The mechanical wearing away of a rock by friction, rubbing, scraping, or grinding. absolute time Geologic time measured in a specific duration of years (in contrast to relative time, which involves only the chronologic order of events).
The part of the ocean floor consisting of hills rising as much as 1000 m above the surrounding floor. They are found seaward of most abyssal plains and occur in profusion in basins isolated from continents by trenches, ridges, or rises.
Flat areas of the ocean floor, having a slope of less than 1:1000. Most abyssal plains lie at the base of a continental rise and are simply areas where abyssal hills are completely covered with sediment.
A fan-shaped deposit of sediment built by a stream where it emerges from an upland or a mountain range into a broad valley or plain (see diagram). Alluvial fans are common in arid and semiarid climates but are not restricted to them.
A small, rocky planetary body orbiting the sun. Asteroids are numbered in the tens of thousands. Most are located between the orbit of Mars and the orbit of Jupiter. Their diameters range downward from 770 km.
The zone in the earth directly below the lithosphere, from 70 to 200 km below the surface. Seismic velocities are distinctly lower in the asthenosphere than in adjacent parts of the earth's interior. The material in the asthenosphere is therefore believed to be soft and yielding to plastic flow.
The smallest unit of an element. Atoms are composed of protons, neutrons, and electrons. attitude The three-dimensional orientation of a bed, fault, dike, or other geologic structure. It is determined by the combined measurements of the dip and the strike of a structure.
1 (crystallography) An imaginary line passing through a crystal around which the parts of the crystal are symmetrically arranged. 2 (fold) The line where folded beds show maximum curvature. The line formed by the intersection of the axial plane with the bedding surface.
1 (structural geology) A circular or elliptical downwarp. After erosion, the youngest beds are exposed in the central part of the structure. 2 (topography) A depression into which the surrounding area drains.
A mixture of various amorphous or crystalline hydrous aluminum oxides and aluminum hydroxides, commonly found as a residual clay deposit in tropical and subtropical regions. Bauxite is the principal commercial source of aluminum.
A large, more or less circular depression or basin associated with a volcanic vent. Its diameter is many times greater than that of the included vents. Calderas are believed to result from subsidence, or collapse, and mayor may not be related to explosive eruptions.
The breaking off of large blocks of ice from a glacier that terminates in a body of water. capacity The maximum quantity of sediment a given stream, glacier, or wind can carry under a given set of conditions.
Chemical reactions that act on rocks exposed to water and the atmosphere so as to change their unstable mineral components to more stable forms. Oxidation, hydrolysis, carbonation, and direct solution are the most common reactions. Synonymous with decomposition.
A large landmass, from 20 to 60 km thick, composed mostly of granitic rock. Continents rise abruptly above the deep-ocean floor and include the marginal areas submerged beneath sea level. Examples: the African continent, the South American continent.
A closed system in which material is transported as a result of thermal convection. Convection currents are characteristic of the atmosphere and of bodies of water. They are believed also to be generated in the interior of the earth. In the plate tectonic theory, convection within the mantle is thought to be responsible for the movement of tectonic plates.
The effect produced by a Coriolis force, namely, the tendency of all particles of matter in motion on the earth's surface to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
A type of landslide in which comparatively dry rock fragments and soil move downslope at speeds ranging from slow to fast. The mass of debris does not show backward rotation (which occurs in a slump) but slides and rolls forward.
A current that flows as a result of differences in density. In oceans, density currents are produced by differences in temperature, salinity, and turbidity (the concentration of material held in suspension).
A veneer of pebbles left in place where wind has removed the finer material. desiccation The process of drying out. With reference to sedimentation, the loss of water from pore spaces by evaporation or compaction.
A planetary body in which various elements and minerals are separated according to density and concentrated at different levels. The earth, for example, is differentiated, with heavy metals (iron and nickel) concentrated in the core, lighter minerals in the mantle, and still lighter materials in the crust, hydrosphere, and atmosphere.
A stream that disappears into an underground channel and does not reappear in the same, or even in an adjacent, drainage basin. In karst regions, streams commonly disappear into sinkholes and follow channels through caves.
A general term for sediment deposited directly on land by glacial ice or deposited in lakes, oceans, or streams as a result of glaciation. drip curtain A thin sheet of dripstone hanging from the ceiling or wall of a cave.
A distinctive group of characteristics within part of a rock body (such as composition, grain size, or fossil assemblages) that differ as a group from those found elsewhere in the same rock unit. Examples: conglomerate facies, shale facies, brachiopod facies.
A long, linear zone of the earth's crust where rocks have been intensely deformed by horizontal stresses and generally intruded by igneous rocks. The great folded mountains of the world (such as the Appalachians, the Himalayas, the Rockies, and the Alps) are believed to have been formed at convergent plate margins.
A fuel containing solar energy that was absorbed by plants and animals in the geologic past and thus is preserved in organic compounds in their remains. Fossil fuels include petroleum, natural gas, and coal.
1 (field geology) A zone where the bedrock is cracked and fractured. 2 (oceanography) A zone of long, linear fractures on the ocean floor, expressed topographically by ridges and troughs. Fracture zones are the topographic expression of transform faults.
An assemblage of late Paleozoicfossil plants named for the seed fern Glossopteris, one of the plants in the assemblage. These flora are widespread in South America, Africa, Australia, India, and Antarctica, and provide important evidence for the theory of continental drift.
A coarse-grained metamorphic rock with a characteristic type of foliation (gneissic layering), resulting from alternating layers of light-colored and darkcolored minerals. Its composition is generally similar to that of granite.
A stream that has attained a state of equilibrium, or balance, between erosion and deposition, so that the velocity of the water is just great enough to transport the sediment load supplied from the drainage basin, and neither erosion nor deposition occurs.
A tributary valley with the floor lying ("hanging") above the valley floor of the main stream or shore to which it flows (see diagram). Hanging valleys commonly are created by deepening of the main valley by glaciation, but they can also be produced by faulting or rapid retreat of a sea cliff.
A state of equilibrium, resembling flotation, in which segments of the earth's crust stand at levels determined by their thickness and density. Isostatic equilibrium is attained by flow of material in the mantle.
A topographic feature or group of features having a linear configuration. Lineaments commonly are expressed as ridges or depressions or as an alignment of features such as stream beds, volcanoes, or vegetation.
A current in the surf zone moving parallel to the shore. Longshore currents occur where waves strike the shore at an angle. The waves push water and sediment obliquely up the beach, and the backwash returns straight down the beach face, so the water' and sediment follow a zigzag pattern, with net movement parallel to the shore.
A general term for the various processes by which early-formed crystals or early-formed liquids are separated and removed from a magma to produce a rock with composition different from that of the original magma. Early-crystallized ferromagnesian minerals commonly are separated by gravitational settling, so that the parent magma is left enriched in silica, sodium, and potassium.
Separation of crystals of certain minerals from a magma as it cools. For example, some minerals (including certain valuable metals) crystallize while other components of the magma are still liquid. These earlyformed crystals can settle to the bottom of a magma chamber and thus become concentrated there, forming an ore deposit.
A naturally occurring inorganic solid having a definite internal structure and a definite chemical composition that varies only within strict limits. Chemical composition and internal structure determine its physical properties, including the tendency to assume a particular geometric form (crystal form).
The first global seismic discontinuity below the surface of the earth. It lies at a depth varying from about 5 to 10 km beneath the ocean floor to about 35 km beneath the continents. Commonly referred to as the Moho.
A general term for any landmass that stands above its surroundings. In the stricter geological sense, a mountain belt is a highly deformed part of the earth's crust that has been injected with igneous intrusions and the deeper parts of which have been metamorphosed. The topography of young mountains is high, but erosion can reduce old mountains to flat lowlands.
The continuous ridge, or broad, fractured topographic swell, that extends through the central part of the Arctic, Atlantic, Indian, and South Pacific oceans. It is several hundred kilometers wide, and its elevation above the ocean floor is 600 m or more. It thus constitutes a major structural and topographic feature of the earth.
The study of ancient magnetic fields, as preserved in the magnetic properties of rocks. It includes studies of changes in the position of the magnetic poles and reversals of the magnetic poles in the geologic past.
A gently sloping erosion surface formed at the base of a receding mountain front or cliff. It cuts across bedrock and can be covered with a veneer of sediment. Pediments characteristically form in arid and semiarid climates.
The processes by which the materials in a planetary body are separated according to density, so that the originally homogeneous body is converted into a zoned or layered (shelled) body with a dense core, a mantle, and a crust.
The theory of global dynamics in which the lithosphere is believed to be broken into individual plates that move in response to convection in the upper mantle. The margins of the plates are sites of considerable geologic activity.
The epoch of geologic time from the end of the Pliocene epoch of the Tertiary period (about 2 million years ago) to the beginning of the Holocene epoch of the Quaternary period (about 10,000 years ago). The major event during the Pleistocene was the expansion of continental glaciers in the Northern Hemisphere. Synonymous with glacial epoch, ice age.
The process of glacial erosion by which large rock fragments are loosened by ice wedging, become frozen to the bottom surface of the glacier, and are torn out of the bedrock and transported by the glacier as it moves. The process involves the freezing of subglacial meltwater that seeps into fractures and bedding planes in the rock.
The division of geologic time from the formation of the earth (about 4.5 billion years ago) to the beginning of the Cambrian period of the Paleozoic era (about 600 million years ago). Also, the rocks formed during that time. Precambrian time constitutes about 90% of the earth's history.
The earliest period of lunar history, extending from the formation of the planet (about 4.5 billion years ago) to the formation of the multiringed basins (about 3.9 billion years ago). Pre-Imbrian system The system of rocks formed on the Moon during the pre-Imbrian period. It includes most of the material on the lunar highlands.
A current formed on the surface of a body of water by the convergence of currents flowing in opposite directions. Rip currents are common along coasts where longshore currents move in opposite directions.
Sedimentary material composed of fragments ranging in diameter from 0.0625 to 2 mm. Sand particles are larger than silt particles but smaller than pebbles. Much sand is composed of quartzgrains, because quartz is abundant and resists chemical and mechanical disintegration, but other materials, such as shell fragments and rock fragments, can also form sand.
Material (such as gravel, sand, mud, and lime) that is transported and deposited by wind, water, ice, or gravity; material that is precipitated from solution; deposits of organic origin (such as coal and coralreefs).
The process in which distinctive sedimentary products (such as sand, shale, and lime) are generated and progressively separated from a rock mass by means of weathering, erosion, transportation, and deposition.
An area where there is very little or no direct reception of seismic waves from a given earthquake because of refraction of the waves in the earth's core. The shadow zone for P waves is between about 103 and 143 degrees from the epicenter.
A set of joints formed essentially parallel to the surface. It allows layers of rock to spall off as the weight of overlying rock is removed by erosion. It is especially well developed in granitic rock.
A large volcano shaped like a flattened dome and built up almost entirely of numerous flows of fluid basalticlava. The slopes of shield volcanoes seldom exceed 10 degrees, so that in profile they resemble a shield or broad dome.
The type of foliation that characterizes slate, resulting from the parallel arrangement of microscopic platy minerals, such as mica and chlorite. Slaty cleavage forms distinct zones of weakness within a rock, along which it splits into slabs.
A stream with a course originally established on a cover of rock now removed by erosion, so that the stream or drainage system is independent of the newly exposed rocks and structures. The stream pattern is thus superposed on, or placed upon, ridges or other structural features that were previously buried.
A low-angle fault (45 degrees or less) in which the hanging wall has moved upward in relation to the footwall. Thrust faults are characterized by horizontal compression rather than by vertical displacement.
A seismic sea wave; a long, low wave in the ocean caused by an earthquake, faulting, or a landslide on the sea floor. Its velocity can reach 800 km per hour. Tsunamis are commonly and incorrectly called "tidal waves."
A current in air, water, or any other fluid caused by differences in the amount of suspended matter (such as mud, silt, or volcanic dust). Marine turbidity currents, laden with suspended sediment, move rapidly down continental slopes and spread out over the abyssal floor.
A pair of thin sedimentary layers, one relatively coarse-grained and light-colored, and the other relatively fine-grained and dark-colored, formed by deposition on a lake bottom during a period of one year (see diagram). The coarse-grained layer is formed during spring runoff, and the fine-grained layer is formed during the winter when the surface of the lake is frozen.
A hard fragment of lava that was liquid or plastic at the time of ejection and acquired its form and surface markings during flight through the air. Volcanic bombs range from a few millimeters to more than a meter in diameter.
The process by which a wave is bent or turned from its original direction. In sea waves, as a wave approaches a shore obliquely, part of it reaches the shallow water near the shore while the rest is still advancing in deeper water; the part of the wave in the shallower water moves more slowly than the part in the deeper water. In seismic waves, refraction results from the wave encountering material with a different density or composition.
The processes by which rocks are chemically altered or physically broken into fragments as a result of exposure to atmospheric agents and the pressures and temperatures at or near the earth's surface, with little or no transportation of the loosened or altered materials.