Oceanography 2

Distribution of major ridges and spreading centres

Oceanic spreading centres are found in all the ocean basins. In the Arctic Ocean a slow-rate spreading centre is located near the eastern side in the Eurasian basin. It can be followed south, offset by transform faults, to Iceland. Iceland has been created by a hot spot (see below) located directly below an oceanic spreading centre. The ridge leading south from Iceland is named the Reykjanes Ridge, and, although it spreads at 20 mm/yr or less, it lacks a rift valley. This is thought to be the result of the influence of the hot spot.

The Mid-Atlantic Ridge extends from south of Iceland to the extreme South Atlantic Ocean near 60 S latitude. It bisects the Atlantic Ocean basin, which led to the earlier designation of mid-ocean ridge for features of this type. The Mid-Atlantic Ridge became known in a rudimentary fashion during the 19th century. In 1855 Matthew Fontaine Maury of the U.S. Navy prepared a chart of the Atlantic in which he identified it as a shallow "middle ground." During the 1950s the American oceanographers Bruce Heezen and Maurice Ewing proposed that it was a continuous mountain range.

In the North Atlantic the ridge spreads slowly and displays a rift valley and mountainous flanks. In the South Atlantic spreading rates are between slow and intermediate, and rift valleys are generally absent, as they occur only near transform faults.

A very slow oceanic ridge, the Southwest Indian Ridge, bisects the ocean between Africa and Antarctica. It joins the Mid-Indian and Southeast Indian ridges east of Madagascar. The Carlsberg Ridge is found at the north end of the Mid-Indian Ridge. It continues north to join spreading centres in the Gulf of Aden and Red Sea. Spreading is very slow at this point but approaches intermediate rates on the Carlsberg and Mid-Indian ridges. The Southeast Indian Ridge spreads at intermediate rates. This ridge continues from the western Indian Ocean in a southeasterly direction, bisecting the ocean between Australia and Antarctica. Rifted crests and rugged mountainous flanks are characteristic of the Southwest Indian Ridge. The Mid-Indian Ridge has fewer features of this kind, and the Southeast Indian Ridge has generally smoother topography. The latter also displays distinct asymmetric seafloor spreading south of Australia. Analysis of magnetic anomalies shows that rates on opposite sides of the spreading centre have been unequal at many times over the past 50 or 60 million years.

The Pacific-Antarctic Ridge can be followed from a point midway between New Zealand and Antarctica northeast to where it joins the East Pacific Rise off the margin of South America. The former spreads at intermediate to fast rates.

The East Pacific Rise extends from this site northward to the Gulf of California, where it joins the transform zone of the Pacific-North American plate boundary. Offshore from Chile and Peru, the East Pacific Rise is currently spreading at fast rates of 159 mm/yr or more. Rates decrease to about 60 mm/yr at the mouth of the Gulf of California. The crest of the ridge displays a low topographic rise along its length rather than a rift valley.

The East Pacific Rise was first detected during the Challenger Expedition of the 1870s. It was described in its gross form during the 1950s and '60s by oceanographers, including Heezen, Ewing, and Henry W. Menard. During the 1980s, Kenneth C. Macdonald, Paul J. Fox, and Peter F. Lonsdale discovered that the main spreading centre appears to be interrupted and offset a few kilometres to one side at various places along the crest of the East Pacific Rise. However, the ends of the offset spreading centres overlap each other by several kilometres. These were identified as a new type of geologic feature of oceanic spreading centres and designated overlapping spreading centres. Such centres are thought to result from interruptions of the magma supply to the crest along its length and define a fundamental segmentation of the ridge on a scale of tens to hundreds of kilometres.

Many smaller spreading centres branch off the major ones or are found behind island arcs. In the western Pacific, spreading centres occur on the Fiji Plateau between the New Hebrides and Fiji Islands and in the Woodlark Basin between New Guinea and the Solomon Islands. A series of spreading centres and transform faults lie between the East Pacific Rise and South America near 40 to 50 S latitude. The Scotia Sea between South America and the Antarctic Peninsula contains a spreading centre. The Gal�pagos spreading centre trends east-west between the East Pacific Rise and South America near the equator.

Three short spreading centres are found a few hundred kilometres off the shore of the Pacific Northwest. These are the Gorda Ridges off northern California, the Juan de Fuca Ridge off Oregon and Washington, and the Explorer Ridge off Vancouver Island. In a careful study of the seafloor spreading history of the Gal�pagos and the Juan de Fuca spreading centres, the American geophysicist Richard N. Hey developed the idea of the propagating rift. In this phenomenon, one branch of a spreading centre ending in a transform fault lengthens at the expense of the spreading centre across the fault. The rift and fault propagate at one to five times the spreading rate and create chevron patterns in magnetic anomalies and the grain of the seafloor topography resembling the wake of a boat.

Spreading centre zones and associated phenomena

From the 1970s highly detailed studies of spreading centres using deeply towed instruments, photography, and manned submersibles have resulted in new revelations about the processes of seafloor spreading. The most profound discoveries have been of deep-sea hydrothermal vents (see below) and previously unknown biological communities.

Spreading centres are divided into several geologic zones. The neovolcanic zone is at the very axis. It is 1 to 2 kilometres wide and is the site of recent and active volcanism and of the hydrothermal vents. It is marked by chains of small volcanoes or volcanic ridges. Adjacent to the neovolcanic zone is one marked by fissures in the seafloor. This may be 1 to 2 kilometres wide. Beyond this point occurs a zone of active faulting. Here, fissures develop into normal faults with vertical offsets.

This zone may be 10 or more kilometres wide. At slow spreading rates the faults have offsets of hundreds of metres, creating rift valleys and rift mountains. At faster rates the vertical offsets are 50 metres or less. A deep rift valley is not formed because the vertical uplifts are cancelled out by faults that downdrop uplifted blocks. This results in linear, fault-bounded abyssal hills and valleys trending parallel to the spreading centre.

Warm springs emanating from the seafloor in the neovolcanic zone were first found on the Gal�pagos spreading centre. These waters were measured to have temperatures about 20 C above the ambient temperature. In 1979 hydrothermal vents with temperatures near 350 C were discovered on the East Pacific Rise off Mexico. Since then, similar vents have been found on the spreading centres off the Pacific Northwest coast of the United States, on the south end of the northern Mid-Atlantic Ridge, and at many locations on the East Pacific Rise.

Hydrothermal vents are localized discharges of heated seawater. They result from cold seawater percolating down into the hot oceanic crust through the zone of fissures and returning to the seafloor in a pipelike flow at the axis of the neovolcanic zone. The heated waters often carry sulfide minerals of zinc, iron, and copper leached from the crust. Outflow of these heated waters probably accounts for 20 percent of the Earth's heat loss. Exotic biological communities exist around the hydrothermal vents. These ecosystems are totally independent of energy from the Sun. They are not dependent on photosynthesis but rather on chemosynthesis by sulfur-fixing bacteria. The sulfide minerals precipitated in the neovolcanic zone can accumulate in substantial amounts and are sometimes buried by lava flows at a later time. Such deposits are mined as commercial ores in ophiolites on Cyprus and in Oman.

Magma chambers have been detected beneath the crest of the East Pacific Rise by seismic experiments. (The principle underlying the experiments is that partially molten or molten rock slows the travel of seismic waves and also strongly reflects them.) The depth to the top of the chambers is about two kilometres below the seafloor. The width is more difficult to ascertain, but is probably one to four kilometres. Their thickness seems to be about two to six kilometres based on studies of ophiolites. The chambers have been mapped along the trend of the crest between 9 and 13 N latitude. The top is relatively continuous, but is apparently interrupted by offsets of transform faults and overlapping spreading centres.

Fracture zones and transform faults

Fracture zones

As was noted above, oceanic ridges (and their associated spreading centres) are offset along their trend by fracture zones. These are ridges and valleys on the order of tens of kilometres wide that cut across the crests of the ridges at approximately right angles and offset their trend (Figure 9). Typically, a regional depth offset is present across a fracture zone, owing to the juxtaposition of crust of different ages (and, therefore, depth) across it. In the Atlantic, on the slow spreading Mid-Atlantic Ridge, fracture zones are numerous and occur every 55 kilometres on average along the trend of the ridge. They offset the crest between 5 and 40 kilometres.

Some of the larger fracture zones in the North Atlantic are the Gibbs at 52 N, the Atlantis at 30 N, and the Vema at 11 N. These and others can be followed across both flanks of the ridge for some 3,000 kilometres. The Vema Fracture Zone offsets the Mid-Atlantic Ridge 320 kilometres to the left. It is marked by a sediment-filled valley more than 5 kilometres deep and 10 to 20 kilometres wide and is flanked by mountains 3,500 metres high. Basalts, gabbros, and serpentinized peridotites (i.e., those peridotites that have been altered in varying degrees to serpentine) of the oceanic crust and mantle have been recovered from the mountain flanks.

Fracture zones occur less frequently on the East Pacific Rise, but they offset the ridge by a greater amount. More than a dozen can be found between 20 N and 30 S. Typical offsets are roughly 100 kilometres. Several fracture zones more than 3,000 kilometres long are found off the shore of western North America. These include the Mendocino, Murray, Molokai, and Clarion fracture zones. They are not associated with a ridge crest. Rather, they occur on the west flank of the defunct Pacific-Farallon oceanic ridge. The Farallon Plate has all but disappeared down a subduction zone that extended along the entire coast of California and Baja California until about 25 to 30 million years ago. Subduction now occurs north of the Mendocino Fracture Zone. These fracture zones off western North America were among the first mapped. Menard has traced them almost 10,000 kilometres westward across the Pacific. The continental margin of northern California is displaced to the right where the Mendocino Fracture Zone and its transform portion, the Gorda Escarpment, intersect it.

Transform faults

The portion of a fracture zone between different offset spreading centres constitutes a transform fault. Transform faults also connect spreading centres to subduction zones (deep-sea trenches). Faults of this kind are the only segments of fracture zones that are seismically active. J. Tuzo Wilson recognized this and other features and explained the phenomenon as a transfer of motion from one spreading centre to another. The American geologist W. Jason Morgan, one of the several outstanding pioneers in plate tectonics, recognized that transform faults are zones where opposing lithospheric plates slip past one another. Morgan proposed that opposing plates along an oceanic ridge crest offset by fracture zones are divided by the spreading centres and transform faults. The inactive portions of the fracture zone on the ridge flanks are scars on the ocean floor created in the transform faults.

This theory made a very dramatic prediction: namely, that the direction of motion on the transform faults was opposite to the offsets of the ridge crests. For example, if a ridge crest was offset to the left by a transform fault, implying leftward movement on a fault joining the offset crests, the movement across the transform fault was instead to the right (Figure 9). This is clear when it is realized that the plate boundaries are confined to the spreading centres and transform faults, not to the inactive part of the fracture zone. Seismic studies of earthquakes from transform faults soon revealed that the motion was opposite, as predicted.

Not everywhere in the ocean basins are plate motions exactly parallel to transform faults. In places where a component of opening motion occurs across the transform, volcanic activity results, and the fracture zone is termed a leaky transform fault. South of New Zealand, between it and the Pacific-Antarctic Ridge, a component of shortening is occurring across a transform called the Macquarie Ridge. Here, subduction may be taking place at a slow rate.

Deep-sea trenches

Types

Although the term trench has been applied to many deep, long linear troughs in the ocean floor, the most common and accurate usage relates it to subduction zones. According to plate tectonic theory, subduction zones are locations where a lithospheric plate bearing oceanic crust slides down into the upper mantle under the force of gravity. The result is a topographic depression where the oceanic plate comes in contact with the overriding plate, which may be either oceanic or continental. If the overriding plate is oceanic, an island arc develops (Figure 10). The trench forms an arc in plan view, and islands with explosive volcanoes develop on the overriding plate.

If the overriding plate is continental, a marginal trench forms where the topographic depression appears to follow the outline of the continental margin. Explosive volcanoes are found here too. Both types of subduction zones are associated with large earthquakes that originate at a depth of as much as 700 kilometres. The deep earthquakes below subduction zones occur in a plane that dips 30 or more under the overriding plate. Typical trench depths are 8 to 10 kilometres. The longest trench is the Peru-Chile Trench, which extends some 5,900 kilometres along the west side of South America. Trenches are relatively narrow, usually less than 100 kilometres wide.

The Pacific basin is rimmed by trenches of both marginal and island arc varieties. Marginal trenches bound the west side of Central and South America from the Gulf of California to southern Chile. Although they are deeply buried in sediment, trenches are found along the western North American continental margin from Cape Mendocino (in northern California) to the Canadian border. The Aleutian Trench extends from the northernmost point in the Gulf of Alaska west to the Kamchatka Peninsula in the Soviet Union. It can be classified as a marginal trench in the east but is more properly termed an island arc west of Alaska.

In the western Pacific, the trenches are associated with island arcs. These include the Kuril, Japan, Bonin, Mariana, Ryukyu, and Philippine trenches that extend from Kamchatka to near the equator. A complex pattern of island arcs is found in Indonesia. The major island arc here is the Java Trench extending from northern Australia to the northwestern end of Sumatra in the northeast Indian Ocean. The region of New Guinea and the Solomon Islands includes the New Britain and Solomon trenches, the latter of which joins the New Hebrides Trench directly to the south. East of this area the Tonga and Kermadec trenches extend south from the Fiji Islands to New Zealand.

Two island arcs occur in the Atlantic Ocean. The South Sandwich Trench is located west of the Mid-Atlantic Ridge between South America and Antarctica. The Puerto Rico Trench joins the Lesser Antilles Island arc in the eastern Caribbean. Some seafloor features bear the name trench and are deep linear troughs but are not subduction zones. The Vema Trench on the Mid-Indian Ridge is a fracture zone. The Vityaz Trench northwest of Fiji is an aseismic (inactive) feature of unknown origin. The Diamantina trench (Diamantina Fracture Zone) extends westward from the southwest coast of Australia. It is a rift valley that was formed when Australia separated from Antarctica between 60 and 50 million years ago.

The deepest water on Earth (11,034 metres) is located in the southern end of the Mariana Trench near Guam. A few trenches are partially filled with sediments derived from the bordering continents. The Aleutian Trench is effectively buried east of Kodiak Island in the Gulf of Alaska. Here, the ocean floor is smooth and flat. To the west farther from the sediment supply on Alaska, the trench reaches depths of more than seven kilometres. The Lesser Antilles trench in the eastern Caribbean also is buried by sediments originating from South America.

Structure

Oceanward of trenches the seafloor is usually bulged upward in an outer ridge or rise of up to 1,000 metres relief. This condition is thought to be the elastic response of the oceanic plate bending down into a subduction zone. The landward or island-arc slope of the trench is often interrupted by a submarine ridge, which sometimes breaks the ocean surface, as in the case of the Java Trench. Such a ridge is constructed from deformed sediments scraped off the top of the descending oceanic plate and is termed an accretionary prism. A line of explosive volcanoes, extruding (erupting) a lava that forms the volcanic rock andesite, is found on the overriding plate usually 100 kilometres or so from the trench. In marginal trenches these volcanoes form mountain chains, such as the Cascades in the Pacific Northwest or the great volcanoes of the Andes. In island arcs they form active volcanic island chains, such as the Mariana Islands.

Behind the volcanic line of island arcs are sometimes found young, narrow ocean basins. These basins are bounded on the opposite side by submarine ridges. Such interarc, or backarc, basins are sites of seafloor spreading directly caused by the dynamics of subduction. They originate at the volcanic line, so that the outer bounding submarine ridge, or third arc, represents an older portion of the volcanic line that has spread away. These backarc basins bear many of the features characteristic of oceanic spreading centres. Well-studied examples of these features are found in the Lau Basin of the Tonga arc and also west of the Mariana Islands. The Sea of Japan originated from backarc spreading behind the Japanese arc that began some 30 million years ago. At least two backarc basins have opened behind the Mariana arc, creating seafloor in two phases from about 30 to 17 million years ago in the western Parece Vela Basin and from 5 million years ago in the Mariana Trough next to the islands.

Aseismic ridges

In some oceans the basin floors are crossed by long, linear and mountainous aseismic ridges. The term aseismic distinguishes these ridges from oceanic spreading centres because the former lack earthquakes. Most aseismic ridges are constructed by volcanism from a hot spot and are composed of coalescing volcanoes of various sizes. A hot spot is a magma-generating centre fixed in the Earth's deep mantle and leaves a trail of volcanic outpourings on the seafloor as an oceanic plate travels over it. This form of volcanism is not associated with the volcanism at spreading centres and is distinct from it chemically in that the magma extruded onto the surface has a higher alkali composition. (For additional information on hot spots, see volcano: Volcanism and tectonic activity: Intraplate volcanism.)

The Hawaiian-Emperor chain is the best displayed aseismic ridge. Earthquakes do occur here, but only at the end of the ridge where volcanism is current--in this case, on the island of Hawaii (commonly known as the Big Island) to the southeast end of the island chain. Taking into account the relief of the island of Hawaii above the seafloor, it is the largest volcanic edifice on Earth. The Hawaiian-Emperor chain stretches from the Big Island to the intersection of the Kuril and Aleutian trenches in the northwest Pacific.

There are roughly 18 volcanoes or seamounts (see below) per 1,000 kilometres along the Hawaiian segment and 13 per 1,000 kilometres on the Emperor portion beyond the bend. The Hawaiian Islands are a part of the chain--the young part--that rises above sea level. The Hawaiian-Emperor chain has two main trends: (1) from the Hawaiian Islands west to the Kammu and Yuryaku seamounts (near 32 N, 168 W), the trend of the Hawaiian portion is just west of northwest; and (2) from this point to the Aleutian Trench, the trend of the Emperor segment is north-northwest. The hot spot interpretation infers that this change in trend is due to a change in the direction of Pacific Plate motion, from north-northwest prior to 38 million years ago (the age of the ridge at the change in trend) to west of northwest until the present day. Radiometric dating of rocks from the ridge indicates that it is 70 million years old at its extreme north end.

Other prominent aseismic ridges include the Ninetyeast Ridge and the Chagos-Laccadive Plateau in the Indian Ocean and the Walvis Ridge and Rio Grande Rise in the South Atlantic. The Ninetyeast Ridge is thought to have originated from hot spot volcanic activity now located at the Kerguelen Islands near Antarctica. These islands lie atop the Kerguelen Plateau, which also originated from volcanism at this hot spot. The Ninetyeast Ridge stretches parallel to 90 E longitude in a long, linear chain of seamounts and volcanic ridges from the Andaman Islands in the Bay of Bengal more than 4,500 kilometres to the south where it intersects Broken Ridge at 30 S latitude. Broken Ridge is an aseismic ridge and was once part of the Kerguelen Plateau. It was split away from the plateau as Australia separated from Antarctica.

Core samples of the seafloor along the Ninetyeast Ridge have been retrieved through deep-sea drilling. Analyses of the samples show that the ridge is slightly less than 30 million years old in the south and about 80 million years old in the north. Additionally, sediments on the ridge indicate that parts of it were above sea level while it was being built near a spreading centre. The ridge then subsided as it rode north on the Indian Plate.

The Walvis Ridge and Rio Grande Rise originated from hot spot volcanism now occurring at the islands of Tristan da Cunha 300 kilometres east of the crest of the Mid-Atlantic Ridge. The Walvis Ridge trends northeast from this location to the African margin. The Rio Grande Rise trends roughly southeast from the South American margin toward the Mid-Atlantic Ridge. Both the Walvis Ridge and Rio Grande Rise began forming from the same hot spot near the spreading centre as the South Atlantic was in its initial opening stages 100 to 80 million years ago. The spreading centre shifted west of the hot spot about 80 million years ago, ending construction of the Rio Grande Rise but continuing to build the Walvis Ridge. Volcanic activity has since diminished, resulting in the younger part of the latter ridge being smaller. The findings of ocean drilling on the Rio Grande Rise show that it was once a volcanic island some two kilometres high.

Seamounts, guyots, and abyssal hills

Seamounts are submarine volcanoes with more than 1,000 metres of relief. Aseismic ridges are built by chains of overlapping seamounts. A seamount is akin to a subaerial shield volcano in that it also has gently sloping sides (5 to 15) and is constructed by nonexplosive eruptions of alkaline basalt lavas that are thought to originate from depths of roughly 150 kilometres. About 2,000 seamounts are known; they are most common in the Pacific and on fast-spreading ridges. Like the Hawaiian-Emperor chain, the lines of seamounts and islands trending northwest-southeast in the central and south Pacific (Marshall Islands, Line Islands, Tuamotu Archipelago, and Cook and Austral Islands) may be due to hot spot volcanism. Isolated seamounts also occur, and many of these are located in the far western Pacific. Another group of smaller seamounts is found in the northeastern Pacific.

Flat-topped seamounts are called guyots. They are particularly abundant in the western Pacific and along the Emperor seamount chain. Bottom samples and drill cores of shallow-water sediments and fossils capping guyots have been retrieved. The presence of such geologic materials suggest that guyots are seamounts that have had their peaks planed off by wave action and have since subsided below sea level. The western Pacific guyots are capped by drowned coral atolls and reefs.

These reefs are generally of Late Cretaceous age (about 95 million years old). The cause of the subsidence is attributed to the sinking of the seafloor as it moves down the flanks of an oceanic ridge. However, the reason for the demise of the coral reefs on the Cretaceous guyots is less clear. Under normal conditions, coral growth can easily keep up with sinking due to seafloor spreading. The Cretaceous guyots may have resulted from the northward drift of seamounts and reefs on the Pacific Plate away from the tropical zone of favourable growth. Another hypothesis is that the reefs were killed by unusually anoxic (oxygen-depleted) conditions that developed suddenly, a situation possibly related to intense seafloor volcanism in the Pacific at this time.

Abyssal hills are low-relief (less than 1 kilometre) features usually 1 to 10 kilometres wide and elongate parallel to spreading centres or to marine magnetic anomalies located in the vicinity of the latter. The tops of the hills are often flat, in which case they have steep sides. Gently sloping sides, however, are equally common. Abyssal hills are extremely numerous, so much so that Menard declared them "the most widespread physiographic forms of the face of the earth." Abyssal hills are most common in the Pacific basin, where they cover 80 to 85 percent of the seafloor. Because they cover the entire flanks and crests of the oceanic ridges, such hills are thought to form during crustal accretion at spreading centres. They are commonly associated with intermediate- and fast-spreading ridges. On slow-spreading ridges, such as the Mid-Atlantic, the topographic features are much larger and have steeper sides. Bottom-sampling and seismic reflection studies reveal that abyssal hills are relief features on the top of the oceanic crust; they are not constructed from ocean-bottom sediments. In areas such as the abyssal plains (see below), abyssal hills are buried by sediments.

Apparently the hills are constructed by two processes: volcanism and block faulting. The relative contribution of each may depend on the spreading rate. At slower rates, faulting of the oceanic crust is a dominant factor in forming the relief, and the relief of the hills is greater as the rate is slower. At the crest of a spreading centre, volcanism in the neovolcanic zone initiates the construction of volcanic hills. The zone of active faulting is where they form or are modified by block faulting. The existence of discrete and separate volcanic hills indicates that volcanism at a spreading centre is episodic.

Deep-sea sediments

The ocean basin floor is everywhere covered by sediments of different types and origins. The only exception are the crests of the spreading centres where new ocean floor has not existed long enough to accumulate a sediment cover. Sediment thickness in the oceans averages about 450 metres. The sediment cover in the Pacific basin ranges from 300 to 600 metres thick, and that in the Atlantic is about 1,000 metres. Generally, the thickness of sediment on the oceanic crust increases with the age of the crust. Oceanic crust adjacent to the continents can be deeply buried by several kilometres of sediment. Deep-sea sediments can reveal much about the last 200 million years of Earth history, including seafloor spreading, the history of ocean life, the behaviour of the Earth's magnetic field, and the changes in the ocean currents and climate.

The study of ocean sediments has been accomplished by several means. Bottom samplers, such as dredges and cores up to 30 metres long, have been lowered from ships by wire to retrieve samples of the upper sediment layers. Deep-sea drilling has retrieved core samples from the entire sediment layer in several hundred locations in the ocean basins. The seismic reflection method has been used to map the thickness of sediments in many parts of the oceans. Besides thickness, seismic reflection data can often reveal sediment type and the processes of sedimentation. (For more information on the equipment and techniques used by investigators to study deep-sea sediments, see undersea exploration.)





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