Geology is the scientific study of the Earth, including its composition, structure, physical properties, and history.

The term geology is broadly inclusive and is often regarded as embracing all of the geologic sciences.

Geology is commonly divided into a number of subdisciplines: (1) those concerned with the chemical makeup of the solid Earth, which include the study of minerals (mineralogy) and rocks (petrology); (2) those having to do with the structure of the solid Earth, as, for example, the study of the relationships of rocks and geologic features in general (structural geology) and the science of volcanic phenomena (volcanology); (3) those concerned with landforms and the processes that produce them (geomorphology and glacial geology); (4) those dealing with geologic history, including the study of fossils and the fossil record (paleontology), the development of sedimentary strata (stratigraphy), and the evolution of planetary bodies and their satellites (astrogeology); and (5) economic geology and its various branches--e.g., mining geology and petroleum geology.

Some major fields closely allied to geology are geodesy, geophysics, and geochemistry.

The various subdisciplines of geology not only intergrade with one another but also with other branches of the Earth sciences and with such fields as physics, chemistry, biology, and mathematics. Paleontology, for instance, at times requires the use of organic chemistry, physical chemistry, and statistics.

The statistical analysis of data also is an important facet of geomorphology and stratigraphy, as is the use of mathematical models.

Besides providing a better understanding of the Earth's evolution and its present features, geology serves society in a variety of practical ways.

Exploration for deposits of commercially valuable minerals is broadly guided by geologic principles and conducted with geophysical and geochemical methods.

The search for fossil fuels (coal, oil, and natural gas) is strongly influenced by those aspects of geology dealing with the deposition and deformation of sedimentary rocks and with the flow of underground fluids. Significant, too, is the contribution of seismological research, whose findings have enabled engineers to design structures that are better able to withstand earthquakes.

Environmental Geology - the field concerned with applying the findings of geologic research to the problems of land use and civil engineering. It is closely allied with urban geology and deals with the impact of human activities on the physical environment (e.g., contamination of water resources by sewage and toxic chemical wastes). Other important concerns of environmental geology include reclaiming mined lands; identifying geologically stable sites for constructing buildings, nuclear power plants, and other facilities; and locating sources of building materials, such as sand and gravel.

Engineering geology, also called GEOLOGICAL ENGINEERING, the scientific discipline concerned with the application of geological knowledge to engineering problems--e.g., to reservoir design and location, determination of slope stability for construction purposes, and determination of earthquake, flood, or subsidence danger in areas considered for roads, pipelines, or other engineering works.

Economic geology - scientific discipline concerned with the distribution of mineral deposits, the economic considerations involved in their recovery, and an assessment of the reserves available.

Economic geology deals with metal ores, fossil fuels (e.g., petroleum, natural gas, and coal), and other materials of commercial value, such as salt, gypsum, and building stone. It applies the principles and methods of various other fields of the geologic sciences, most notably geophysics, structural geology, and stratigraphy. Its chief objective is to guide the exploration for mineral resources and help determine which deposits are economically worthwhile to mine. Specialists in economic geology often assist in the extraction of the mineral commodities as well.

Marine geology - also called Geologic Oceanography, scientific discipline that is concerned with all geological aspects of the continental shelves and slopes and the ocean basins. In practice, the principal focus of marine geology has been on marine sedimentation and on the interpretation of the many bottom samples that have been obtained through the years. The advent of the concept of seafloor spreading in the 1960s, however, broadened the scope of marine geology considerably. Many investigations of midoceanic ridges, remanent magnetism of rocks on the seafloor, geochemical analyses of deep brine pools, and of seafloor spreading and continental drift may be considered within the general realm of marine geology.

Structural geology - the scientific discipline that is concerned with rock deformation on both a large and a small scale. Its scope of study is vast, ranging from submicroscopic lattice defects in crystals to fault structures and fold systems of the Earth's crust.

The methods of structural geology are nearly as diverse as those of the geologic sciences as a whole. Small-scale structural features may be studied using the same general techniques that are employed in petrology, in which sections of rock mounted on glass slides are ground very thin and are then examined with polarizing microscopes.

On a larger scale, the techniques of field geology are used. These include plotting the orientation of such structural features as faults, joints, cleavage, and small folds. In most cases, the objective is to interpret the structure beneath the surface by using information available at the surface. Where mountains, continents, ocean basins, and other large-scale features are involved, the methods employed are chiefly those of geophysics and include the use of seismological, magnetic, and gravitational techniques.

Furthermore, since the processes that cause rocks to deform can rarely be observed directly, it is necessary to study them by means of computer models in which they are represented mathematically.