Interval of time occupied by the Earth's geologic history, extending from circa 3.9 billion years ago (corresponding to the age of the oldest known rocks) to the present day. It is, in effect, the part of the Earth's history that is recorded in rock strata. The geologic time scale is classified in nested intervals distinguished by characteristic geologic and biologic features. From longest to shortest duration, the intervals are eon, era, period, and epoch.
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Scientific discipline concerned with rock deformation on both small and large scales. Its scope ranges from submicroscopic lattice defects in crystals to fault structures and fold systems of the Earth's crust. Depending on the scale, the general techniques used are the same as those used in petrology, field geology, and geophysics. Furthermore, since the processes that cause rocks to deform can rarely be observed directly, computer models are also used.
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Scientific discipline concerned with all geologic aspects of the continental shelves and slopes and the ocean basins. Marine geology originally focused on marine sedimentation and the interpretation of bottom samples. The advent of the concept of seafloor spreading, however, broadened its scope. Many investigations of the oceanic ridge system, the magnetism of rocks on the seafloor, geochemical analyses of deep brine pools, and seafloor spreading and continental drift may be considered within the general realm of marine geology.
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Scientific study of the Earth, including its composition, structure, physical properties, and history. Geology is commonly divided into subdisciplines concerned with the chemical makeup of the Earth, including the study of minerals (mineralogy) and rocks (petrology); the structure of the Earth (structural geology) and volcanic phenomena (volcanology); landforms and the processes that produce them (geomorphology and glaciology); geologic history, including the study of fossils (paleontology), the development of sedimentary strata (stratigraphy), and the evolution of planetary bodies and their satellites (astrogeology); and economic geology and its various branches, such as mining geology and petroleum geology. Some major fields closely allied to geology are geodesy, geophysics, and geochemistry. Seealso environmental geology.
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Scientific 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. 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.
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Scientific discipline concerned with the application of geologic knowledge to engineering problems such as 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, bridges, dams, or other engineering works.
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Scientific discipline concerned with the distribution of mineral deposits, the economic considerations involved in their recovery, and assessment of the reserves available. Economic geology deals with metal ores, fossil fuels, and other materials of commercial value, such as salt, gypsum, and building stone. It applies the principles and methods of various other fields, especially geophysics, structural geology, and stratigraphy.
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Areas identified as head include deposits of aeolian origin such as blown sand and loess, slope deposits such as gelifluctates and solifluctates as well as recently eroded soil material, called colluvium.
With geologists becoming more interested in studying the near surface environment and its related processes, the term head is becoming more and more obsolete.