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geology

The science of the Earth. The study of the Earth's materials and of the processes that shape them is known as physical geology. Historical geology is the record of past events.  See also: Earth; Earth sciences

Energy from two sources continually produces changes in the Earth. Radiant energy from the Sun causes ocean currents, winds, waves, rainfall, weathering, soil formation, and a myriad of other physical and chemical changes in the outermost rocky portion of the solid Earth (lithosphere), in the fluid envelopes of water (hydrosphere) and air (atmosphere), and in the totality of living matter (the biosphere). Heat energy inside the Earth causes slow convective movements deep in the Earth's interior. The internal motions break the rigid lithosphere into large fragments called tectonic plates, which move laterally at velocities up to around 5 in. (12 cm) a year. Collisions and other interactions between moving plates of lithosphere produce the Earth's gross topography—the ocean basins, mountain ranges, even the shapes of the continents themselves.  See also: Atmosphere; Biosphere; Hydrosphere; Insolation

Geologists examine rocks exposed at the Earth's surface and samples recovered from drilling. However, the radius of the Earth is 3982 mi (6371 km), and so the inner portions of the Earth must be studied remotely by means of the Earth's magnetic, electrical, gravitational, elastic, and other physical properties.

Following the spacecraft landings on the Moon, and space exploration of the other planets and their moons, the study methods of geology have been used in comparative planetology, in which the origin, development, and history of all solid bodies in the solar system are compared. Geology became a universal science in the second half of the twentieth century, and an understanding of the geological evolution of the Moon, Mars, Venus, and other planetary bodies has provided a new perspective on the Earth's history.  See also: Planetary physics

Physical and historical geology

Geology is an interdisciplinary subject that overlaps and depends on other scientific disciplines. Physical geology is concerned primarily with the Earth's materials (minerals, rocks, soils, water, ice, and so forth) and the processes of their origin and alteration. Chemistry and physics are the two scientific disciplines most closely related—study of the chemistry of the Earth's materials is geochemistry, study of the physical properties of the Earth is geophysics.  See also: Geochemistry; Geophysics

Historical geology is based on two complementary disciplines, stratigraphy and paleontology. Stratigraphy is the systematic study of stratified rocks through geologic time. The stratigraphic record reveals the sequence of events that have affected the Earth through eons of time. Absolute dates for the stratigraphic record are provided from geochemical studies of naturally occurring radioactive isotopes. Paleontology is the study of fossilized plants and animals with regard to their distribution in space and time. The fossil record is much more abundant in strata deposited during the present, or the Phanerozoic Eon, covering the past 545 million years. Paleontology is closely related to biology. The distinctions between physical and historical geology are more matters of convenience than substance, because it is increasingly clear, within the framework of plate tectonics, that all aspects of geology are interrelated.  See also: Paleobotany; Paleontology; Stratigraphy

Internal structure and plate tectonics

The solid Earth is layered with respect to both composition and the physical properties (Fig. 1). The outermost compositional layer, the crust, is of two kinds: the oceanic crust, averaging about 6 mi (10) in thickness, everywhere underlies the ocean basins; the continental crust, averaging about 24 mi (40 km) in thickness, everywhere underlies the continents. Although the oceanic and continental crusts differ in composition, both are richer in silicon, aluminum, potassium, and sodium than the mantle which lies below. The mantle cannot be sampled directly, and its compositional heterogeneity is still uncertain. The innermost compositional layer is the core, a metallic mass composed largely of iron and nickel, with small amounts of silicon, oxygen, sulfur, and other chemical elements.

Fig. 1  Layers in the Earth. (a) Comparison of compositional layers and physical property changes in the Earth; temperature and pressure increase with depth. (b) Continental crust and oceanic crust.

Layering of the Earth's physical properties arises from changes in pressure and temperature with depth. The outermost 60 mi (100 km) of the Earth, the lithosphere, is rigid and tough, and it can be fractured readily. From a depth of about 60–220 mi (100–350 km) is the asthenosphere, a region where temperatures are sufficiently high that rocks in the mantle are weak. Instead of fracturing, rocks in the asthenosphere flow and deform plastically. Beneath the asthenosphere is the mesosphere, a region where plastic properties decrease. Another distinct physical-property boundary occurs within the core—the outer core is molten, while the inner core, which has the same composition as the outer core, is solid.  See also: Asthenosphere; Earth interior; Lithosphere

Plate tectonics is one result of the Earth's layering of physical properties. Rigid plates of lithosphere float on the fluidlike asthenosphere. Continental crust and oceanic crust ride on top of lithospheric plates. Magma rising from deep in the mantle creates new oceanic crust where plates move apart. Where plates converge, the old oceanic crust is resorbed into the interior. Continental crust is characterized by a low density and cannot be dragged down into the mantle and resorbed in the same way as high-density oceanic crust. Instead, continental crust is continually moved around, colliding to form larger continents, fragmenting to form smaller units, in a fashion that endlessly changes the shapes of continents and the disposition of continents and ocean basins. Continental masses have collided and broken apart repeatedly throughout the Earth's history (Fig. 2). Geological activity is concentrated along plate boundaries and is most obviously manifested in earthquakes, volcanic eruptions, and the formation of mountain ranges such as the Himalaya and the Alps.  See also: Continent; Continental drift; Continents, evolution of; Earth, convection in; Earth crust; Earthquake; Magma; Orogeny; Plate tectonics; Volcano

Fig. 2  Continents 250 million years ago, when they were assembled into a single large landmass called Pangaea. The northern portion of Pangaea is called Laurasia, the southern portion Gondwanaland. (After B. J. Skinner and S. C. Porter, Physical Geology, Wiley, 1987)

Mineralogy

Minerals are the basic building blocks of rocks. About 3600 minerals have been identified, but fewer than 50 are common constituents in the types of rocks that are abundant in the Earth. The most common minerals in the crust are feldspars, quartz, micas, amphiboles, pyroxenes, olivine, and calcite. Each mineral is characterized by a distinct geometric packing of its constituent atoms, known as its crystal structure. Under ideal growth conditions the atomic packing is expressed as a crystal, which is a natural geometric solid bounded by plane faces. When a substance crystallizes in bulk, crowding of grains growing from neighboring centers prevents formation of recognizable crystals. An interlocking aggregate of irregularly shaped grains results. Most rocks have such an interlocking fabric of grains. Modern laboratories have effective devices for resolving the mineral content of rock materials; even the ultramicroscopic particles in clays are clearly defined under the electron microscope.  See also: Crystal structure; Electron microscope; Mineral; Mineralogy

Petrology

This is the study of rocks, their physical and chemical properties, and their modes of origin. Rocks are divisible into three primary families and three secondary ones. The primary families are igneous rocks, which have solidified from molten matter (magma); sedimentary rocks, made of fragments derived by weathering of preexisting rocks, of chemical precipitates from sea or lake water, and of organic remains; and metamorphic rocks derived from igneous or sedimentary rocks under conditions that brought about changes in mineral composition, texture, and internal structure (fabric). The secondary rock families are pyroclastic rocks, which are partly igneous and partly sedimentary rocks because they are composed largely or entirely of fragments of igneous matter erupted explosively from a volcano; diagenetic rocks are transitional between sedimentary and metamorphic rocks because their textures or compositions were affected by low-temperature, postsedimentation processes below conditions of metamorphism; migmatites are transitional between metamorphic and igneous rocks because they form when metamorphic rocks are raised to temperatures and pressures so that small localized fractions of the rock start to melt but the melting is insufficient for a large body of magma to develop.  See also: Petrography; Petrology; Rock

Igneous rocks

Igneous rocks are formed as either extrusive or intrusive masses; that is, they are solidified at the Earth's surface or deep underground. Both kinds range widely in composition; silica, the most abundant ingredient, varies from about 40% to more than 75%. The most silica-rich varieties of igneous rock such as granite and granodiorite tend to be found in the continental crust. Basalt and gabbro, both containing about 50% silica, are the primary igneous rocks of the oceanic crust.  See also: Silica minerals

Intrusive bodies (plutons) of granite and other silica-rich igneous rocks that formed at various depths are most numerous in mountain zones for two reasons: first, mountain belts have been much deformed, and abundant evidence indicates that crustal disturbance has favored igneous action; second, uplifts in mountain lands have permitted erosion to great depths so that plutonic masses are exposed. Evidence suggests that many large plutons solidified in magma reservoirs that formerly supplied volcanoes in action.  See also: Pluton

Volcanic materials are erupted through two kinds of openings—central vents and long fissures. Central eruptions build up conical mountains; the materials are in part products of explosion, pyroclastic rocks, and partly interspersed lava flows. Lavas issuing from fissures do not build up volcanic cones but instead form vast fields of volcanic rock, chiefly basalt. The most extensive and most active volcanic fissures lie beneath the sea along the centers of the mid-ocean ridges. There, basaltic magma formed in the mantle rises and creates new oceanic crust.  See also: Basalt; Igneous rocks; Lava; Mid-Oceanic Ridge

Sedimentary rocks

Bedrock exposed to the atmosphere and hydrosphere is broken into pieces, large and small, which are moved by running water and other agents to lower ground, and spread in sheets over river floodplains, lake bottoms, and sea floors. Dissolved matter is carried to seas and other bodies of water, and some of it is precipitated chemically and by action of organisms. The material deposited in various ways becomes compacted, and in time much of it is cemented into firm sedimentary rock. Generally, the deposition is not continuous but recurrent, and sheets of sediment representing separate events come to form distinct layers of sedimentary rock. The individual layers are beds or strata, and the rocks are said to be stratified (Fig. 3).

Fig. 3  Nearly horizontal sedimentary rocks approximately 4000 ft (1200 m) thick along the Colorado River Canyon, Arizona. Some tilting of layers is discernible in the left section. (Spence Air Photos)

Large areas in every continent are underlain by sedimentary rocks that represent deposits during many periods of the Earth's history. In part, these bedded rocks are nearly horizontal, as they were originally; but in many places, particularly along the margins of present or past tectonic plates, elongate belts of bent and fractured strata can be observed in mountain belts (Fig. 4).

Fig. 4  Strata deformed by the compressive forces that created the Alps, near Val Malenco, northern Italy. (Courtesy of Kurt Bucher)

The principal kinds of sedimentary rock are conglomerate, sandstone, siltstone, shale, limestone, and dolostone. There are many other kinds, and some, though less important quantitatively, have large practical value; examples include common salt, gypsum, rock phosphate, iron ore, and coal.  See also: Sedimentary rocks

Metamorphic rocks

These rocks have been developed from earlier igneous and sedimentary rocks by heat and pressure, especially in mountain zones found along plate margins, and near large masses of intrusive igneous rock. Thermal metamorphism adjacent to plutons results from rising temperatures, often with addition of new elements by circulating fluids, but without pronounced deformation of strata. Common effects are hardening and crystallizing of the affected rock, with changes in mineral composition.

Dynamic metamorphism involves elevated temperatures but also results from shearing stresses in rocks subjected to high pressure; effects are development of cleavage planes and growth of platy minerals in parallel arrangement. Dynamic metamorphism is most commonly observed in mountain belts where the crust is thickened and deformed due to plate collisions.

The common metamorphic rocks are in the two general classes, foliated (including slate, phyllite, schist, and gneiss) and nonfoliated (including marble and quartzite).  See also: Metamorphic rocks; Metamorphism

Regolith

Bedrock at and near the Earth's surface is subject to mechanical and chemical changes in a complex group of processes called weathering. Blocks and small chips that become detached due to mechanical fragmentation are especially vulnerable to chemical attack, which makes radical changes in the mineral content. Some soluble products of alteration are removed by percolating water; and in the less soluble residue the most common constituents are quartz and clay, which are the basic minerals of soil. The effectiveness of weathering and the nature of its products are controlled by climate, topography, kinds of bedrock, and other variables. Organic processes play a major role in chemical weathering, which is most effective under conditions that favor development of bacteria, plants, and ground-dwelling animals. The blanket of weathered material that covers most of the Earth's surface is called the regolith.

Weathering prepares the way for removal of rock materials and reshaping of land surfaces by several agents of erosion. The most obvious of these agents is running water, which during a single rainstorm may cut deep gullies into plowed fields and sweep vast quantities of soil, sand, and coarser debris into brooks and eventually into channels of major streams. Abrasion by such moving loads deepens and widens stream channels in hard bedrock. Study of drainage systems brings conviction that even the largest and deepest valleys have been fashioned by the action of running water.  See also: Weathering processes

The pull of gravity causes the regolith to move downslope, from high points to low. Soil on slopes, even those covered with grass and other vegetation, creeps slowly downward. Blocks dislodged from cliffs build steep masses of sliderock which slowly migrate downslope. Frequently, in mountain lands great masses, including loose material and bedrock, rush down as landslides. All downslope movements caused by gravity are termed mass wasting.  See also: Landslide; Mass wasting; Regolith

Water moving through underground openings dissolves and carries away great quantities of material. Caverns, large and small, are a conspicuous result. In high latitudes and in some mountain regions, glaciers are powerful eroding agents. In arid regions, quantities of sand and dust are moved by wind. Large-scale erosion along the coasts of seas and lakes is performed by waves and currents.  See also: Cave; Glaciated terrain

Each major agent of erosion fashions characteristic features in landscapes; the subdiscipline of geology devoted to studies of these features is geomorphology. The net tendency of erosion is to reduce the height of landmasses. Various stages in the history of reduction are indicated by forms of valleys and slopes and by relations of land surfaces to the underlying bedrock. Some wide regions have been uplifted, and the streams have been rejuvenated after an advanced stage was reached in a cycle of erosion. Rejuvenation of an eroded land surface is believed to result, at least in part, from plate tectonics.  See also: Erosion; Geomorphology

Sediments and sedimentation

Sediment now deposited provides essential clues to understanding processes of past sedimentation and hence of the origin of sedimentary rock. Sediment and sedimentary rock are all important in geology. On the basis of depositional environment, sediments are assigned to three categories: terrestrial, those laid down on land; marine, those deposited on sea floors; and mixed terrestrial-marine, those laid down in transitional zones such as deltas, marine estuaries, and areas between high and low tide. In each major group the sediments are further described as clastic (consisting of rock fragments) and chemical (formed either as inorganic precipitates or partly through organic agencies). Study of modern sediments in the several environments takes account of physical peculiarities and the included remains of organisms.

Terrestrial sediments are extremely variable. They are laid down chiefly through the agencies of mass wasting, running water, glacier ice, and wind. The deposits are in large part temporary, as the tendency is for them to shift downslope and seaward in continued erosion of the lands. Marine sediments, deposited beneath oceans, comprise material derived from the land, from shells and skeletons of marine animals and plants, from the ocean by chemical precipitation, and from space (particles of meteorites).  See also: Marine sediments; Nearshore processes

Tectonics and structural geology

Through careful study of rock masses, it is possible to distinguish primary structures (such as bedding in sedimentary rocks, and flow structures in igneous rocks) that were acquired as a result of the genesis of the rock, from secondary structures that result from later deformation. Significant features in sedimentary rocks make them especially valuable for registering later changes in form.

Deformation may proceed rapidly or slowly. Rapid deformation usually involves large fractures (faults) with instantaneous displacements of several meters (Fig. 5). Rapid deformation is generally attended by strong earthquakes. Slow deformation leads to broad-scale warping, bending, and folding. The principal kinds of structural features that record past deformation are folds, joints, faults, cleavage, and unconformities. Tectonics is closely related to structural geology but deals with regional features, such as mountain ranges.  See also: Fault and fault structures; Fold and fold systems; Geodynamics; Structural geology

Fig. 5  Fault breaking through to the surface, near Hilo, Hawaii. (R. S. Fiske, U.S. Geological Suvey)

The most pronounced crustal deformation tends to be found in mountain belts, where erosion has exposed exceptionally thick sections of sedimentary rocks. These rocks record long histories of slow subsidence and sedimentation, interrupted by large-scale deformation and uplift. The deformation is due to lateral compressive forces arising from plate tectonic movements.  See also: Cordilleran belt; Mountain systems

Major relief features of the Earth reflect differences in density of the underlying rocks. Continental rocks have appreciably lower average density than the basaltic rocks of the oceanic crust. Continents therefore stand high, while ocean basins are low. Great mountain blocks, such as the Alps and the Himalaya, represent thickened parts of continental crust. The condition of approximate balance among diverse parts of the crust is called isostasy (equal standing).  See also: Isostasy

Stratigraphy

Studies of sedimentary rocks in comparison with the many kinds of modern sediments provide a basis for recognizing conditions under which the rocks were formed. Generally, a close interpretation is possible; bodies of rock are confidently classified as deposits on floodplains, at margins of glaciers, in large lakes, in shallow seas near shore, or in deep-sea troughs. Each distinctive type of deposit represents a facies. A rock type that is essentially uniform over a considerable area constitutes a lithofacies. An assemblage of fossils that is nearly uniform in a large unit of sedimentary deposits, indicating an environment suited to certain forms of life, is a biofacies. Deposits formed at the same time may differ in both lithofacies and biofacies, reflecting differences in topography, climate, and other items of environment.  See also: Facies (geology)

A fundamental principle in stratigraphic studies, known as the law of superposition, is that in a normal sequence of strata any layer is older than the layer next above it. This elementary law is of importance in studies of many belts of highly deformed rocks where thick sections of strata have been overturned, even completely inverted, and can be resolved only through criteria that indicate original tops of beds. Close matching of the many kinds of modern sediments with materials in sedimentary rocks formed over an immense span of time has established the uniformitarian principle, which holds that processes now operating on the Earth have operated in similar fashion through the ages.

The history of the Earth's crust is encoded chiefly in sedimentary rocks, which record a sequence of events, changing physical environments, developments in plant and animal life, and effects of crustal movements. Additional records are supplied by volcanic rocks, which in many areas are interlayered with, and grade into, sedimentary strata; by relations of intrusive igneous bodies to older and younger rocks; and by erosion surfaces, some displayed in present landscapes, others revealed in exposures of unconformities. The long history includes radical changes in physical geography, featuring a contest between land and sea; the birth, rise, and wasting away of successive mountain systems; and the evolution of living forms in seas and on lands.  See also: Unconformity

Each continent (other than ice-buried Antarctica) displays a wide lowland or platform that was occupied repeatedly by seas and is now mantled with little-deformed marine strata. The total deposit on each platform represents a long span of time and ranges from a few tens to a few thousand meters thick. The join between continental crust and oceanic crust marks the geological boundary between an ocean basin and a continent. The boundary is beneath the sea, at the foot of the continental slope, and it is here that sediment eroded from the continent is accumulated. The sites of such accumulations of sediment classically were called geosynclines. When continental collisions occur, the wedge of sedimentary strata draped along and over the continental margin becomes deformed through folding and faulting, and elevated into a mountain range (Fig. 5). Examples of this process can be found in the Appalachian Mountains and the Alpine-Himalayan chain. One of the triumphs of plate tectonics is the illumination it has thrown on this long-puzzling aspect of mountain range development.  See also: Continental margin; Geosyncline; Physical geography

Geologic column and geologic time scale

At any one locality a sequence of sedimentary beds, from older to younger, can be determined through physical evidence. Persistence of some peculiar units may establish approximate correlations through moderate distances, occasionally hundreds of miles. But the key to relative dating of stratigraphic units and to confident worldwide correlations is provided by fossils of animals and plants, which record progressive evolution in living forms from ancient to recent times. Through correlation a worldwide composite diagram has been developed that combines, in chronological order, the succession of known strata on the basis of fossil contents or other evidence. This composite diagram is the geologic column.

Many of the oldest known sedimentary rocks are now highly metamorphosed, and any fossils they may have held must have been obliterated. Some thick sections of old strata that are not appreciably altered have yielded only sparse indications of life, such as patterns of marine algae and burrows made by worms or other lowly forms. Successively younger groups of strata hold abundant fossils of marine invertebrates, marine fishes, land plants that progressed from primitive to more modern kinds, reptiles, birds, and small mammals, followed by diverse and generally more advanced kinds of mammals, primitive humans, and finally modern humans. Some forms evolved rapidly, and short-lived species that were equipped to become widely dispersed are of greatest value for correlation.

Absolute dates can be given to points on the geologic column, and a geologic time scale developed thereby, through the use of certain naturally occurring radioactive isotopes. Such dating is known as geochronology. The names given to units of the geologic column (see table) are derived largely from the paleontologic record—or its absence.  See also: Geochronometry; Geologic time scale

Geologic mapping

A geologic map represents the lithology and, so far as possible, the geologic age of every important geologic unit in a given area. Each distinctive unit that can be shown effectively to the scale of the map is a geologic formation. A good topographic base map is essential for representing relations of bedrock to land surface forms. Cooperation of workers with specialized qualifications—for example, in petrology, paleontology, or structural geology—required for accurate mapping and description of complex areas. Large organizations, such as federal geological surveys and some commercial firms, possess diversified personnel, laboratories equipped for varied analyses, and special field equipment. Aerial photographic surveys serve as a guide in field work and help in plotting accurate locations. Satellite imagery has provided vast quantities of data for mapping.  See also: Aerial photography; Remote sensing; Topographic surveying and mapping

A completed geologic map should indicate important structural details, such as inclinations of strata, locations of faults, and axial traces of folds. Usually the map is supplemented by vertical sections on which structural features seen at the surface are projected to limited depths. Maps of small scale may represent sedimentary rocks only according to the systems to which they belong. With larger scale a given system may be represented by several formations, each recording an important episode in the history of the region. In some European countries, geologic mapping has been completed to fairly large scales; but in all continents great areas have been mapped only in reconnaissance fashion.

Economic geology

A general knowledge of geology has many practical applications, and large numbers of geologists receive special training for service in solving problems met in the mining of metals and nonmetals, in discovering and producing petroleum and natural gas, and in engineering projects of many kinds. Human use of materials has become so great that waste materials are influencing natural geological processes. As a result, a new discipline, environmental geology, is starting to emerge.