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 general term for any sedimentary accumulations deposited by comparatively recent action of rivers. It thus includes sediment laid down in river beds, flood plains, and alluvial fans.
A fine-grained igneous rock composed mostly of plagioclasefeldspar and from 25% to 40% amphibole and biotite, but no quartz or K-feldspar. It is abundant in mountains bordering the Pacific Ocean, such as the Andes Mountains of South America, from which the name was derived. Andesitic magma is believed to originate from fractionation of partially melted basalt.
A rock texture in which individual crystals are too small to be identified without the aid of a microscope. In hand specimens, aphanitic rocks appear to be dense and structureless.
The geographic area in an island arcdeep-sea trench system that separates the arc of volcanoes from the trench. In most cases, the gap is about 100 km wide.
Ground water confined in an aquifer and under pressure great enough to cause the water to rise above the top of the aquifer when it is tapped by a well.
A turbulent blend of unsorted pyroclastic material (mostly fine-grained) mixed with high-temperature gases ejected explosively from a fissure or crater.
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 mixture of gases surrounding a planet. The earth's atmosphere consists chiefly of oxygen and nitrogen, with minor amounts of other gases. Synonymous with air.
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.
The level below which a stream cannot effectively erode. Sea level is the ultimate base level, but lakes form temporary base levels for inland drainage systems.
A series of igneous and metamorphic rocks lying beneath the oldest stratified rocks of a region. In shields, the basement complex is exposed over large areas.
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 narrow, usually submerged ridge of sand or gravel deposited across the mouth of a bay by longshore drift. Baymouth bars commonly are formed by extension of spits along embayed coasts.
Material transported by currents along the bottom of a stream or river by rolling or sliding, in contrast to material carried in suspension or in solution.
The solid zone of accumulation underlying the A horizon of a soil profile. Some of the material dissolved by leaching in the A horizon is deposited in the B horizon.
A general term for material originating from organisms. Examples: fossils (shells, bones, leaves), peat, coal. biosphere The totality of life on or near the earth's surface.
An intrusive rock that was once exposed at the surface by erosion and was subsequently covered by younger sediment. The relative age of the intrusion thus falls between, or is bracketed by, the ages of the younger and older sedimentary deposits.
A stream with a complex of converging and diverging channels separated by bars or islands. Braided streams form where more sediment is available than can be removed by the discharge of the stream.
A general term for sediment consisting of angular fragments in a matrix of finer particles. Examples: sedimentary breccias, volcanic breccias, fault breccias, impact breccias.
A somewhat isolated hill, usually capped with a resistant layer of rock and bordered by talus. A butte is an erosion remnant of a formerly more extensive slope.
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.
The belief that geologic history consists of major catastrophic events involving processes that were far more intense than any we observe now. Contrast with uniformitarianism.
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.
1 Pertaining to fragments (such as mud, sand, and gravel) produced by the mechanical breakdown of rocks. 2 A sedimentary rock composed chiefly of consolidated clastic material.
A rock fragment with a diameter between 64 mm (about the size of a tennis ball) and 2567 mm (about the size of a volleyball). Cobbles are larger than pebbles but smaller than boulders.
A stream that has a course determined by! or directly resulting from, the original slope on which it developed. contact The surface separating two different rock bodies.
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.
The submerged margin of a continental mass extending from the shore to the first prominent break in slope, which usually occurs at a depth of about 120 m.
The slope that extends from a continental shelf down to the ocean deep. In some areas, such as off eastern North America, the continental slope grades into the more gently sloping continental rise.
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 period of lunar history during which rayed craters, such as Copernicus, and their associated rim deposits were formed (from 2 billion years ago to the present).
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.
Stratification inclined to the original horizontal surface upon which the sediment accumulated. It is produced by deposition on the slope of a dune or sand wave.
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 cone-shaped or fan-shaped deposit of land-derived sediment located seaward of large rivers or submarine canyons. Synonymous with abyssal cone, abyssalfan, submarine cone.
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).
The combined action of all of the various processes that cause the wearing away and lowering of the land, including weathering, mass wasting, stream action, and ground-water activity.
A distinctively disordered drainage pattern formed in a recently glaciated area. It is characterized by irregular direction of stream flow, few short tributaries, swampy areas, and many lakes.
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.
Variation in the rate of erosion on different rock masses. As a result of differential erosion, resistant rocks form steep cliffs, whereas nonresistant rocks form gentle slopes.
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 sudden or rapid change in physical properties of rocks within the earth. Discontinuities are recognized by seismic data. See also Mohorovicic discontinuity.
A plate margin formed where the lithosphere splits into plates that drift apart from one another. Divergent plate boundaries are areas subject to tension, where new crust is generated by igneous activity. Synonymous with spreading center. See also oceanic ridge.
1 (structural geology) An uplift that is circular or elliptical in map view, with beds dipping away in all directions from a central area. 2 (topography) A general term for any dome-shaped landform.
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 low mound of fine-grained material that accumulates as a result of sediment transport in a current system. Dunes have characteristic geometric forms that are maintained as they migrate. Sand dunes are commonly classified according to shape. See also barchan dune, longitudinal dune, parabolic dune, self dune, star dune, and transverse dune.
A series of elastic waves propagated in the earth, initiated where stress along a fault exceeds the elastic limit of the rock so that sudden movement occurs along the fault.
Rock material (crushed rock, large blocks, breccia, and dust) ejected from an impact crater or explosion crater and deposited over the surrounding area.
The sedimentary environment of deserts, where sediment is transported and deposited primarily by wind. epicenter The area on the earth's surface that lies directly above the focus of an earthquake.
The period of lunar when large craters, the rays of which are no longer visible, such as Eratosthenes, were formed (from 3.1 billion to 0.8 billion years ago).
The processes that loosen sediment and move it from one place to another on the earth's surface. Agents of erosion include water, ice, wind, and gravity.
A bay at the mouth of a river formed by subsidence of the sand or by a rise in sea level. Fresh water from the river mixes with and dilutes seawater in an estuary.
Bedrock not covered with soil or regolith; outcrop. extrusive rock A rock formed from a mass of magma that flowed out on the surface of the earth. Example: basalt.
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 mineral group consisting of silicates of aluminum and one or more of the metals potassium, sodium, or calcium. Examples: K-feldspar, Ca-plagioclase, Na-plagioclase.
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.
The state of matter in which a substance has neither independent shape nor independent volume. Gases can readily be compressed and tend to expand indefinitely.
A hollow nodule of rock lined with crystals; when separated from the rock body by weathering, it appears as a hollow, rounded shell partly filled with crystals.
1 A state of matter in which a substance displays many properties of a solid but lacks crystal structure. 2 An amorphous igneous rock formed from a rapidly cooling magma.
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.
The ancient continental landmass that is thought to have split apart during Mesozoic time to form the present day continents of South America, Africa, India, Australia, and Antarctica.
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.
1 (mineralogy) The measure of the resistance of a mineral to scratching or abrasion. 2 (water) A property of water resulting from the presence of calcium carbonate and magnesium carbonate in solution.
1 (geologic) A plane of stratification assumed to have been originally horizontal. 2 (soil) A layer of soil distinguished by characteristic physical properties. Soil horizons generally are designated by letters (for example, A horizon, B horizon, C horizon).
The pressure exerted by a fluid at a given depth beneath its surface. It is proportional to the 'height of the fluid's surface above the area where the pressure is measured.
Chemical combination of water with other substances. hydrosphere The waters of the earth, as distinguished from the rocks (lithosphere), the air (atmosphere), and (biosphere).
The period of lunar history during which the large multiringed basins, such as Mare Imbrium, were formed and the marebasalts extruded (from 3.9 billion to 3.1 billion years ago).
Igneous rock that, while it was fluid, penetrated into or between other rocks and solidified. It can later be exposed at the earth's surface after erosion of the overlying rock.
