(Greek, ge, “Earth”; -logia, “knowledge of”), field
of science concerned with the origin of the planet Earth,
its history, its shape, the materials forming it, and the processes that
are acting and have acted on it. It is one of several related subjects
commonly grouped as the Earth sciences, or geoscience, and geologists are
Earth scientists concerned primarily with rocks and derivative materials
that make up the outer part of the Earth. To understand these materials,
geologists make use of knowledge from other fields, such as physics,
chemistry, and biology; thus, geological fields such as geochemistry,
geochronology (which uses dating
methods), and palaeontology,
now important disciplines in their own right, incorporate other sciences,
enabling geologists to understand better the working of Earth processes
Although each Earth science has a particular focus, they all frequently overlap with geology. Thus, the study of the Earth's waters in relation to geological processes involves knowledge of hydrology and oceanography, and the measurement and mapping of the Earth's surface forms involve knowledge of cartography (maps) and geodetics (surveying). Clues to the origin of the Earth are also sought by the study of extraterrestrial bodies, especially the Moon, Mars, and Venus. Originally limited to Earth-based telescopic observation, such studies were given a powerful impetus by the space exploration that began in the 1960s.
As a major science, geology not only involves the study of landforms and other surface features of the Earth but also is concerned with the structure and inner parts of the planet. Such knowledge is of basic scientific interest, but it is also placed at the service of humanity. Thus, the focus of applied geology is on the search for useful minerals within the Earth, the identification of geologically stable environments for human constructions, and the foreknowledge of natural hazards associated with the geodynamic forces described below.
Records of the Earth's geological history are obtained from four major types of rock, each produced by a different kind of crustal activity: (1) Erosion and sedimentation produce successive layers of sedimentary rocks; (2) molten rock, pushed upwards from deep-lying magma chambers, cools and forms surface rocks or the upper part of the Earth's crust, providing records of volcanic activity; (3) geological structures developed from pre-existing rocks form records of past deformations; and (4) records of plutonism, or magmatic activity deep within the Earth, are supplied by studying the deep-lying metamorphic and granitic rocks. A time chart of the Earth's geological events is developed by dating these past geological episodes by using various radiometric and relativistic methods.
The divisions of the resulting geological timescale are based primarily on changes in fossil forms found from one stratum to the next. The first five-sixths of the estimated 4 to 6 billion years of the Earth's history, however, is recorded in rocks that contain almost no fossils; an adequate fossil record for stratigraphic correlation exists only for the past 600 million years, beginning at the time when Lower Cambrian deposits were laid. Scientists therefore conveniently separate the Earth's vast span of existence into two major time divisions: the Cryptozoic (hidden life), or Precambrian; and the Phanerozoic (obvious life), or Cambrian, and the more recent time divisions.
Fundamental differences in the fossil assemblages of early, middle, and late Phanerozoic rocks gave rise to the designation of three great eras: the Palaeozoic (ancient life), the Mesozoic (middle life), and the Cenozoic (recent life). The principal divisions of time in each of these eras constitute geological periods, during which rocks of corresponding systems were laid down throughout the world. The periods are generally named after the regions where rocks of the period in question are well exposed; for example, the Permian period is named after the European province of Perm in Russia. Some periods are named instead after typical deposits, such as the Carboniferous period for its coal beds; or for ancient peoples, such as the Ordovician and Silurian periods, named after the Ordovices and Silures of ancient Britain and Wales. The Cenozoic's Tertiary and Quaternary periods are further divided into epochs and ages, from the Palaeocene to the Holocene, or most recent time. Besides these time periods, geologists also use time-rock divisions called systems; such systems are similarly divided into series and, sometimes, still smaller units called stages.
The discovery of radioactivity enabled 20th-century geologists to devise new dating methods and thereby assign absolute ages, in millions of years, to the divisions of the timescale. The following is an overview of these divisions and the life forms on which they are based. The scantier fossil record of Precambrian times, as stated, does not permit similar clear divisions.
(570-510 million years ago). An explosion of life populated the seas, but land areas remained barren. Animal life was wholly invertebrate, and the most common animals were arthropods called trilobites (now extinct), with species numbering in the thousands. Multiple collisions between the Earth's crustal plates gave rise to the first supercontinent, known as Gondwanaland.
(510-439 million years ago). The predecessor of today's Atlantic Ocean began to shrink as the continents of that time drifted closer together. Trilobites were still abundant; important groups making their first appearance included the corals, crinoids, bryozoans, and pelecypods. Armoured, jawless fishes—the oldest known vertebrates—made their appearance as well; their fossils are found in ancient estuary beds in North America.
(439-408.5 million years ago). Life ventured on to land in the form of simple plants called psilophytes, with a vascular system for circulating water, and scorpion-like animals akin to now extinct marine arthropods called eurypterids. Trilobites decreased in number and variety, but the seas teemed with reef corals, cephalopods, and jawed fishes.
(408.5-362.5 million years ago). This period is also known as the age of fishes, because of their abundant fossils in Devonian rocks. Fishes had also become adapted to fresh water as well as to salt water. They included a diversity of both jawless and jawed armoured fishes, early sharks, and bony fishes, from the last of which amphibians evolved. (One subdivision of the sharks of that time is still extant.) On land areas, giant ferns were widespread.
(362.5-290 million years ago). Trilobites were almost extinct, but corals, crinoids, and brachiopods were abundant, as were all groups of the molluscs. Warm, humid climates fostered lush forests in swamplands, where the major coal beds of today were formed. Dominant plants included tree-like lycopods, horsetails, ferns, and extinct plants called pteridosperms, or seed ferns. Amphibians spread and gave rise to reptiles, the first vertebrates to live entirely on land; and winged insects such as the dragonfly appeared.
(290-245 million years ago). The Earth's land areas became welded into a single land mass that geologists call Pangaea, and in the North American region the Appalachians were formed. Cycad-like plants and true conifers appeared in the northern hemisphere, replacing the coal forests. Environmental changes resulting from the redistribution of land and sea triggered the greatest mass extinction of all time. Trilobites and many fishes and corals died out as the Palaeozoic era came to an end.
(245-208 million years ago). The beginning of the Mesozoic era was marked by the reappearance of Gondwanaland, as Pangaea split apart into northern (Laurasia) and southern (Gondwanaland) supercontinents. Forms of life changed considerably in the Mesozoic, known as the age of reptiles. New pteridosperm families appeared, and conifers and cycads became major floral groups, along with ginkgos and other genera. Such reptiles as dinosaurs and turtles appeared, as did mammals.
(208-145.6 million years ago). As Gondwanaland rifted apart, the North Atlantic Ocean widened and the South Atlantic was born. Giant dinosaurs ruled on land, while marine reptiles such as ichthyosaurs and plesiosaurs increased in number. Primitive birds appeared, and modern reef-building corals grew in coastal shallows. Crab-like and lobster-like animals evolved among the arthropods.
(145.6-65 million years ago). Dinosaurs flourished and evolved into highly specialized forms, but they abruptly disappeared at the end of the period, along with many other kinds of life. (Theories to account for these mass extinctions are currently of great scientific interest.) The floral changes that took place in the Cretaceous were the most marked of all alterations in the organic world known to have occurred in the history of the Earth. Gymnosperms were widespread, but in the later part of the period angiosperms (flowering plants) appeared.
(65-1.64 million years ago). In the Tertiary, North America's land link to Europe was broken, but its ties to South America were forged towards the end of the period. During Cenozoic times, life forms both on land and in the sea became more like those of today. Grasses became more prominent, leading to marked changes in the dentition of plant-eating animals. With most of the dominant reptile forms having vanished at the end of the Cretaceous, the Cenozoic became the age of mammals. Thus, in the Eocene epoch, new mammal groups developed such as small, horse-like animals; rhinoceroses; tapirs; ruminants; whales; and the ancestors of elephants. Members of the cat and dog families appeared in the Oligocene epoch, as did species of monkeys. In Miocene times, marsupials were numerous, and anthropoid (human-like) apes first appeared. Placental mammals reached their zenith, in numbers and variety of species, in the Pliocene, extending into the Quaternary period.
(1.64 million years ago to present). Intermittent continental ice sheets covered much of the northern hemisphere. Fossil remains show that many primitive pre-human types existed in south-central Africa, China, and Java by Lower and middle Pleistocene times; but modern humans (Homo sapiens) did not appear until the later Pleistocene. Late in the period, humans crossed over into the New World by means of the Bering land bridge. The ice sheets finally retreated, and the modern age began.
The discipline of geology deals with the history of the Earth, including the history of life, and covers all physical processes at work on the surface and in the crust of the Earth. Broadly, geology thereby includes studies of interactions between the Earth's rocks, soils, waters, atmosphere, and life forms. In practice, geologists specialize in a branch of either physical or historical geology. Physical geology, including fields such as geophysics, petrology, and mineralogy, focuses on the processes and forces that shape the exterior of the Earth and operate within the interior, while historical geology is concerned primarily with the evolution of the Earth's surface and its life forms through time, and involves investigations into palaeontology, stratigraphy, palaeogeography, and geochronology.
The aim of geophysics is to deduce the physical properties of the Earth, along with its internal composition, from various physical phenomena. For example, geophysicists study the geomagnetic field, palaeomagnetism in rocks and soils, heat-flow phenomena within the Earth, the force of gravity, and the propagation of seismic waves (seismology). As a subfield, applied geophysics investigates relatively small-scale and shallow structural features within the Earth's crust, such as salt domes, synclines, and faults, for human-related purposes. Exploration geophysics also combines physics with geological information to solve practical problems related to searching for oil and gas, locating water-bearing strata, detecting new metal-ore deposits, and various forms of civil engineering.
Geochemistry is concerned with the chemistry of the Earth as a whole, but the subject is further divided into such areas as sedimentary geochemistry, organic geochemistry, the new field of environmental geochemistry, and several others. Of great interest for the geochemist are the origin and evolution of the Earth's elements and the major classes of rocks and minerals. The geochemist specifically studies the distribution and amounts of the chemical elements in minerals, rocks, soils, life forms, water, and the atmosphere. Knowledge of the circulation of the elements in nature—for example, the carbon, nitrogen, phosphorus, and sulphur geochemical cycles—is of practical significance, as is the study of the distribution and abundance of isotopes and of their stability in nature. Exploration geochemistry, or geochemical prospecting, is the practical application of theoretical geochemical principles to mineral exploration.
Petrology deals with the origin, occurrence, structure, and history of rocks, particularly igneous and metamorphic rocks. (The study of the petrology of sediments and sedimentary rocks is known more particularly as sedimentary petrology.) Petrography, a related discipline, is concerned with the description and characteristics of crystalline rocks as determined by microscopic examination under polarized light. Petrologists study changes that occur spontaneously in rock masses when magmas solidify, when solid rocks melt partly or wholly, or when sediments undergo chemical or physical transformation. Workers in this field are specifically concerned with the crystallization of minerals and solidification of glass from molten materials at high temperatures (igneous processes), the recrystallization of minerals at high temperatures without the intervention of a molten phase (metamorphic processes), the exchange of ions between minerals of solid rocks and migrating fluid phases (metasomatic and diagenetic processes), and sedimentary processes including weathering, transport, and deposition.
The science of mineralogy deals with minerals in the Earth's crust and those found outside the Earth, such as lunar samples or meteorites. (Crystallography, a branch of mineralogy, involves the study of the external form and internal structure of natural and artificial crystals.) Mineralogists study the formation, occurrence, chemical and physical properties, composition, and classification of minerals. Determinative mineralogy is the science of identifying a mineral from its physical and chemical properties. Economic mineralogy focuses on the geological processes responsible for the formation of ore minerals, especially those with industrial or strategic importance.
Originally concerned with analysing the deformation of sedimentary strata, structural geologists now study the distortions of rocks in general. Commonly investigated structural forms or shapes lead to a comparison of observed features and, eventually, to the classification of related types. Comparative structural geology, concerned with large external features, contrasts with theoretical and experimental approaches, which employ the microscopic study of mineral grains in deformed rocks. Oil and coal geologists must employ structural geology in their daily work, especially in petroleum exploration, where the detection of structural traps that can hold petroleum is an important source of information to the geologists.
Also referred to as sedimentary geology, this study of sedimentary deposits and their origins deals with ancient and recent marine and terrestrial deposits and their faunas, floras, minerals, textures, and evolution in time and space. Sedimentologists study numerous intricate features of soft and hard rocks in their natural sequences, with the goal of restructuring the Earth's earlier environments in their stratigraphic and tectonic frameworks. The study of sedimentary rocks includes data and methods borrowed from other branches of geology, such as stratigraphy, marine geology, geochemistry, mineralogy, and environmental geology.
Palaeontology, the study of prehistoric life, deals with fossil animals (palaeozoology) and fossil plants (palaeobotany) in relation to existing plants and animals. Investigation of microscopic fossils (micropalaeontology) involves techniques different from that of larger specimens. Fossils, the remains of or indications of life in the geological past, as preserved by natural means in the Earth's crust, are the chief data of palaeontology. Palaeontography is the formal, systematic description of fossils (plants and animals), and invertebrate palaeontology is frequently regarded as a separate subdiscipline from vertebrate palaeontology.
Meaning “form and development of the Earth”, geomorphology involves the attempt to furnish a working model for the outer part of the Earth. Geomorphologists explain Earth-surface morphologies in terms of established principles related to glacial action, fluvial processes, wind transport and deposition, and erosion and weathering. Major subfields focus on tectonic influences on landforms (morphotectonics), the influence of climate on morphogenetic processes and associated landform assemblages (climatic geomorphology), and the measurement and statistical analysis of landform data (quantitative geomorphology).
This major branch of geology is geared to the analysis, exploration, and exploitation of geological materials of use to humans, such as fuels, metals and non-metallic minerals, water, and geothermal energy. Kindred fields include the science of locating economic or strategic minerals (exploration geology), processing ores (metallurgy), and the practical application of geological theories to mining (mining geology).
The rifting of the great lithospheric plates, the continual drifting of continental crust, and the expansion of oceanic crust from midoceanic spreading centres all set deep-seated dynamic forces into action. Diastrophism is a general term for all crustal movements produced by endogenic Earth forces that produce ocean basins, continents, plateaux, and mountains. The so-called geotectonic cycle relates these larger structural features to gross crustal movements and to the kinds of rocks that form various stages of their development.
Orogenesis, or mountain building, tends to be a localized process that distorts pre-existing strata. Epeirogeny affects large parts of the continents and oceans, primarily through upwards or downward movements, and produces plateaux and basins. Slow, gradual displacement of crustal units particularly affects cratons, or stable regions of the crust. Rock fractures and displacements that range in scale from a few centimetres to several kilometres are called faults. Faulting is commonly associated with plate boundaries that glide past one another—for example, the San Andreas Fault—and with sites at which continents are rifted apart, such as the Eastern Rift Valley, in East Africa. Geysers and hot springs, like volcanoes, are often found in tectonically unstable areas.
Volcanoes are produced by outpouring of lavas from deep within the Earth. The Columbia plateau of the western United States is overlaid by volcanic basalts that are more than 3,000 m (10,000 ft) thick and cover 52,000 sq km (20,000 sq mi). Such plateau basalts are derived from fissure volcanoes. Other kinds of volcanoes include shield volcanoes, which are broad and convex in profile, such as those forming the Hawaiian Islands, and strato volcanoes, such as Mount Fuji or Mount St Helens, which are composed of interleaved layers of different materials.
Earthquakes are caused by the abrupt release of slowly accumulated strain by faulting or volcanic activity, or both. Sudden motion at the Earth's surface is a manifestation of endogenic processes that can wreak havoc through seismic sea waves (tsunamis), landslides, surface collapse or subsidence, and related phenomena.
Weathering is a collective name for a group of processes responsible for the disintegration and decomposition of rock in place. Physical, chemical, or biological weathering is a prerequisite to erosion. Mass wasting (the gravitative transfer of material downslope) involves creep and such actions as earthflow, debris avalanches, and landslides. Hydraulic action is the sweeping away of loose material by running water; the companion process performed by wind is known as deflation. The action of ice moving over a land surface is sometimes called scouring; plucking and gouging are erosional processes restricted to glaciers. Aggradation, or the accumulation of sediments, contributes to the general levelling of the Earth's surface as a result of deposition, which occurs when the medium transporting the sediments loses power.
"Geology," Microsoft® Encarta® Online