Geology (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, geophysics, geochronology (see Dating Methods), and paleontology, now important disciplines in their own right, incorporate other sciences, enabling geologists to understand better the working of earth processes through time.

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 (see Water) and oceanography (see Ocean and Oceanography), and the measurement and mapping of the earth's surface forms involve knowledge of cartography (see Map) and geodetics (see 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 (see Space Exploration: Space Programs—Unmanned).

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.

History of Geological Thought

Ancient peoples considered many geological features and processes as the work of gods and goddesses, and they regarded the natural environment with fear and wonder as dangerous and mysterious. Thus, the ancient Sumerians, Babylonians, and other peoples, although they made remarkable discoveries in mathematics and astronomy, went astray in geological inquiries by simply personifying geological processes. Irish legends, for example, suggest that giants were responsible for certain natural phenomena such as a weathered formation of basaltic columns, now known as the Giant's Causeway. Such mythology was also popular among the civilizations of the New World; for example, furrows on the flanks of what came to be known as Devils Tower in Wyoming were thought by Native Americans to be the claw marks of a giant bear.

Ancient to Medieval Times

wpe10.gif (79950 bytes)

Similarly, in ancient Greece and Rome, many of the gods were identified with geological processes. For example, volcanic eruptions in Sicily were ascribed to the local Roman volcano god, Vulcan. The Greek philosopher Thales of Miletus, in the 6th century BC, has been credited with making the first clean break with this traditional mythologizing. He regarded geological processes as natural and orderly events that could be studied in the light of reason, rather than as supernatural interventions. The Greek philosopher Democritus advanced this naturalistic philosophy with the theory that all matter is composed of atoms. Building on his atomic theory, he offered rational explanations of all manner of geologic processes: earthquakes, volcanic eruptions, the hydrologic cycle, erosion, and sedimentation. His teachings, as expounded by the Roman poet Lucretius in his poem On the Nature of Things, are readily available in English translation. Aristotle, the most influential natural philosopher of ancient times, recognized in the 4th century BC that fossil seashells embedded in sedimentary rock strata were similar to shells found along the beach. From this observation he surmised that the relative positions of land and sea must have fluctuated in the past, and he also realized that such changes would require great lengths of time. Theophrastus, Aristotle's pupil, contributed to geological thought by writing the first book on mineralogy. Called Concerning Stones, it formed the basis of most mineralogies throughout the Middle Ages and even later.

The Renaissance

The Renaissance was truly a new beginning for the earth sciences; people began to observe geological processes much as the ancient Greeks had done. Were Leonardo da Vinci not better known as a painter and engineer, he might still be recognized as a pioneer of natural science. He realized, for example, that landscapes are sculptured by erosive processes and that fossil shells in Apennine limestones were the remains of marine organisms that had lived on the floor of a former sea that must have extended over Italy.

Following Leonardo, the French natural philosopher Bernard Palissy wrote on the nature and scientific study of soils, groundwater, and fossils. The classic works on minerals written in this period, however, were by Georgius Agricola, a German mining expert who published De Re Metallica (1556) and De Natura Fossilium (1546). Agricola recorded the most recent developments in geology, mineralogy, mining, and metallurgy at that time, and his works were widely translated.

17th Century

Niels Stensen, a Dane—better known by the Latinized version of his name, Nicolaus Steno—stands prominent among 17th-century geoscientists. In 1669 he showed that the interfacial angles of quartz crystals were constant, regardless of the shape and size of the crystals, and that by extension, the structure of other crystal species should also be constant. Thus, by drawing attention to the significance of crystal form, Steno laid the foundation for the science of crystallography. Steno's observations on the nature of rock strata led him to formulate the law of superposition, one of the basic principles of stratigraphy (see below).

18th and 19th Centuries

Geological thought during the 18th century was characterized by debates between contrasting schools. Plutonists, who proposed that the earth's rocks were all originally solidified from a molten mass and later altered by other processes, were opposed by Neptunists, whose leading exponent was the German geologist Abraham Gottlob Werner. Werner hypothesized that the earth's crust is a series of layers derived from mechanical and chemical sedimentary deposits laid down by a vast ocean, in a regular sequence, like the layers of an onion. By contrast, the Scottish geologist James Hutton and the Plutonists, as his followers were called, distinguished sedimentary rocks from intrusive rocks of volcanic origin.

In 1785, Hutton introduced the concept of uniformitarianism, according to which the history of the earth can be interpreted solely on the basis of everyday geologic processes familiar to modern observers. He reasoned that most such processes, operating as slowly as they do today, would have taken millions of years to produce the modern landscape. This theory set him at odds with theologic opinions of the day, which held that the earth was barely 4000 years old. Hutton's antagonists, led by the French naturalist Georges Cuvier, believed that abrupt, violent changes—natural catastrophes such as floods and earthquakes—were responsible for the earth's geologic features. For this reason, they were known as catastrophists.

The debate that raged between these two schools began to tip in favor of the uniformitarians with the publication of Charles Lyell's Principles of Geology (1830-33). Born in 1797, the year Hutton died, Lyell became a major influence on modern geologic theory, courageously attacking theological prejudices concerning the age of the earth and rejecting attempts to interpret geology in the light of scripture.

In the American colonies, the noted surveyor, draftsman, and mapmaker Lewis Evans had already made remarkable contributions to American geological knowledge before Lyell's influential work. For Evans, river erosion and fluvial deposition were self-evident processes that had been at work in the past. Through the work of Evans, in addition, the concept of isostasy—that the density of the earth's crust decreases as its thickness increases—also appeared for the first time in American geological writings.

Besides Lyell's work, the primary 19th-century developments in geology were the following: new reactions to traditional geological concepts, the fostering of glacial theory, the beginnings of American geomorphology, theories of mountain building, the advent of marine exploration, and the development of the so-called structuralist school (see below). Geological explorations, mainly in the American West, were major scientific events.

Glacial Theory

The glacial theory drew on the work of Lyell and many others. First propounded about 1840 and later universally accepted, the theory states that glacial drift had been deposited by glaciers and ice sheets moving slowly from higher to lower latitudes during the Pleistocene epoch (see Quaternary Period). The Swiss naturalist Horace Bénédict de Saussure had been among the first to credit glaciers in the Alps with the power to move large boulders. The Swiss-American naturalist Louis Agassiz correctly interpreted the environmental impact of this erosive and transporting agent and, with his colleagues, accumulated diverse forms of evidence that supported concepts of glacial advance and retreat for continental and mountain glaciers.


Advances in stratigraphy were made by the English geologist William Smith, who traced out the strata of England and represented them on a geological map that remains substantially unchanged today. Smith first traced strata over relatively short distances; he then correlated stratigraphic units of the same age but of different rock content. After the development of evolutionary theory by Charles Darwin later in the 19th century, this knowledge led to the principle of faunal succession. According to this principle the life in each period of earth history is unique for that specific period, fossil remains provide a basis for recognizing contemporaneous deposits around the world, and fossils can be used to assemble scattered fragments of the record into a chronological sequence known as the geologic time scale (see below).

Cycles of Geologic Activity

Many 19th-century geologists came to understand the earth as a thermally and dynamically active planet, internally as well as externally. Those known as structuralists or neocatastrophists believed that catastrophic or structural upheavals accounted for the formation of the earth's topographic features. Thus, the English geologist William Buckland and his followers postulated frequent changes of sea level and upheaval of landmasses to explain geological successions and breaks, or unconformities, in stratigraphic sequences. Hutton, by contrast, regarded earth history in terms of overlapping, successive cycles of geological activity. He referred to long belts of folded rocks, which were taken to be the result of a variety of such cycles, as orogenic belts, and he referred to mountain formation through the processes of folding and uplift as orogenesis. Other geologists later supported these orogenic concepts, and they distinguished four major orogenic periods: the Huronian (end of the Precambrian time); the Caledonian (Lower Paleozoic era); the Hercynian, or Variscan (end of the Paleozoic era); and the Alpine (end of the Cretaceous period).


Exploration of the western U.S. in the 19th century provided a whole new body of geological data that had an immediate effect on geomorphological theory. Early survey parties to the American West, under the auspices of the government, were headed by such figures as Clarence King, Ferdinand Vandeever Hayden, and John Wesley Powell, among others. Grove Karl Gilbert, the most outstanding of Powell's associates, recognized a form of topography caused by faults in the earth's crust, and he deduced a system of laws governing landform development.

20th Century

Technological advances made in the 20th century provided new, sophisticated tools for geologists, enabling them to measure and monitor earth processes with a precision previously unattainable. In terms of basic theory, the field of geology underwent a major revolution with the introduction and development of the plate tectonics hypothesis, that the earth's crust is divided into a number of plates that move about, collide, and separate over geologic time. The great crustal plates of the earth are now understood to begin at midocean and other ridges, or spreading centers, and to move toward submarine trenches, or subduction zones, where the crustal material again descends. The places on the earth where major earthquakes occur tend to outline the boundaries between these crustal plates, suggesting that seismic activity can be interpreted as the result of the horizontal movements of the crustal plates.

This hypothesis is related to the concept of continental drift, first proposed in modern form by the German geophysicist Alfred Wegener in 1912. The hypothesis gained support later in the century as deep-sea exploration provided evidence for seafloor spreading—the outflow of new crustal material along midocean ridges. The concept of plate tectonics has since been related to the origin and growth of continents, the generation of continental as well as oceanic crust, and the nature of the earth's underlying layers and their evolution through time. Thus, 20th-century geologists have developed a theory that unifies many of the major processes that have shaped the earth and its landforms.

The Geologic Time Scale

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 (see Sedimentary Rock); (2) molten rock, pushed upward 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 preexisting 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 geologic time scale 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 Paleozoic (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 worldwide. The periods generally are named for the regions where rocks of the period in question are well exposed; for example, the Permian period is named for the European province of Perm’ in Russia. Some periods are named instead for 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 Paleocene 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 time scale. 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. For a description of the earth's formation and earlier history, along with a discussion of the origin of life on earth, see Earth; Evolution; Life.

Cambrian Period

(570-500 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.

Ordovician Period

(500-430 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. Armored, jawless fishes—the oldest known vertebrates—made their appearance as well; their fossils are found in ancient estuary beds in North America.

Silurian Period

(430-395 million years ago). Life ventured onto land in the form of simple plants called psilophytes, with a vascular system for circulating water, and scorpionlike 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.

Devonian Period

(395-345 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 armored 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.

Carboniferous Period

(345-280 million years ago). Trilobites were almost extinct, but corals, crinoids, and brachiopods were abundant, as were all groups of the mollusks. Warm, humid climates fostered lush forests in swamplands, where the major coal beds of today were formed. Dominant plants included treelike lycopods (see Lycopsid), 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.

Permian Period

(280-225 million years ago). The earth's land areas became welded into a single landmass that geologists call Pangaea, and in the North American region the Appalachians were formed. Cycadlike 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 Paleozoic era came to an end.

Triassic Period

(225-195 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.

Jurassic Period

(195-136 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. Crablike and lobsterlike animals evolved among the arthropods.

Cretaceous Period

(136-65 million years ago). The Rocky Mountains began to rise in North America. 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.

Tertiary Period

(65-2.5 million years ago). In the Tertiary, North America's land link to Europe was broken, but its ties to South America were forged toward 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, horselike 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 (humanlike) apes first appeared. Placental mammals reached their zenith, in numbers and variety of species, in the Pliocene, extending into the Quaternary period.

Quaternary Period

(2.5 million years ago to present). Intermittent continental ice sheets covered much of the northern hemisphere. Fossil remains show that many primitive prehuman 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.

Fields of Geological Study

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 primarily concerned with the evolution of the earth's surface and its life forms through time, and involves investigations into paleontology, stratigraphy, paleogeography, 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, paleomagnetism in rocks and soils, heat-flow phenomena within the earth, the force of gravity, and the propagation of seismic waves (see 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 sulfur 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 (see Microscope). Petrologists study changes that occur spontaneously in rock masses when magmas solidify, when solid rocks melt partially 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 also 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 (and art) 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.

Structural Geology



Originally concerned with analyzing 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.




Paleontology, the study of prehistoric life, deals with fossil animals (paleozoology) and fossil plants (paleobotany) in relation to existing plants and animals. Investigation of microscopic fossils (micropaleontology) involves techniques different from that of larger specimens. Fossils, the remains of or indications of life in the geologic past, as preserved by natural means in the earth's crust, are the chief data of paleontology. Paleontography is the formal, systematic description of fossils (plants and animals), and invertebrate paleontology is frequently regarded as a separate subdiscipline from vertebrate paleontology.


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 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).

Economic Geology

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 nonmetallic minerals, water, and geothermal energy. Kindred fields include the science of locating economic or strategic minerals (exploration geology), processing ores (see Metallurgy), and the practical application of geological theories to mining (mining geology).

Engineering (Environmental) Geology

Engineering geologists apply geologic principles in investigating the natural materials—soil, rock, surface water, and groundwater—that impinge on the design, construction, and operation of civil engineering projects. Representative of such projects are dams, bridges, highways, pipelines, housing developments, and waste-management systems. A recent offshoot, environmental geology, involves collection and analysis of geologic data for the purpose of resolving problems created by human use of the natural environment. Chief among such problems are the risks to life and property that result from building homes and other structures in areas subject to geologic hazards, particularly earthquakes (see Earthquake), landslides (see Landslide), coastal erosion, and flooding. The scope of environmental geology is exceptionally broad, comprising as it does physical sciences such as geochemistry and hydrology, as well as biological and social sciences. See also Engineering: Geological and Mining Engineering.

Geological Processes

Geological processes may conveniently be divided into those that originate within the earth (endogenic processes) and those that originate externally (exogenic processes).

Endogenic Processes

The rifting of the great lithospheric plates, the continual drifting of continental crust, and the expansion of oceanic crust from midoceanic spreading centers 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, plateaus, 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 preexisting strata. Epeirogeny affects large parts of the continents and oceans, primarily through upward or downward movements, and produces plateaus 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 centimeters to several kilometers are called faults. Faulting is commonly associated with plate boundaries that glide past one another—for example, the San Andreas Fault—and with sites where 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 U.S. is overlaid by volcanic basalts that are more than 3000 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 Fuji or Mount Saint 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.

Exogenic Processes

Any natural medium capable of picking up and moving earth material is referred to as a geomorphic agent. Running water, groundwater, glaciers, wind, and movements within bodies of standing water (such as tides, waves, and currents) are all primary geomorphic agents. Because they originate outside the earth's crust, these geological processes are designated as epigene or exogenic.

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 leveling of the earth's surface as a result of deposition, which occurs when the medium transporting the sediments loses power.


Numerous geological organizations provide their members with a wide range of services. Primarily they act as forums for the dissemination of knowledge, by means of professional journals, letters, and other communications. In addition they provide codes of professional conduct, short courses of practical instruction, job placement services, and certify specialists. Representative American organizations include the Geological Society of America, the American Geological Institute, the American Geophysical Union, the American Institute of Mining, Metallurgy, and Petroleum Engineers, the American Petroleum Institute, the American Association of Petroleum Geologists, and the Association of Exploration Geophysics. Other organizations include the Association of Geoscientists for International Development, the Geological Association of Canada, the Geological Society of London, the Geological Society of Australia, the Geoscience Information Society, the International Union of Geological Sciences, the Society of Economic Geologists, and the Society of Economic Paleontologists and Mineralogists.

For additional information on individual scientists, see biographies of those whose names are not followed by dates.



Contributed by:

Charles W. Finkl, Jr.