in Mexico's Yucatan peninsula. Called the Chicxulub crater, it is the best candidate for the K–T meteor impact site envisioned by Luis Alvarez and others.
1995 U.S. and French geophysicists discover that the Indo-Australian plate split in two in the middle of the Indian Ocean about 8 million years ago.
1996 U.S. geophysicists discover that the earth's core spins slightly faster than the rest of the planet.
June 25, 1997 A volcanic eruption in the Soufrière Hills on the British dependency of Montserrat in the Leeward Islands, West Indies, kills 23 people.
July 24, 1997 Canadian researcher Richard Bottomley and colleagues date the 100-km/62-mi-wide Popigai impact crater in Siberia, thought to be the fifth largest impact crater on earth, to 35.7 million years old. They suggest that the meteorite that created it may be responsible for the mass extinction that occurred at the end of the Eocene and the start of the Oligocene geological periods, which is dated to about the same time.
July 25, 1997 U.S. researcher Joseph L. Kirschvink and colleagues, by examining the record of remnant magnetism in very ancient rocks, discover that the outer layers of the earth shifted by 90 degrees relative to the core between about 535 and 520 million years ago. This major reorganization of the continents they suggest may have led to the Cambrian Explosion—the rapid appearance of abundant fossils in the geological record in the Cambrian period.





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  Agassiz, (Jean) Louis Rodolphe (1807–1873) Swiss-born U.S. paleontologist and geologist who established his name through his work on the classification of fossil fishes. His book Researches on Fossil Fish (1833–44) described and classified over 1,700 species. Unlike Charles Darwin, he did not believe that individual species themselves changed, but that new species were created from time to time.  
  Alvarez, Luis Walter (1911–1988) U.S. physicist. In 1980 he was responsible for the theory that dinosaurs disappeared because a meteorite crashed into the earth 70 million years ago, producing a dust cloud that blocked out the sun for several years, causing dinosaurs and plants to die.  
  Bakker, Robert T. (1945– ) U.S. paleontologist who in 1975 published the theory that dinosaurs were warm-blooded. He also views them as fast, agile, and colorful. He believes that the extinction of the dinosaurs was caused by disease carried across the new land bridges that appeared at the end of the Cretaceous period. He has popularized his work on television and in books such as The Dinosaur Heresies (1986).  
  Boltwood, Bertram (1870–1927) U.S. radiochemist who pioneered the use of radioactive elements as tools for dating rocks. By studying abundances of radioactive elements in ores, he deduced that the radium present in an ore was the product of the breakdown of uranium in the ore and that uranium ultimately would decay to lead. In 1907 he demonstrated that by knowing the rate at which uranium decays (its half-life) he could calculate the age of a mineral by measuring the relative proportions of its uranium and lead. He dated rocks from several localities using his uranium—lead technique, obtaining ages between 410 million to 2.2 billion years old, and determined the age of the earth to be at least 2.2 billion years old, significantly older than previously thought.  
  Bowen, Norman Levi (1887–1956) Canadian geologist whose work helped found modern petrology. His findings on the experimental melting and crystallization behavior of silicates and similar mineral substances were published from 1912 onwards. He demonstrated the principles governing the formation of magma by partial melting, and the fractional crystallization of magma; his book The Evolution of Igneous Rocks (1928) deals particularly with magma.  
  Bridgman, Percy Williams (1882–1961) U.S. physicist. His research into machinery producing high pressure led in 1955 to the creation of synthetic diamonds by General Electric. His technique for synthesizing diamonds was used to synthesize many more minerals, and a new school of geology developed, based on experimental work at high pressure and temperature. Because the pressures and temperatures that Bridgman achieved simulated those deep in the earth, his discoveries gave an insight into the geophysical processes that take place within the earth. His book Physics of High Pressure (1931) remains a basic work. He was awarded the Nobel Prize for Physics in 1946.  
  Brongniart, Alexandre (1770–1847) French naturalist and geologist who first used fossils to date strata of rock and was the first scientist to arrange the geological formations of the Tertiary period in chronological order. He introduced the idea of geological dating according to the distinctive fossils found in each stratum. From 1804–11 Brongniart and Georges c0016-01.gifCuvier studied the fossils deposited in the Paris basin and showed that the fossils had been laid down during alternate fresh and salt-water conditions.  
  Buckland, William (1784–1856) English geologist and paleontologist, a pioneer of British geology. He contributed to the descriptive and historical stratigraphy of the British Isles, inferring from the vertical succession of the strata a stage-by-stage temporal development of the earth's crust. His interest in catastrophic transformations of the earth's surface in the geologically recent past, as indicated by such features as fossil bones and erratic boulders, led him to become an early British exponent of the glacial theory of Louis c0016-01.gifAgassiz.  
  Buffon, Georges-Louis Leclerc, comte de Buffon (1707–1788) French naturalist and author of the 18th century's most significant work of natural history, the 44-volume Histoire naturelle generale et particulière (1749–67). In The Epochs of Nature, one of the volumes, he questioned biblical chronology for the first time, and raised the earth's age from the traditional figure of 6,000 years to the seemingly colossal estimate of 75,000 years.  
  Bullard, Edward Crisp (1907–1980) English geophysicist who, with U.S. geologist Maurice c0016-01.gifEwing, founded the discipline of marine geophysics. He pioneered the application of the seismic method to study the sea-floor. He also studied continental drift before the theory became generally accepted. Bullard's earliest work was to devise a technique to measure minute gravitational variations in the East African Rift Valley. He then investigated the rate of efflux (outflow) of the earth's interior heat through the land surface; later he devised apparatus for measuring the flow of heat through the deep seafloor. According to his "dynamo" theory of geomagnetism, the earth's magnetic field results from convective movements of molten material within the earth's core.  
  Chamberlin, Thomas Chrowder (1843–1928) U.S. geophysicist who asserted that the earth was far older than then believed. He developed the planetesimal hypothesis for the origin of the earth and other planetary bodies—that they had been formed gradually by accretion of particles.  
  Cuvier, Georges Léopold Chrétien Frédéric Dagobert, Baron Cuvier (1769–1832) French comparative anatomist, the founder of paleontology. In 1799 he showed that some species have become extinct by reconstructing extinct giant animals that he believed were destroyed in a series of giant deluges. These ideas are expressed in Recherches sur les ossiments fossiles de quadrupèdes/Researches on the Fossil Bones of Quadrupeds (1812) and Discours sur les révolutions de la surface du globe (1825). He was the first to relate the structure of fossil animals to that of their living relatives.  
  De la Beche (born Beach), Henry Thomas (1796–1855) English geologist. He secured the founding of the Geological Survey in 1835, a government-sponsored geological study of Britain, region by region. He wrote books of descriptive stratigraphy, above all on the Jurassic and Cretaceous rocks  




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  of the Devon and Dorset area. He also conducted important fieldwork on the Pembrokeshire coast and in Jamaica. His main work is The Geological Observer (1851).  
  Desmarest, Nicolas (1725–1815) French champion of volcanist geology, who countered the widely held belief that all rocks were sedimentary. Studying the large basalt deposits of central France, he traced their origin to ancient volcanic activity in the Auvergne region. In 1768 he produced a detailed study of the geology and eruptive history of the volcanoes responsible. However, he did not believe that all rocks had igneous origin, and emphasized the critical role of water in the shaping of the earth's history.  
  Du Toit, Alexander Logie (1878–1948) South African geologist. His work was to form one of the foundations for the synthesis of continental drift theory and plate tectonics that created the geological revolution of the 1960s. The theory of continental drift put forward by Alfred c0016-01.gifWegener inspired Du Toit's book A Geological Comparison of South America and South Africa (1927), in which he suggested that they had probably once been joined. In Our Wandering Continents (1937) he maintained that the southern continents had, in earlier times, formed the supercontinent of Gondwanaland, which was distinct from the northern supercontinent of Laurasia.  
  Edinger, Tilly (Johanna Gabrielle Ottilie) (1897–1967) German-born U.S. paleontologist. Her work in vertebrate paleontology laid the foundations for the study of paleoneurology. She demonstrated that the evolution of the brain could be studied directly from fossil cranial casts.  
  Elsasser, Walter Maurice (1904–1991) German-born U.S. geophysicist. He pioneered analysis of the earth's former magnetic fields, which are frozen in rocks. His research in the 1940s yielded the dynamo model of the earth's magnetic field. The field is explained in terms of the activity of electric currents flowing in the earth's fluid metallic outer core. The theory premises that these currents are magnified through mechanical motions, rather as currents are sustained in power-station generators.  
  Eskola, Pentti Eelis (1883–1964) Finnish geologist. He was one of the first to apply physicochemical postulates on a far-reaching basis to the study of metamorphism, thereby laying the foundations of most subsequent studies in metamorphic petrology. His approach enabled comparison of rocks of widely differing compositions in respect of the pressure and temperature under which they had originated.  
  Ewing, (William) Maurice (1906–1974) U.S. geologist. His studies of the ocean floor provided crucial data for the plate tectonics revolution in geology in the 1960s. Ewing ascertained that the crust of the earth under the ocean is much thinner (5–8 km/3–5 mi thick) than the continental shell (about 40 km/25 mi thick). He demonstrated that mid-ocean ridges, with deep central canyons, are common to all oceans, and his studies of ocean sediment showed that its depth increases with distance from the mid-ocean ridge, which gave clear support for the hypothesis of seafloor spreading.  
  Gardner, Julia (Anna) (1882–1960) U.S. geologist and paleontologist. Her work was important for petroleum geologists establishing standard stratigraphic sections for Tertiary rocks in the southern Caribbean.  
  Goldring, Winifred (1888–1971) U.S. paleontologist. Her research focused on Devonian fossils and during the late 1920s and the 1930s, as well as geologically mapping the Coxsackie and Berne quadrangles of New York, she developed and maintained the State Museum's public program in paleontology. Her works include The Devonian Crinoids of the State of New York (1923) and Handbook of Paleontology for Beginners and Amateurs (1929–31). She did much to popularize geology.  
  Guettard, Jean-Etienne (1715–1786) French pioneer of geological mapping, who studied the origin of various types of rock. Research in the field suggested that the rocks of the Auvergne district of central France were volcanic in nature, and Guettard boldly identified several peaks in the area as extinct volcanoes, though he later had doubts about this hypothesis. Of basalt, he originally took the view that it was not volcanic in origin, but changed his mind after visits to Italy in the 1770s.  
  Gutenberg, Beno (1889–1960) German-born U.S. geophysicist who determined the depth of earth's core and contributed to the understanding of earth's deep interior. As a student in 1914, he used velocities of seismic waves to calculate the depth of earth's core at 2,900 km/1,812 mi. This boundary between earth's mantle and its core is called the Gutenberg discontinuity. In 1948 he suggested the existence of a low-velocity zone approximately 60–150 km/38–94 mi below the earth's surface in which seismic waves travel more slowly. This zone is now known to be the asthenosphere, the more ductile layer of the mantle on which the earth's lithospheric plates ride.  
  Hall, James (1761–1832) Scottish geologist. He was one of the founders of experimental geology and provided evidence in support of the theories of James c0016-01.gifHutton regarding the formation of the earth's crust. By means of furnace experiments, he showed that Hutton had been correct to maintain that igneous rocks would generate crystalline structures if cooled very slowly. Hall also demonstrated that there was a degree of interconvertibility between basalt and granite rocks, and that, even though subjected to immense heat, limestone would not decompose if sustained under suitable pressure.  
  Haüy, René-Just (1743–1822) French mineralogist, the founder of modern crystallography. He regarded crystals as geometrically structured assemblages of units (integrant molecules), and developed a classification system on this basis. He proposed six primary forms: parallelepiped, rhombic dodecahedron, hexagonal dipyramid, right hexagonal prism, octahedron, and tetrahedron. His two major works are Traité de minéralogie/Treatise of Mineralogy (1801) and Treatise of Crystallography (1822).  
  Hess, Harry Hammond (1906–1969) U.S. geologist who in 1962 proposed the notion of seafloor spreading. This played a key part in the acceptance of plate tectonics as an explanation of how the earth's crust is formed and moves. Building on the recognition that certain parts of the ocean floor were anomalously young, and the discovery of the global distribution of mid-ocean ridges and central rift valleys, Hess suggested that convection within the earth was continually creating new ocean floor, rising at mid-ocean ridges, and then flowing horizontally to form new oceanic crust. It would  




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  follow that the further from the mid-ocean ridge, the older would be the crust—an expectation confirmed by research in 1963.  
  Holmes, Arthur (1890–1965) English geologist who helped develop interest in the theory of continental drift. In 1928 he proposed that convection currents within the earth's mantle, driven by radioactive heat, might furnish the mechanism for the continental drift theory broached a few years earlier by Alfred c0016-01.gifWegener. He also pioneered the use of radioactive decay methods for rock dating, giving the first reliable estimate of the age of the earth.  
  Hutton, James (1726–1797) Scottish geologist, known as the "founder of geology," who formulated the concept of uniformitarianism, suggesting that past events could be explained in terms of processes that work today. For example, the kind of river current that produces a certain settling pattern in a bed of sand today must have been operating many millions of years ago, if that same pattern is visible in ancient sandstones. In 1785 he developed a theory of the igneous origin of many rocks. His Theory of the earth (1788) proposed that the earth was incalculably old.  
  Knopf (born Bliss), Eleanora Frances (1883–1974) U.S. geologist who studied metamorphic rocks. She introduced the technique of petrofabrics, which had been developed in Austria, to the United States.  
  Lyell, Charles (1797–1875) Scottish geologist. In his Principles of Geology (1830–33), he opposed Georges c0016-01.gifCuvier's theory that the features of the earth were formed by a series of catastrophes, and expounded James c0016-01.gifHutton's concept of uniformitarianism, that past events were brought about by the same processes that occur today—a view that influenced Charles Darwin's theory of evolution. He suggested that the earth was as much as 240 million years old (in contrast to the 6,000 years of prevalent contemporary theory), and provided the first detailed description of the Tertiary period, dividing it into the Eocene, Miocene, and older and younger Pliocene periods.  
  Le Pichon, Xavier (1937– ) French geophysicist who worked out the motions of earth's six major lithospheric plates. His work was instrumental in the development of plate tectonics. In 1968 he published Sea-floor spreading and continental drift in which he depicted earth's lithosphere divided into six major plates. The boundaries between the plates were shown to have high seismic activity and occurred along mid-ocean ridges, island arcs, active orogenic (mountain-building) belts and transform faults.  
  Libby, Willard Frank (1908–1980) U.S. chemist whose development in 1947 of radiocarbon dating as a means of determining the age of organic or fossilized material won him a Nobel prize in 1960.  
  Matsuyama, Motonori (1884–1958) Japanese geophysicist who determined that earth's magnetic field reverses its polarity periodically throughout its geological history. The Matsuyama Epoch, a major polar reversal occurring approximately 0.5–2.5 million years ago, is named for him. He also pioneered the use of gravimetry in finding geological structures below the earth's surface.  
  Mercator, Gerardus (Latinized form of Gerhard Kremer) (1512–1594) Flemish mapmaker who devised the first modern atlas, showing Mercator's projection in which the parallels and meridians on maps are drawn uniformly at 90°. It is often used for navigational charts, because compass courses can be drawn as straight lines, but the true area of countries is increasingly distorted the further north or south they are from the Equator.  
  Mohorovicic, Andrija (1857–1936) Croatian seismologist and meteorologist who discovered the Mohorovicic discontinuity, the boundary between the earth's crust and the mantle. In 1909, after a strong earthquake occurred in the Kulpa Valley south of Zagreb, he discovered two distinct sets of P and S seismic waves—one set arriving earlier than the other. He deduced that one set of waves was slower than the other because it had traveled through denser material. He proposed that the earth's surface consists of an outer layer of rocky material approximately 30 km/19 mi thick, which overlies a denser mantle.  
  Murchison, Roderick Impey (1792–1871) Scottish geologist responsible for naming the Silurian period, based on studies of slate rocks in south Wales. Expeditions to Russia 1840–45 led him to define another worldwide system, the Permian, named for the strata of the Perm region. He believed in a universal order of the deposition of strata, indicated by fossils rather than solely by lithological features. With Charles c0016-01.gifLyell, he also established the Devonian system in southwest England.  
  Oldham, Richard Dixon (1858–1936) Irish seismologist who discovered the earth's core and first distinguished between primary and secondary seismic waves. While analyzing seismic records in 1906, he noticed an area on the globe in which P-waves were not detected. Every time an earthquake occurred, this P-wave "shadow zone" appeared on the opposite side of the globe. Oldham demonstrated that the earth had a core that was causing the primary waves to refract (bend) away, leaving a seismic shadow. In 1919 he suggested that the core may be liquid.  
  Richter, Charles Francis (1900–1985) U.S. seismologist, deviser of the Richter scale used to measure the strength of the waves from earthquakes.  
  Saussure, Horace Bénédict de (1740–1799) Swiss geologist who made the earliest detailed and first-hand study of the Alps. The results of his Alpine survey appeared in his classic work Voyages des Alpes/Travels in the Alps (1779–86).  
  Sedgwick, Adam (1785–1873) English geologist who contributed greatly to understanding the stratigraphy of the British Isles, using fossils as an index of relative time. In the 1830s he unraveled the stratigraphic sequence of fossil-bearing rocks in north Wales, naming the oldest of them the Cambrian period. In south Wales, Roderick c0016-01.gifMurchison had concurrently developed the Silurian system. The question of where the boundary lay between the older Cambrian and the younger Silurian sparked a dispute that was not resolved until 1879, when Charles Lapworth (1842–1920) coined the term Ordovician for the middle ground. Together with Murchison, Sedgwick identified the Devonian system in southwest England.  




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  Smith, William (1769–1839) English geologist. He produced the first geological maps of England and Wales, setting the pattern for geological cartography. Often called the founder of stratigraphical geology, he determined the succession of English strata across the whole country, from the Carboniferous up to the Cretaceous. He also established their fossil specimens.  
  Sorby, Henry Clifton (1826–1908) English geologist whose discovery in 1863 of the crystalline nature of steel led to the study of metallography. Thin-slicing of hard minerals enabled him to study the constituent minerals microscopically in transmitted light. He later employed the same techniques in the study of iron and steel under stress. He published On the Microscopical Structure of Crystals (1858).  
  Steno, Nicolaus (Latinized form of Niels Steensen) (1638–1686) Danish anatomist and naturalist, one of the founders of stratigraphy. To illustrate his ideas, he sketched what are probably the earliest geological sections. His examination of quartz crystals disclosed that, despite differences in the shapes, the angle formed by corresponding faces is invariable for a particular mineral. This constancy is known as Steno's law. In his Sample of the Elements of Myology (1667) Steno championed the organic origin of fossils.  
  Strabo (c. 63 B.C.A.D. c. 24) Greek geographer and historian who traveled widely to collect first-hand material for his Geography.  
  Suess, Eduard (1831–1914) Austrian geologist who helped pave the way for the theories of continental drift. He suggested that there had once been a great supercontinent, made up of the present southern continents; this he named Gondwanaland, after a region of India. In his book The Face of the earth (1885–1909) he offered an encyclopedic view of crustal movement, the structure and grouping of mountain chains, sunken continents, and the history of the oceans. He also made significant contributions to rewriting the structural geology of each continent.  
  Wegener, Alfred Lothar (1880–1930) German meteorologist and geophysicist whose theory of continental drift, expounded in Origin of Continents and Oceans (1915), was originally known as "Wegener's hypothesis." He supposed that a united supercontinent, Pangea, had existed in the Mesozoic era. This had developed numerous fractures and had drifted apart some 200 million years ago. During the Cretaceous period, South America and Africa had largely been split, but not until the end of the Quaternary had North America and Europe finally separated; the same was true of the break between South America and Antarctica. Australia had been severed from Antarctica during the Eocene. His ideas can now be explained in terms of plate tectonics, the idea that the earth's crust consists of a number of plates, all moving with respect to one another.  
  Werner, Abraham Gottlob (1749–1817) German geologist, one of the first to classify minerals systematically. He also developed the later discarded theory of neptunism—that the earth was initially covered by water, with every mineral in suspension; as the water receded, layers of rocks "crystallized." His geology was particularly important for establishing a physically based stratigraphy, grounded on precise mineralogical knowledge. He linked the order of the strata to the history of the earth, and related studies of mineralogy and strata.  
  Wilson, John Tuzo (1908–1993) Canadian geologist and geophysicist who established and brought about a general understanding of the concept of plate tectonics. He pioneered hands-on interactive museum exhibits, and could explain complex subjects like the movement of continents, the spreading of ocean floors, and the creation of island chains by using astonishingly simple models.  
substance used for cutting and polishing or for removing small amounts of the surface of hard materials. There are two types: natural and artificial abrasives, and their hardness is measured using the Mohs scale. Natural abrasives include quartz, sandstone, pumice, diamond, emery, and corundum; artificial abrasives include rouge, whiting, and carborundum.
  aclinic line
the magnetic equator, an imaginary line near the Equator, where a compass needle balances horizontally, the attraction of the north and south magnetic poles being equal.
cryptocrystalline (with crystals too small to be seen with an optical microscope) c0016-01.gifsilica composed of cloudy and banded c0016-01.gifchalcedony, sometimes mixed with c0016-01.gifopal, that forms in rock cavities.
naturally occurring fine-grained white or lightcolored translucent form of gypsum, often streaked or mottled. A soft material, it is easily carved, but seldom used for outdoor sculpture.
variety of quartz, colored violet by the presence of small quantities of impurities such as manganese or iron; used as a semiprecious stone. Amethysts are found chiefly in the Ural Mountains, India, the United States, Uruguay, and Brazil.
aluminum silicate (Al
2SiO5), a white to pinkish mineral crystallizing as square- or rhombus-based prisms. It is common in metamorphic rocks formed from clay sediments under low pressure conditions. Andalusite, kyanite, and sillimanite are all polymorphs (see c0016-01.gifpolymorphism) of Al2SiO5.
hard, dense, shiny variety of c0016-01.gifcoal, containing over 90% carbon and a low percentage of ash and impurities, which causes it to burn without flame, smoke, or smell. Because of its purity, anthracite gives off relatively little sulfur dioxide when burned.
places at opposite points on the globe.
blue variety of the mineral beryl. A semiprecious gemstone, it is used in jewelry.
group of islands, or an area of sea containing a group of islands. The islands of an archipelago are usually volcanic in origin, and they sometimes represent the tops of peaks in areas around continental margins flooded by the sea.




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any of several related minerals of fibrous structure that offer great heat resistance because of their nonflammability and poor conductivity. Commercial asbestos is generally either made from serpentine ("white" asbestos) or from sodium iron silicate ("blue" asbestos). The fibers are woven together or bound by an inert material. Over time the fibers can work loose and, because they are small enough to float freely in the air or be inhaled, asbestos usage is now strictly controlled; exposure to its dust can cause cancer.
mineral mixture containing semisolid brown or black c0016-01.gifbitumen, used in the construction industry. Asphalt is mixed with rock chips to form paving material, and the purer varieties are used for insulating material and for waterproofing masonry. It can be produced artificially by the distillation of c0016-01.gifpetroleum.
continuous or broken circle of c0016-01.gifcoral reef and low coral islands surrounding a lagoon.
large, irregular, deep-seated mass of intrusive igneous rock, usually granite, with an exposed surface of more than 100 sq km/40 sq mi. The mass forms by the intrusion or upswelling of magma (molten rock) through the surrounding rock. Batholiths form the core of some large mountain ranges like the Sierra Nevada of western North America.
principal ore of aluminum, consisting of a mixture of hydrated aluminum oxides and hydroxides, generally contaminated with compounds of iron, which give it a red color. It is formed by the chemical weathering of rocks in tropical climates. Chief producers of bauxite are Australia, Guinea, Jamaica, Russia, Kazakhstan, Suriname, and Brazil.
in geology, a single sedimentary rock unit with a distinct set of physical characteristics or contained fossils, readily distinguishable from those of beds above and below. Well-defined partings called bedding planes separate successive beds or strata.
  Benioff zone
seismically active zone inclined from a deep sea trench beneath a continent or continental margin. Earthquakes along Benioff zones define the top surfaces of plates of lithosphere that descend in to the mantle beneath another, overlying plate. The zone is named for Hugo Benioff (1899–1968), a U.S. seismologist who first described this feature.
mineral, beryllium aluminum silicate (Be
3A12Si6O18), which forms crystals chiefly in granite. It is the chief ore of the metallic element beryllium. Two of its gem forms are aquamarine (light-blue crystals) and emerald (dark-green crystals).
impure mixture of hydrocarbons, including such deposits as petroleum, asphalt, and natural gas, although sometimes the term is restricted to a soft kind of pitch resembling asphalt.
very large basin-shaped crater. Calderas are found at the tops of volcanoes, where the original peak has collapsed into an empty chamber beneath. The basin, many times larger than the original volcano vent, may be flooded, producing a crater lake, or the flat floor may contain a number of small volcanic cones, produced by volcanic activity after the collapse.
art and practice of drawing c0016-01.gifmaps.
any bonding agent used to unite particles in a single mass or to cause one surface to adhere to another. In geology, cement refers to a chemically precipitated material such as carbonate that occupies the interstices of clastic rocks.
form of the mineral quartz in which the crystals are so fine-grained that they are impossible to distinguish with a microscope (cryptocrystalline). Agate, onyx, and carnelian are c0016-01.gifgem varieties of chalcedony.
soft, fine-grained, whitish sedimentary rock composed of calcium carbonate (CaCO
3), extensively quarried for use in cement, lime, and mortar, and in the manufacture of cosmetics and toothpaste. Blackboard chalk in fact consists of gypsum (calcium sulfate, CaSO4.2H2O).
mercuric sulfide mineral (HgS), the only commercially useful ore of mercury. It is deposited in veins and impregnations near recent volcanic rocks and hot springs. The mineral itself is used as a red pigment, commonly known as vermilion. Cinnabar is found in the United States (California), Spain (Almadén), Peru, Italy, and Slovenia.
very fine-grained sedimentary deposit that has undergone a greater or lesser degree of consolidation. When moistened it is plastic, and it hardens on heating, which renders it impermeable. It may be white, gray, red, yellow, blue, or black, depending on its composition. Clay minerals consist largely of hydrous silicates of aluminum and magnesium together with iron, potassium, sodium, and organic substances. The crystals of clay minerals have a layered structure, capable of holding water, and are responsible for its plastic properties. According to international classification, in mechanical analysis of soil, clay has a grain size of less than 0.002 mm/0.00008 in.
black or blackish mineral substance formed from the compaction of ancient plant matter in tropical swamp conditions. It is used as a fuel and in the chemical industry. Coal is classified according to the proportion of carbon it contains. The main types are c0016-01.gifanthracite (shiny, with about 90% carbon), bituminous coal (shiny and dull patches, about 75% carbon), and lignite (woody, grading into peat, about 50% carbon). Coal burning is one of the main causes of acid rain.
  continental drift
in geology, the theory that, about 250–200 million years ago the earth consisted of a single large continent (Pangaea), which subsequently broke apart to form the continents known today. The theory was proposed in 1912 by German meteorologist Alfred Wegener, but such vast continental movements could not be satisfactorily explained until the study of plate tectonics in the 1960s.
marine invertebrate of the class Anthozoa in the phylum Cnidaria, which also includes sea anemones and jellyfish. It has a skeleton of lime (calcium carbonate) extracted from the surrounding water. Corals exist in warm seas, at moderate depths with sufficient light. Some coral is valued for decoration or jewelry, for example, Mediterranean red coral Corallum rubrum.
native aluminum oxide (Al
2O3), the hardest naturally occurring mineral known apart from diamond (corundum rates 9 on the Mohs scale of hardness); lack of cleavage




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  also increases its durability. Its crystals are barrel-shaped prisms of the trigonal system. Varieties of gem-quality corundum are ruby (red) and sapphire (any color other than red, usually blue). Poorer-quality and synthetic corundum is used in industry, for example as an c0016-01.gifabrasive.  
bowl-shaped depression in the ground, usually round and with steep sides. Craters are formed by explosive events such as the eruption of a volcano or the impact of a meteorite.
  deep-sea trench
another term for ocean trench.
sheet of igneous rock created by the intrusion of magma (molten rock) across layers of pre-existing rock. (By contrast, a sill is intruded between layers of rock.) It may form a ridge when exposed on the surface if it is more resistant than the rock into which it intruded.
the angle at which a structural surface, such as a fault or a bedding plane, is inclined from the horizontal. Measured at right angles to the c0016-01.gifstrike of that surface, it is used together with strike to describe the orientation of geological features in the field. Rocks that are dipping have usually been affected by folding.
white mineral with a rhombohedral structure, calcium magnesium carbonate (CaMg (CO
3)2). Dolomites are common in geological successions of all ages and are often formed when limestone is changed by the replacement of the mineral calcite with the mineral dolomite.
clear, green gemstone variety of the mineral beryl. It occurs naturally in Colombia, the Ural Mountains in Russia, Zimbabwe, and Australia. The green color is caused by the presence of the element chromium in the beryl.
black to grayish form of impure c0016-01.gifcorundum that also contains the minerals magnetite and hematite. It is used as an c0016-01.gifabrasive.
  Equator, or terrestrial equator,
the great circle whose plane is perpendicular to the earth's axis (the line joining the poles). Its length is 40,092 km/24,901.8 mi, divided into 360 degrees of longitude. The Equator encircles the broadest part of the earth, and represents 0° latitude. It divides the earth into two halves, called the northern and the southern hemispheres.
  extrusive rock, or volcanic rock,
igneous rock formed on the surface of the earth; for example, basalt. It is usually finegrained (having cooled quickly), unlike the more coarse-grained c0016-01.gifintrusive rocks. The magma (molten rock) that cools to form extrusive rock may reach the surface through a crack such as the constructive margin at the Mid-Atlantic Ridge, or through the vent of a volcano. Extrusive rock can be either lava (solidified from a flow) or a pyroclastic deposit (hot rocks or ash).
  felsic rock
plutonic rock composed chiefly of light-colored minerals, such as quartz, feldspar, and mica. It is derived from feldspar, lenad (meaning feldspathoid), and silica. The term felsic also applies to light-colored minerals as a group, especially quartz, feldspar, and feldspathoids.
compact, hard, brittle mineral (a variety of chert), brown, black, or gray in color, found as nodules in limestone or shale deposits. It consists of cryptocrystalline (grains too small to be visible even under a light microscope) c0016-01.gifsilica, principally in the crystalline form of quartz. Implements fashioned from flint were widely used in prehistory.
bend in c0016-01.gifbeds or layers of rock. If the bend is arched in the middle it is called an anticline; if it sags downwards in the middle it is called a syncline. The line along which a bed of rock folds is called its axis. The axial plane is the plane joining the axes of successive beds.
any of a group of silicate minerals with the formula X
3Y3(SiO4)3, where X is calcium, magnesium, iron, or manganese, and Y is usually aluminum or sometimes iron or chromium. Garnets are used as semiprecious gems (usually pink to deep red) and as abrasives. They occur in metamorphic rocks such as gneiss and schist.
mineral valuable by virtue of its durability (hardness), rarity, and beauty, cut and polished for ornamental use, or engraved. Of 120 minerals known to have been used as gemstones, only about 25 are in common use in jewelry today; of these, the diamond, emerald, ruby, and sapphire are classified as precious, and all the others semiprecious; for example, the topaz, amethyst, opal, and aquamarine.
methods of surveying the earth for making maps and correlating geological, gravitational, and magnetic measurements. Geodesic surveys, formerly carried out by means of various measuring techniques on the surface, are now commonly made by using radio signals and laser beams from orbiting satellites.
study of the earth's surface; its topography, climate, and physical conditions, and how these factors affect people and society. It is usually divided into physical geography, dealing with landforms and climates, and human geography, dealing with the distribution and activities of peoples on earth.
science of the earth, its origin, composition, structure, and history. It is divided into several branches: mineralogy (the minerals of earth), petrology (rocks), stratigraphy (the deposition of successive beds of sedimentary rocks), paleontology (fossils), and tectonics (the deformation and movement of the earth's crust).
  Gondwanaland, or Gondwana,
southern landmass formed 200 million years ago by the splitting of the single world continent c0016-01.gifPangea. (The northern landmass was Laurasia.) It later fragmented into the continents of South America, Africa, Australia, and Antarctica, which then moved slowly to their present positions. The baobab tree found in both Africa and Australia is a relic of this ancient landmass.
blackish-gray, laminar, crystalline form of carbon. It is used as a lubricant and as the active component of pencil lead.
coarse c0016-01.gifsediment consisting of pebbles or small fragments of rock, originating in the beds of lakes and streams or on beaches. Gravel is quarried for use in road building, railroad ballast, and for an aggregate in concrete. It is obtained from quarries known as gravel pits, where it is often found mixed with sand or clay.
principal ore of iron, consisting mainly of iron(III) oxide, Fe
2O3. It occurs as specular hematite (dark, metallic




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  lustre), kidney ore (reddish radiating fibers terminating in smooth, rounded surfaces), and a red earthy deposit.  
  hot spot
isolated rising plume of molten mantle material that may rise to the surface of the earth's crust creating features such as volcanoes, chains of ocean islands, sea mounts, and rifts in continents. Hot spots occur beneath the interiors of tectonic plates and so differ from areas of volcanic activity at plate margins. Examples of features made by hot spots are Iceland in the Atlantic Ocean, and in the Pacific Ocean the Hawaüan Islands and Emperor Seamount chain, and the Galápagos Islands.
  International Date Line (IDL)
imaginary line that approximately follows the 180° line of longitude. The date is put forward a day when crossing the line going west, and back a day when going east. The IDL was chosen at the International Meridian Conference in 1884.
  intrusive rock
igneous rock formed beneath the earth's surface. Magma, or molten rock, cools slowly at these depths to form coarse-grained rocks, such as granite, with large crystals. A mass of intrusive rock is called an intrusion. Intrusion features include vertical cylindrical structures such as stocks, pipes, and necks; sheet structures such as dikes that cut across the strata and sills that push between them; laccoliths, which are blisters that push up the overlying rock; and batholiths, which represent chambers of solidified magma and contain vast volumes of rock.
  island arc
curved chain of islands produced by volcanic activity at a destructive margin (where one tectonic plate slides beneath another). Island arcs are common in the Pacific where they ring the ocean on both sides; the Aleutian Islands off Alaska are an example.
semiprecious stone consisting of either jadeite (NaAlSi
2O6, a pyroxene), or nephrite (Ca2(Mg,Fe)5Si8O22(OH,F)2, an amphibole), ranging from colorless through shades of green to black according to the iron content. Jade ranks 5.5–6.5 on the Mohs scale of hardness.
white or grayish c0016-01.gifclay mineral, hydrated aluminum silicate (Al
2Si2O5(OH)4), formed mainly by the decomposition of feldspar in granite. It is made up of platelike crystals, the atoms of which are bonded together in two-dimensional sheets, between which the bonds are weak, so that they are able to slip over one another, a process made more easy by a layer of water. China clay (kaolin) is derived from it. It is mined in France, the United States, Germany, China, and the U.K.
aluminum silicate (Al
2SiO5), a pale-blue mineral occurring as blade-shaped crystals. It is an indicator of highpressure conditions in metamorphic rocks formed from clay sediments. Andalusite, kyanite, and sillimanite are all polymorphs (see c0016-01.gifpolymorphism).
mudflow formed of a fluid mixture of water and volcanic ash. During a volcanic eruption, melting ice may combine with ash to form a powerful flow capable of causing great destruction. The lahars created by the eruption of Nevado del Ruiz in Colombia, South America, in 1985 buried 22,000 people in 8 m/26 ft of mud.
  lapis lazuli
rock containing the blue mineral lazurite in a matrix of white calcite with small amounts of other minerals. It occurs in silica-poor igneous rocks and metamorphic limestones found in Afghanistan, Siberia, Iran, and Chile. Lapis lazuli was a valuable pigment of the Middle Ages, also used as a gemstone and in inlaying and ornamental work.
  latitude and longitude
imaginary lines used to locate position on the globe. Lines of latitude are drawn parallel to the Equator, with 0° at the Equator and 90° at the north and south poles. Lines of longitude are drawn at right angles to these, with 0° (the Prime Meridian) passing through Greenwich, England.
molten rock (usually 800–1,100°C/ 1,500–2,000°F) that erupts from a volcano and cools to form extrusive igneous rock. It differs from magma in that it is molten rock on the surface; magma is molten rock below the surface. Lava that is viscous and sticky does not flow far; it forms a steep-sided conical composite volcano. Less viscous lava can flow for long distances and forms a broad flat shield volcano.
type of fertile soil, a mixture of sand, silt, clay, and organic material. It is porous, which allows for good air circulation and retention of moisture.
geological deposit rich in certain minerals, generally consisting of a large vein or set of veins containing ore minerals. A system of veins that can be mined directly forms a lode, for example the mother lode of the California gold rush.
see c0016-01.giflatitude and longitude.
  mafic rock
plutonic rock composed chiefly of dark-colored minerals containing abundant magnesium and iron, such as olivine and pyroxene. It is derived from magnesium and ferric (iron). The term mafic also applies to dark-colored minerals rich in iron and magnesium as a group.
molten rock material beneath the earth's (or any of the terrestrial planets) surface from which igneous rocks are formed. c0016-01.gifLava is magma that has extruded onto the surface.
diagrammatic representation of an area—for example, part of the earth's surface or the distribution of the stars. Modern maps of the earth are made using satellites in low orbit to take a series of overlapping stereoscopic photographs from which a three-dimensional image can be prepared. The earliest accurate large-scale maps appeared about 1580.
  mass extinction
an event that produces the extinction of many species at about the same time. One notable example is the boundary between the Cretaceous and Tertiary periods (known as the K-T boundary) that saw the extinction of the dinosaurs and other big reptiles, and many of the marine invertebrates as well. Mass extinctions have taken place frequently during earth's history.
half a great circle drawn on the earth's surface passing through both poles and thus through all places with the same longitude. Terrestrial longitudes are usually measured from the Greenwich Meridian.
any of a group of silicate minerals that split easily into thin flakes along lines of weakness in their crystal structure (perfect basal cleavage). They are glossy, have a pearly luster, and are found in many igneous and metamorphic rocks. Their good thermal and electrical insulation qualities make them valuable in industry.




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study of minerals. The classification of minerals is based chiefly on their chemical compostion and the kind of chemical bonding that holds these atoms together. The mineralogist also studies their crystallographic and physical characters, occurrence, and mode of formation.
  niter, or saltpeter,
potassium nitrate (KNO
3), a mineral found on and just under the ground in desert regions; used in explosives. Niter occurs in Bihar, India, Iran, and Cape Province, South Africa. The salt was formerly used for the manufacture of gunpowder, but the supply of niter for explosives is today largely met by making the salt from nitratine (also called Chile saltpeter, NaNO3). Saltpeter is a preservative and is widely used for curing meats.
  nuée ardente
rapidly flowing, glowing white-hot cloud of ash and gas emitted by a volcano during a violent eruption. The ash and other pyroclastics in the lower part of the cloud behave like an ash flow. In 1902 a nuée ardente produced by the eruption of Mount Pelee in Martinique swept down the volcano in a matter of seconds and killed 28,000 people in the nearby town of St. Pierre.
black or dark-colored glassy volcanic rock, chemically similar to granite, but formed by cooling rapidly on the earth's surface at low pressure.
greenish mineral, magnesium iron silicate ((Mg,Fe)
2SiO4). It is a rock-forming mineral, present in, for example, peridotite, gabbro, and basalt. Olivine is called peridot when pale green and transparent, and used in jewelry.
semiprecious variety of chalcedonic c0016-01.gifsilica in which the crystals are too fine to be detected under a microscope, a state known as cryptocrystalline. It has straight parallel bands of different colors: milk-white, black, and red.
form of hydrous c0016-01.gifsilica, (SiO
2.nH2O), often occurring as stalactites and found in many types of rock. The common opal is translucent, milk-white, yellow, red, blue, or green, and lustrous. Precious opal is opalescent, the characteristic play of colors being caused by close-packed silica spheres diffracting light rays within the stone.
body of rock, a vein within it, or a deposit of sediment, worth mining for the economically valuable mineral it contains. The term is usually applied to sources of metals. Occasionally metals are found uncombined (native metals), but more often they occur as compounds such as carbonates, sulfides, or oxides. The ores often contain unwanted impurities that must be removed when the metal is extracted.
  orogeny, or orogenesis,
the formation of mountains. It is brought about by the movements of the rigid plates making up the earth's crust and upper-most mantle (described by plate tectonics). Where two plates collide at a destructive margin rocks become folded and lifted to form chains of mountains (such as the Himalayas).
study of ancient life, encompassing the structure of ancient organisms and their environment, evolution, and ecology, as revealed by their fossils. The practical aspects of paleontology are based on using the presence of different fossils to date particular rock strata and to identify rocks that were laid down under particular conditions; for instance, giving rise to the formation of oil.
single land mass, made up of all the present continents, believed to have existed between 300 and 200 million years ago; the rest of the earth was covered by the Panthalassa ocean. Pangaea split into two land masses—Laurasia in the north and c0016-01.gifGondwanaland in the south—which subsequently broke up into several continents. These then drifted slowly to their present positions.
rock consisting largely of the mineral olivine; pyroxene and other minerals may also be present. Peridotite is an ultramafic rock containing less than 45% silica by weight. It is believed to be one of the rock types making up the earth's upper mantle, and is sometimes brought from the depths to the surface by major movements, or as inclusions in lavas.
yellow, brown, or grayish-black orthorhombic mineral (CaTiO
3), which sometimes contains cerium. Other minerals that have a similar structure are said to have the perovskite structure. The term also refers to MgSiO3 with the perovskite structure, the principle mineral that makes up the earth's lower mantle.
  petroleum, or crude oil,
natural mineral oil, a thick greenish-brown flammable liquid found underground in permeable rocks. Petroleum consists of hydrocarbons mixed with oxygen, sulfur, nitrogen, and other elements in varying proportions. It is thought to be derived from ancient organic material that has been converted by, first, bacterial action, then heat, and pressure (but its origin may be chemical also).
  From crude petroleum, various products are made by distillation and other processes; for example, fuel oil, gasoline, kerosene, diesel, and lubricating oil. Petroleum products and chemicals are used in large quantities in the manufacture of detergents, artificial fibers, plastics, insecticides, fertilizers, pharmaceuticals, toiletries, and synthetic rubber.  
branch of geology that deals with the study of rocks, their mineral compositions, and their origins.
either of the geographic north and south points of the axis about which the earth rotates. The geographic poles differ from the magnetic poles, which are the points toward which a freely suspended magnetic needle will point.
in mineralogy, the ability of a substance to adopt different internal structures and external forms, in response to different conditions of temperature and/or pressure. For example, diamond and graphite are both forms of the element carbon, but have very different properties and appearance.
in geology, pertaining to fragments of solidified volcanic magma, ranging in size from fine ash to large boulders, that are extruded during an explosive volcanic eruption; also the rocks that are formed by consolidation of such material. Pyroclastic rocks include tuff (ash deposit) and agglomerate (volcanic breccia).
any one of a group of minerals, silicates of calcium, iron, and magnesium with a general formula X,YSi
2O6, found in igneous and metamorphic rocks. The internal structure is based on single chains of silicon and oxygen. Diopside (X = Ca, Y = Mg) and augite (X = Ca, Y = Mg,Fe,Al) are common pyroxenes.




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igneous rock, the fine-grained volcanic (extrusive) equivalent of granite.
red transparent gem variety of the mineral c0016-01.gifcorundum. Small amounts of chromium oxide (Cr
2O3), substituting for aluminum oxide, give ruby its color. Natural rubies are found mainly in Myanmar (Burma), but rubies can also be produced artificially and such synthetic stones are used in lasers.
  salt, common, or sodium chloride
(NaCl), white crystalline solid found dissolved in sea water and as rock salt (the mineral halite) in large deposits and salt domes. Common salt is used extensively in the food industry as a preservative and for flavoring, and in the chemical industry in the making of chlorine and sodium.
  San Andreas fault
geological fault stretching for 1,125 km/700 mi northwest—southeast through the state of California, United States. It marks a conservative plate margin, where two plates slide past each other.
loose grains of rock, sized 0.0625–2.00 mm/ 0.0025–0.08 in in diameter, consisting most commonly of quartz, but owing their varying color to mixtures of other minerals. Sand is used in cement-making, as an abrasive, in glass-making, and for other purposes.
deep-blue, transparent gem variety of the mineral c0016-01.gifcorundum. Small amounts of iron and titanium give it its color. A corundum gem of any color except red (which is a ruby) can be called a sapphire; for example, yellow sapphire.
  scarp and dip
in geology, the two slopes formed when a sedimentary bed outcrops as a landscape feature. The scarp is the slope that cuts across the bedding plane; the dip is the opposite slope which follows the bedding plane. The scarp is usually steep, while the dip is a gentle slope.
metamorphic rock containing mica or another platy or elongate mineral, whose crystals are aligned to give a foliation (planar texture) known as schistosity. Schist may contain additional minerals such as c0016-01.gifgarnet.
  seafloor spreading
growth of the ocean crust outward (sideways) from ocean ridges. The concept of seafloor spreading has been combined with that of continental drift and incorporated into plate tectonics.
any loose material that has ''settled"—deposited from suspension in water, ice, or air, generally as the water current or wind speed decreases. Typical sediments are, in order of increasing coarseness, clay, mud, silt, sand, gravel, pebbles, cobbles, and boulders.
study of earthquakes and how their shock waves travel through the earth. By examining the global pattern of waves produced by an earthquake, seismologists can deduce the nature of the materials through which they have passed. This leads to an understanding of the earth's internal structure.
silicon dioxide (SiO
2), the composition of the most common mineral group, of which the most familiar form is quartz. Other silica forms are c0016-01.gifchalcedony, chert, opal, tridymite, and cristobalite. Common sand consists largely of silica in the form of quartz.
sheet of igneous rock created by the intrusion of magma (molten rock) between layers of pre-existing rock. An example of a sill in the United States is the Palisades Sill along the Hudson river north of New York City.
aluminum silicate (Al
2SiO5), a mineral that occurs either as white to brownish prismatic crystals or as minute white fibers. It is an indicator of high temperature conditions in metamorphic rocks formed from clay sediments. Andalusite, kyanite, and sillimanite are all polymorphs (see c0016-01.gifpolymorphism) of Al2SiO5.
the compass direction of a horizontal line on a planar structural surface, such as a fault plane, bedding plane, or the trend of a structural feature, such as the axis of a fold. Strike is 90
° from c0016-01.gifdip.
branch of geology that deals with the sequence of formation of sedimentary rock layers and the conditions under which they were formed. Its basis was developed by the English geologist William Smith. Stratigraphy in the interpretation of archeological excavations provides a relative chronology for the levels and the artifacts within them. The basic principle of superimposition establishes that upper layers or deposits have accumulated later in time than the lower ones.
mound produced in shallow water by mats of algae that trap mud particles. Another mat grows on the trapped mud layer and this traps another layer of mud and so on. The stromatolite grows to heights of a yard or so. They are uncommon today but their fossils are among the earliest evidence for living things—over 2,000 million years old.
  subduction zone
region where two plates of the earth's rigid lithosphere collide, and one plate descends below the other into the weaker asthenosphere. Subduction occurs along ocean trenches, most of which encircle the Pacific Ocean; portions of the ocean plate slide beneath other plates carrying continents.
accurate measuring of the earth's crust, or of land features or buildings. It is used to establish boundaries, and to evaluate the topography for engineering work. The measurements used are both linear and angular, and geometry and trigonometry are applied in the calculations.
in geology, the study of the movements of rocks on the earth's surface. On a small scale tectonics involves the formation of folds and faults, but on a large scale plate tectonics deals with the movement of the earth's surface as a whole.
mineral, aluminum fluorosilicate (Al
2(F2SiO4)). It is usually yellow, but pink if it has been heated, and is used as a gemstone when transparent. It ranks 8 on the Mohs scale of hardness.
surface shape and composition of the landscape, comprising both natural and artificial features, and its study. Topographical features include the relief and contours of the land; the distribution of mountains, valleys, and human settlements; and the patterns of rivers, roads, and railroads.
minor earthquake.




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mineral, hydrous basic copper aluminum phosphate (CuAl
6(PO4)4(OH)85H2 O). Blue-green, blue, or green, it is a gemstone. Turquoise is found in Australia, Egypt, Ethiopia, France, Germany, Iran, Turkestan, Mexico, and the south-western United States. It was originally introduced into Europe through Turkey, from which its name is derived.
  volcanic rock
another name for c0016-01.gifextrusive rock, igneous rock formed on the earth's surface.
zirconium silicate (ZrSiO
4), a mineral that occurs in small quantities in a wide range of igneous, sedimentary, and metamorphic rocks. It is very durable and is resistant to erosion and weathering. It is usually colored brown, but can be other colors, and when transparent may be used as a gemstone.
  Further Reading  
  Ager, D. V. The Nature of the Stratigraphical Record (1992)  
  Allen, K., and Briggs, D. (eds.) Evolution and the Fossil Record (1989)  
  Bailey, E. B. James Hutton (1967)  
  Barnes, J. W. Ores and Minerals: Introducing Economic Geology (1988)  
  Benton, M. J. Vertebrate Paleontology (1990)  
  Borchardt-Ott, W. Crystallography (1993)  
  Briggs, David Fundamentals of the Physical Environment (1992)  
  Brown, G. A., and others Understanding the Earth (1992)  
  Brown, G. C., and Mussett, A. E. The Inaccessible Earth (1981)  
  Calder, Nigel Restless Earth (1972)  
  Carroll, R. L. Vertebrate Paleontology and Evolution (1988)  
  Chisholm, Michael Human Geography: Evolution or Revolution? (1975)  
  Cocks, L. R. M. The Evolving Earth (1981)  
  Condie, Kent C. Plate Tectonics and Crustal Evolution (1997, 4th edition)  
  Cowen, Richard History of Life (1990)  
  Cox, Allan Plate Tectonics: How It Works (1986)  
  Cresser, M. Killham, K. and Edwards, T. Soil Chemistry and its Applications (1993)  
  Decker, R. W., and Decker, B. B. Mountains of Fire: The Nature of Volcanoes (1991)  
  Dietrich, R. V., and Skinner, B. J. Gems, Granites and Gravels: Knowing and Using Rocks and Minerals (1990)  
  Dineen, Jacqueline Natural Disasters: Volcanoes (1991)  
  Dixon, Dougal The Practical Geologist (1992)  
  Dunning, F. W., and others Britain Before Man (1978), The Story of the Earth (1981)  
  Emery, Dominic, and Meyers, Keith Sequence Stratigraphy (1996)  
  Emery, D., and Robinson, A. Inorganic Geochemistry (1994)  
  Erickson, Jon Plate Tectonics: Unravelling the Mysteries of the earth (1992), Rock Formations and Unusual Geologic Structures: Exploring the Earth's Surface (1993)  
  Farndon, J. How the Earth Works (1992)  
  Fortey, Richard Fossils: The Key to the Past (1991)  
  Fowler, C. M. R. The Solid Earth: An Introduction to Global Geophysics (1990)  
  Fuller, Sue Rocks and Minerals (1995)  
  Goudie, Andrew, and Gardner, Rita Discovering Landscape (1985)  
  Gould, Stephen Jay Wonderful Life (1989), (ed.) The Book of Life (1993)  
  Gubbins, David Seismology and Plate Tectonics (1990)  
  Hall, C Gemstones (1994)  
  Halstead, L. B. Hunting the Past (1982)  
  Harper, David, and Owen, Alan Fossils of the Upper Ordovician (1998)  
  Holdsworth, R. V. Strachan, R. A. and Dewey, J. F. (eds.) Continental Transpressional Tectonics and Transtensional Tectonics (1998)  
  Holmes, Arthur The Principles of Physical Geology (1965)  
  Killops, S. D., and Killops, V. J. An Introduction to Organic Geochemistry (1994)  
  Klein, C., and Hurlbutt, C. S. Manual of Mineralogy (1994)  
  MacKenzie, W. S., and Adams, A. E. A Colour Atlas of Rocks and Minerals in Thin Section (1994)  
  Mayr, H. Fossils (1992)  
  Milner, Angela The Natural History Museum Book of Dinosaurs (1995)  
  Myers, Norman The Gaia Atlas of Planet Management (1994)  
  Navrotsky, Alexandra Physics and Chemistry of Earth Materials (1994)  
  Nelson, C. R. (ed.) Chemistry of Coal Weathering (1989)  
  Nield, E. W. Drawing and Understanding Fossils: A Theoretical and Practical Guide for Beginners with Self-Assessment (1987)  
  Payne, K. R. Chemicals from Coal: New Processes (1987)  
  Pellant, Chris Rocks, Minerals and Fossils of the World (1990), Rocks and Minerals (1992)  
  Playfair, John Illustrations of the Huttonian Theory of the earth (1802; reprinted 1956)  
  Raup, David The Nemesis Affair. A Story of the Death of Dinosaurs and Ways of Science (1986)  




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  Reading, Harold Sedimentary Environments: Processes, Facies and Stratigraphy (1996)  
  Roberts, J. L. The Macmillan Field Guide to Geological Structures (1989)  
  Roberts, Willard L., and others Encyclopedia of Minerals (1990)  
  Robinson, George Minerals: An Illustrated Exploration of the Dynamic World of Minerals and Their Properties (1995)  
  Roger, Jacques Buffon: a Life in Natural History (1998)  
  Ross, Simon Hazard Geography (1987), The Challenge of the Natural Environment (1989), Exploring Geography (1991)  
  Rudwick, Martin The Meaning of Fossils (1972), Georges Cuvier, Fossil Bones, and Geological Catastrophes (1998)  
  Russell, D. A. An Odyssey in Time: The Dinosaurs of North America (1989)  
  Sabins, Floyd Remote Sensing (1996)  
  Smith, James Introduction to Geodesy: the History and Concepts of Modern Geodesy (1998)  
  Stanley, Steven M. Earth and Life Through Time (1986)  
  Stefoff, Rebecca Maps and Mapmaking (1995)  
  Tassy, Pascal The Message of Fossils (1991; translated 1993)  
  Thackray, John The Age of the Earth (1980)  
  Tooley, R. V. Maps and Map-Makers (1987)  
  Trueman, A. E. Geology and Scenery in England and Wales (1971)  
  van Rose, Susanna earthquakes (1983), Darling Kindersley Eyewitness Guides: Volcano (1992)  
  Walker, C., and Ward, D. Fossils, A Visual Guide to over 500 Fossils Genera from Around the World (1992)  
  Waltham, A. C. Foundations of Engineering Geology (1994)  
  Wendt, Herbert Before the Deluge (1968; translated 1970)  
  Whittow, John Disasters (1980)