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.
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.
One of the several forms of a chemical element that have the same number of protons in the nucleus but differ in the number of neutrons and thus differ in atomic weight.
A concordant igneous intrusion that has arched up the strata into which it was injected, so that it forms a pod-shaped or lensshaped body with a generally horizontal floor.
Any feature of the earth's surface having a distinct shape and origin. Landforms include major features (such as continents, ocean basins, plains, plateaus, and mountain ranges) and minor features (such as hills, valleys, slopes, drumlins, and dunes). Collectively, the landforms of the earth constitute the entire surface configuration of the planet.
An accumulation of till deposited along the side margins of a valley glacier. It accumulates as a result of mass movement of debris on the sides of the glacier.
A solution produced by leaching. Example: water that has seeped through a waste disposal site and thus contains in solution various substances derived from the waste material.
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.
The state of matter in which a substance flows freely and lacks crystal structure. Unlike a gas, a liquid retains the same volume independent of the shape of its container.
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 buoyant mass of hot mantle material that rises to the base of the lithosphere. Mantle plumes commonly produce volcanic activity and structural deformation in the central part of lithospheric plates.
The transfer of rock and soil downslope by direct action of gravity without a flowing medium (such as a river or glacial ice). Synonymous with mass wasting.
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.
A hot cloud of volcanic fragments and superheated gases that flows as a mass, because it is denser than air. Upon cooling, it forms a type of rock called tuff, including ash-flow tuff and welded tuff.
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.
An ancient current, which existed in the geologic past, with a direction of flow that can be inferred from cross-bedding, ripple marks, and other sedimentary structures.
The study of geography in the geologic past, including the patterns of the earth's surface, the distribution of land and ocean, and ancient mountains and other landforms.
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.
An ancient wind, existing in the geologic past, the direction of which can be inferred from patterns of ancient ash falls, orientation of cross-bedding, and growth rates of colonial corals.
The process by which minerals with low melting points liquefy within a rock body as a result of an increase in temperature or a decrease in pressure (or both) while other minerals in the rock are still solid. If the liquid (magma) is removed before other components of the parent rock have melted, the composition of the magma can be quite different from that of the parent rock. Partial melting is believed to be important in the generation of basaltic magma from peridotite at spreading centers and in the generation of granitic magma from basalticcrust at subduction zones.
An accumulation of partly carbonized plant material containing approximately 60% carbon and 30% oxygen. It is considered an early stage, or rank, in the development of coal.
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.
Deep-sea sediment composed of fine-grained detritus that slowly settles from surface waters. Common constituents are clay, radiolarian ooze, and foraminiferal ooze.
A crystal that is significantly larger than the crystals surrounding it. Phenocrysts form during an early phase in the cooling of a magma when the magma cools relatively slowly.
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 inclination, with respect to the horizontal plane, of any linear structural element of a rock. The plunge of a fold is the inclination of the axis of the fold.
A lake that was created under former climatic conditions, at a time when rainfall in the region was more abundant than it is now. Pluvial lakes were common in arid regions during the Pleistocene.
A relatively brief interval of time within a polarity epoch; during a polarity event, the polarity of the earth's magnetic field is reversed with respect to the prevailing polarity of the epoch.
The spaces within a rock body that are unoccupied by solid material. Pore spaces include spaces between grains, fractures, vesicles, and voids formed by dissolution.
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 coast shaped by subaerialerosion, deposition, volcanism, or tectonic activity. primary sedimentary structure A structure of sedimentary rocks (such as cross-bedding, ripple marks, or mud cracks) that originates contemporaneously with the deposition of the sediment (in contrast to a secondary structure, such as a joint or fault, which originates after the rock has been formed).
A group of rock-forming silicateminerals composed of single chains of silicon-oxygen tetrahedra. Compare with amphibole, which is composed of double chains.
Determination of the age in years of a rock or mineral by measuring the proportions of an original radioactive material and its decay product. Synonymous with radioactive dating.
Determination of the chronologie order of a sequence of events in relation to one another without reference to their ages measured in years. Relative geologic dating is based primarily on superposition, faunal succession, and crosscutting relations.
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.
An abraded knob of bedrock formed by an overriding glacier. It typically is striated and has a gentle slope facing the upstream direction of ice movement.
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.
The type of foliation that characterizes schist, resulting from the parallel arrangement of coarse-grained platy minerals, such as mica, chlorite, and talc.
The theory that the sea floor spreads laterally away from the oceanic ridge as new lithosphere is created along the crest of the ridge by igneous activity.
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.
A place where sediment is deposited and the physical, chemical, and biological conditions that exist there. Examples: rivers, deltas, lakes, shallow-marine shelves.
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.
An extensive area of a continent where igneous and metamorphic rocks are exposed and have approached equilibrium with respect to erosion and isostasy. Rocks of the shield are usually very old (that is, more than 600 million years old).
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 zone between the waterline at high tide and the waterline at low tide. A narrow strip of land immediately bordering a body of water, especially a lake or an ocean.
Sedimentary material composed of fragments ranging in diameter from 1/265 to 1/16 mm. Silt particles are larger than clay particles but smaller than sand particles.
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 type of mass movement in which material moves slowly downslope in areas where the soil is saturated with water. It commonly occurs in permafrost areas.
A valley produced by solution activity, either by dissolution of surface materials or by removal of subsurface materials such as limestone, gypsum, or salt.
The process by which corners and edges of a rock body become rounded as a result of exposure to weathering on all sides, so that the rock acquires a spheroidal or ellipsoidal shape.
The imaginary axis through the earth about which a set of tectonic plates moves. The motion of a diverging plate can be described as rotation around a spreading axis.
A pole of the imaginary axis about which a set of tectonic plates moves. The spreading poles are the two points at which a spreading axis intersects the earth's surface.
The hierarchical number of a stream segment. The smallest tributary has the order number of 1, and successively larger tributaries have progressively higher numbers.
Diversion of the headwaters of one stream into another stream. The process occurs by headward erosion of a stream having greater erosive power than the stream it captures.
One of a series of level surfaces in a stream valley representing the dissected remnants of an abandoned flood plain, stream bed, or valley floor produced in a previous stage of erosion or deposition.
Occurring beneath the atmosphere or in the open air, with reference to conditions or processes (such as erosion) that occur on the land. Contrast with submarine and subterranean.
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.
The part of a stream's load that is carried in suspension for a considerable period of time, without contact with the stream bed. It consists mainly of mud, silt, and sand. Contrast with bed load and dissolved load.
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 large, nearly horizontal area of land covered with water at high tide and exposed to the air at low tide. Tidal flats consist of fine-grained sediment (mostly mud, silt, and sand).
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 interval of time required for a wave crest to travel a distance equal to one wavelength; the interval of time required for two successive wave crests to pass a fixed point.
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.
A tributary stream that flows parallel to the main stream for a considerable distance before joining it. Such a tributary is forced to flow along the base of a natural levee formed by the main stream.
