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دیکشنری تخصصی زمین شناسی


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Aa: A blocky and fragmented form of lava occurring in flows with fissured and angular surfaces

A-horizon: The uppermost layer of a soil, containing organic material and leached minerals

 

Algal mat: A layered communal growth of algae observed in fossils an in present day tidal zones associated with carbonate sedimentation

 

Alkali metal: A strongly basic metal like potassium or sodium

 

 

Alluvial fan: A low, cone shaped deposit of terrestrial sediment formed where a stream undergoes an abrupt reduction of slope

 

 

Alluvium: Unconsolidated terrestrial sediment composed of sorted or unsorted sand, gravel, and clay that has been deposited by water

 

Angle of repose: The steepest slope angle in which particular sediment will lie without cascading down

 

 

Angstrom: A length of 10 to the minus tenth meter or one hundred millionth of a centimeter

 

 

Angular unconformity: An unconformity in which the bedding planes of the rocks above and below are not parallel

 

 

Anthracite: The most highly metamorphosed form of coal, containing 92 to 98 percent of fixed carbon. It is black, hard, and glassy

 

 

Aquifer: A permeable formation that stores and transmits groundwater in sufficient quantity to supply wells

 

 

Arkose: A variety of sandstone containing abundant feldspar and quartz, frequently in angular, poorly sorted grains

 

 

Arroyo: A steep-sided and flat-bottomed gulley in an arid region that is occupied by a stream only intermittently, after rains

 

 

Artesian well: A well that penetrates an aquiclude to reach an aquifer containing water under pressure. Thus water in the well rises above the surrounding water table

 

Astrobleme: A circular erosional feature that has been ascribed to the impact of a meteorite or comet

 

 

Atmosphere (unit): A unit of pressure equal to 101,325 newtons per square meter, or about 14.7 pounds per square inch

 

 

Atoll: A continuous or broken circle of coral reef and low coral islands surrounding a central lagoon

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Backwash: The return flow of water down a beach after a wave has broken

 

Banded iron ore: A sediment consisting of layers of chert alternating with bands of ferric iron oxides (hematite and limonite) in valuable concentrations

 

Bankfull stage: The height of water in a stream that just corresponds to the level of the surrounding floodplain

 

Bar: A unit of pressure equal to 10 to the sixth dynes/square centimeter; approximately one atmosphere

 

Bar (stream): An accumulation of sediment, usually sandy, which forms at the borders or in the channels of streams or offshore from a beach

 

Barchan: A crescent-shaped sand dune moving across a clean surface with its convex face upwind and its concave slip face downwind

 

Bar-finger sand: An elongated lens of sand deposited during the growth of a distributary in a delta. The bar at the distributary

mouth is the growing segment of the bar finger

 

Barrier island: A long, narrow island parallel to the shore, composed of sand and built by wave action

 

Basalt: A fine-grained, dark, mafic igneous rock composed largely of plagioclase feldspar and pyroxene

 

Base-level: The level below which a stream cannot erode; usually sea level sometimes locally the level of a lake or resistant formation

 

Basement: The oldest rocks recognized in a given area, a complex of metamorphic and igneous rocks that underlies all the sedimentary formations. Usually Precambrian or Paleozoic in age

 

Basic rock: Any igneous rock containing mafic minerals rich in iron and magnesium, but containing no quartz and little sodium rich plagioclase feldspar

 

Basin: In tectonics, a circular, syncline-like depression of strata. In sedimentology, the site of accumulation of a large thickness of sediments

 

Batholith: A great irregular mass of coarse-grained igneous rock with an exposed surface of more than 100 square kilometers, which has either intruded the country rock or been derived from it through metamorphism

 

Bathymetry: The study and mapping of sea-floor topography

 

Bauxite: A rock composed primarily of hydrous aluminum oxides and formed by weathering in tropical areas with good drainage; a major ore of aluminum

 

Bedding: A characteristic of sedimentary rocks in which parallel planar surfaces separating different grain sizes or compositions indicate successive depositional surfaces that existed at the time of sedimentation

 

Bed-load: The sediment that a stream moves along the bottom of its channel by rolling and bouncing

 

Beta-particle: An electron emitted with high energy and velocity from a nucleus undergoing radioactive decay

 

B-horizon: The intermediate layer in a soil, situated below the A-horizon and consisting of clays and oxides. Also called the zone of accumulation

 

Biochemical precipitate: A sediment, especially of limestone or iron, formed from elements extracted from seawater by living organisms

 

Bituminous coal: A soft coal formed by an intermediate degree of metamorphism and containing 15 to 20 percent volatiles. The most common grade of coal

 

Block fault: A structure formed when the crust is divided into blocks of different elevation by a set of normal faults

 

Blowout: A shallow circular or elliptical depression in sand or dry soil formed by wind erosion

 

Bolson: In arid regions, a basin filled with alluvium and intermittent playa lakes and having no outlet

 

Bond: The force that holds together two atoms in a compound. It may be derived from the sharing of electrons (covalent) or from electrostatic attraction between ions

 

 

Butte: A steep sided and flat topped hill formed by erosion of flat laying strata where remnants of a resistant layer protect the softer rocks underneath

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Caldera: A large, circular depression in a volcanic terrain, typically originating in collapse, explosion, or erosion

Carbonate ion: The anion group CO3 with a charge of minus two

 

Carbonate platform: A submarine or intertidal shelf whose elevation is maintained by active shallow water carbonate deposition

 

Carbonate rock: A rock composed of carbonate minerals, especially limestone and dolomite

 

Carbonic acid: The weak acid H2CO3 formed by the dissolution of CO2 in water

 

Cataclastic rock: A breccia of powdered rock formed by crushing and shearing during tectonic movements

 

Cation: Any ion with a positive electric charge

 

Central vent: The largest vent of a volcano, situated at the center of its cone

 

Chemical sediment: One that is formed at or near its place of deposition by chemical precipitation, usually from seawater

 

Chemical weathering: The total set of all chemical reactions that act on rock exposed to water and atmosphere and so change it minerals to stable forms

 

Chert: A sedimetary form of amorphous or extremely fine-grained silica, partially hydrous, found in concretions and beds

 

C-horizon: The lowest layer of soil, consisting of fragments of rock and their chemically weathered products

 

Cinder cone: A steep, conical hill built up about a volcanic vent and composed of coarse pyroclasts expelled from the vent by escaping gases

 

Cirque: The head of a glacial valley, usually with the form of one half of an inverted cone. The upper edges have the steepest slopes, approaching vertical, and the base may be flat or hollowed out and occupied by a small lake or pond

 

Clastic rock: A sedimentary rock formed from mineral particles (clasts) that were mechanically transported

 

Clay: Any of a number of hydrous aluminosilicate minerals formed by weathering and hydration of other silicates; also, any mineral fragment smaller than 1/255 mm

 

Coal: The metamorphic product of stratified plant remains. It contains more than 50 percent carbon compounds and burns readily

 

Coastal plain: A low plain of little relief adjacent to the ocean and covered with gently dipping sediments

 

Composite cone: The volcanic cone of a stratovolcano, composed of both cinders and lava flows

 

Contact metamorphism: Mineralogical and textural changes and deformation of rock resulting from the head and pressure of an igneous intrusion in the near vicinity

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Datum plane: An artificially established, well surveyed horizontal plane against which elevations, depths, tides, etc. are measured (for example mean sea-level

)

Daughter element: Also "daughter product". An element that occurs in a rock as end product of the radioactive decay of another element

 

Debris avalanche: A fast downhill mass movement of soil and rock

 

Declination: At any place on Earth, the angle between the magnetic and rotational poles

 

Deflation: The removal of clay and dust from dry soil by strong winds

 

Delta: A body of sediment deposited in an ocean or lake at the mouth of a stream

 

Delta kame: A deposit having the form of a steep, flat topped hill, left at the front of a retreating continental glacier

 

Dendritic drainage: A stream system that branches irregularly and resembles a branching tree in plan

 

Density: The mass per unit volume of a substance, commonly expressed in grams/ cubic centimeter

 

Density current: A subaqueous current that flows on the bottom of a sea or lake because entering water is denser due to temperature or suspended sediments

 

Deposition: A general term for the accumulation of sediments by either physical or chemical sedimentation

 

Deposition remnant magnetization: A weak magnetization created in sedimentary rocks by the rotation of magnetic crystals into line with the ambient field during settling

 

Desert pavement: A residual deposit produced by continued deflation, which removes the fine grains of a soil and leaves a surface covered with closely packed cobbles

 

Detrital sediment: A sediment deposited by a physical process

 

Diagenesis: The physical and chemical changes undergone by a sediment during lithification and compaction, excluding erosion and metamorphism

 

Diatom: A one celled plant that has a siliceous framework and grows in oceans and lakes

 

Diatomite: A siliceous chert-like sediment formed from the hard parts of diatoms

 

Diatom ooze: A fine muddy sediment consisting of the hard parts of diatoms

 

Diatreme: A volcanic vent filled with breccia by the explosive escape of gases

 

Differentiated planet: One that is chemically zoned because heavy materials have sunk to the center and light materials have accumulated in a crust

 

Dip: The angle by which a stratum or other planar feature deviates from the horizontal. The angle is measured in a plane perpendicular to the strike

 

Divide: A ridge of high ground separating two drainage basins emptied by different streams

 

Dome: In structural geology, a round or elliptical upwarp of strata resembling a short anticline

 

Drainage basin: A region of land surrounded by divides and crossed by streams that eventually converge to one river or lake

 

Drift (glacial): A collective term for all the rock, sand, and clay that is transported and deposited by a glacier either as till or as outwash

 

Drumlin: A smooth, streamlined hill composed of till

 

Dry wash: An intermittent streambed in an arroyo or canyon that carries water only briefly after a rain

 

Dune: An elongated mound of sand formed by wind or water

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Earthflow: A detachment of soil and broken rock and its subsequent downslope movement at slow or moderate rates in a stream- or tongue like form.

 

Earthquake: The violent oscillatory motion of the ground caused by the passage of seismic waves radiating from a fault along which sudden movement has taken place.

Ebb tide: The part of the tide cycle during which the water level is falling.

Echo-sounder: An oceanographic instrument that emits sound pulses into the water and measures its depth by the time elapsed before they return.

Ecliptic: The plane that contains the Earth's orbit around the Sun.

 

Eclogite: An extremely high-pressure metamorphic rock containing garnet and pyroxene.

Ecology: The science of the life cycles, populations, and interactions of various biological species as controlled by their physical environment, including also the effect of life forms upon the environment.

Elastic limit: The maximum stress that can be applied to a body without resulting in permanent strain.

Elastic rebound theory: A theory of fault movement and earthquake generation that holds that faults remain locked while strain energy accumulates in the country rock, and then suddenly slip and release this energy.

Electron: A negatively charged particle with negligible mass orbiting around the nucleus of an atom.

Elevation: The vertical height of one point on the Earth above a given datum plane, usually sea level.

 

Elliptical orbit: An orbit with the shape of a geometrical ellipse. All orbits are elliptical or hyperbolic, with the Sun occupying one focus.

Eolian: Pertaining to or deposited by wind.

Eon: The largest division of geologic time, embracing several Eras, for example, the Phanerozoic, 600 m.y. ago to present); also any span of one billion years.

Epicenter: The point on the Earth's surface directly above the focus or hypocenter of an Earthquake.

Epoch: One subdivision of a geologic period, often chosen to correspond to a stratigraphic series. Also used for a division of time corresponding to a paleomagnetic interval.

Era: A time period including several periods, but smaller than an eon. Commonly recognized eras are Precambrian, Paleozoic, Mesozoic, and Cenozoic.

Erosion: The set of all processes by which soil and rock are loosened and moved downhill or downwind.

Eskar: A glacial deposit in the form of a continuous, winding ridge, formed from the deposits of a stream flowing beneath the ice.

Eugeosyncline: The seaward part of a geosyncline; characterized by clastic sediments and volcanism.

Eustatic change: Sea level changes that affect the whole Earth.

Eutrophication: A superabundance of algal life in a body of water; caused by an unusual influx of nitrate, phosphate, or other nutrients.

 

Evaporite: A chemical sedimentary rock consisting of minerals precipitated by evaporating waters, especially salt and gypsum.

Exfoliation: A physical weathering process in which sheets of rock are fractured and detached from an outcrop.

Exobiology: The study of life outside the Earth.

Extinction angle: The angle between a crystallographic direction, such as a face or cleavage plane, and the direction in which all light is blocked by a pair of crossed polarizers.

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Facies: The set of all characteristics of a sedimentary rock that indicates its particular environment of deposition and which distinguish it from other facies in the same rock.

 

Fault: A planar or gently curved fracture in the Earth's crust across which there has been relative displacement

 

Fault-block mountain: A mountain or range formed as a horst when it was elevated between parallel normal faults

 

Fault plane: The plane that best approximates the fracture surface of a fault

 

Faunal succession: The evolutionary sequence of life forms, especially as recorded by the fossil remains in a stratigraphic sequence.

 

Felsic: An adjective used to describe a light-colored igneous rock poor in iron and magnesium content, abundant in feldspars and quartz.

 

Fiord: A former glacial valley with steep walls and a U-shaped profile now occupied by the sea

 

Fissure: An extensive crack, break, or fracture in the rocks

 

Fissure vein: A cleft or crack in the rock material of the earth's crust, filled with mineral matter different from the walls and precipitated therin from aqueous solution.

 

Flood basalt: A plateau basalt extending many kilometers in flat, layered flows originating in fissure eruptions

 

Flood plain: A level plain of stratified alluvium on either side of a stream; submerged during floods and built up silt and sand carried out of the main channel.

 

Flood tide: The part of the tide cycle during which the water is rising or leveling off at high water

 

Flow cleavage: In a metamorphic rock, the parallel arrangement of all planar or linear crystals as a result of rock flowage during metamorphism.

 

Fluid inclusion: A small body of fluid that is entrapped in a crystal and has the same composition as the fluid from which the crystal formed.

 

Flume: A laboratory model of stream flow and sedimentation consisting of a rectangular channel filled with sediment and running water.

 

Focus (earthquake): The point at which the rupture occurs; synonymous with hypocenter

 

Fold: A planar feature, such as a bedding plane, that has been strongly warped, presumably by deformation

 

Foliation: Any planar set of minerals or banding of mineral concentrations including cleavage, found in a metamorphic rock

 

Foraminifera: A class of oceanic protozoa most of which have shells composed of calcite

 

Foraminiferal ooze: A calcareous sediment composed of the shells of dead Foraminifera

 

Forset bed: One of the inclined beds found in crossbedding; also an inclined bed deposited on the outer front of a delta

 

Formation: The basic unit for the naming of rocks in stratigraphy: a set of rocks that are or once were horizontally continuous, that share some distinctive feature of lithology, and are large enough to be mapped.

 

Fossil: An impression, cast, outline, or track of any animal or plant that is preserved in rock after the original organic material is transformed or removed.

 

Fossil fuel: A general term for combustible geologic deposits of carbon in reduced (organic) form and of biological origin, including coal, oil, natural gas, oil shales, and tar sands.

 

Free oscillation: The ringing or periodic deformation of the whole Earth at characteristic low frequencies after a major earthquake.

 

Friction breccia: A breccia formed in a fault zone or volcanic pipe by the relative motion of two rock bodies

 

Fringing reef: A coral reef that is directly attached to a landmass not made of coral

 

Fumarole: A small vent in the ground from which volcanic gases and heated groundwater emerge, but not lava

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Gabbro: A black, coarse-grained, intrusive igneous rock, composed of calcic feldspars and pyroxene. The intrusive equivalent of basalt

 

Geochronology: The science of absolute dating and relative dating of geologic formations and events, primarily through the measurement of daughter elements produced by radioactive decay in minerals

 

Geologic cycle: The sequence through which rock material passes in going from its sedimentary form, through diastrophism and deformation of sedimentary rock, then through metamorphism and eventual melting and magma formation, then through volcanism and plutonism to igneous rock formation, and finally through erosion to form new sediments

 

Geomorphic cycle: An idealized model of erosion wherein a plain is uplifted epeirogenically, then dissected by rapid streams (youth), then rounded by d0wnslope movements into a landscape of steep hills (maturity), and finally reduced to a new peneplain at sea level (old age)

 

Geomorphology: The science of surface landforms and their interpretation on the basis of geology and climate

 

Geosyncline: A major downwarp in the Earth's crust, usually more than 1000 kilometers in length, in which sediments

accumulate to thicknesses of many kilometers. The sediments may eventually be deformed and metamorphosed during a mountain-building episode

 

Geotherm: A curving surface within Earth along which the temperature is constant

 

Gey ser: A hot spring that throws hot water and steam into the air. The heat is thought to result from the contact of groundwater with magma bodies

 

Glacial rebound: Epeirogenic uplift of the crust that takes place after the retreat of a continental glacier, in response to earlier subsidence under the weight of the ice

 

Glacial striations: Scratches left on bedrock and boulders by overriding ice, and showing the direction of motion

 

Glacial valley: A valley occupied or formerly occupied by a glacier, typically with a U-shaped profile

 

Glacier: A mass of ice and surficial snow that persists throughout the year and flows downhill under its own weight. The size range is from 100 meters to 10,000 kilometers

 

Glacier surge: A period of unusually rapid movement of one glacier, sometimes lasting more than a year

 

Glass: A rock formed when magma is too rapidly cooled (quenched) to allow crystal growth

 

Glassiness: The content of extent of glass in an igneous rock

 

Gneiss: A coarse-grained regional metamorphic rock that shows compositional banding and parallel alignment of minerals

 

Graben: A downthrown block between two normal faults of parallel strike but converging dips; hence a tensional feature. See also horst

 

Graded bedding: A bed in which the coarsest particles are concentrated at the bottom and grade gradually upward into fine silt, the whole bed having been deposited by a waning current

 

Graded stream: A stream whose smooth profile is unbroken by resistant ledges, lakes, or waterfalls, and which maintains exactly the velocity required to carry the sediment provided to it

 

Granite: A coarse-grained, intrusive igneous rock composed of quartz, orthoclase feldspar, sodic plagioclase feldspar, and micas. Also sometimes a metamorphic product

 

Granitization: Gneiss:Graben:Graded bedding:Graded stream:Granite:itization:The formation of metamorphic granite from other rocks by recrystallization with or without complete melting

 

Granular snow: Snow that has been metamorphosed into small granules of ice

 

Granulite: A metamorphic rock with coarse interlocking grains and little or no foliation

 

Gravel: The coarsest of alluvial sediments, containing mostly particles larger than 2 mm in size and including cobbles and boulders

 

Gravity anomaly: The value of gravity left after subtracting from a gravity measurement the reference value based on latitude, and possibly the free-air and Bouguer corrections

 

Gravity survey: The measurement of gravity at regularly spaced grid points with repetitions to control instrument drift

 

Greenhouse effect: The heating of the atmosphere by the absorption of infrared energy re-emitted by the Earth as it receives light energy in the visible band from the Sun

 

Greenschist: A metamorphic schist containing chlorite and epidote (which are green) and formed by low-temperature, low-pressure metamorphism

 

Ground moraine: A glacial deposit of till with no marked relief, interpreted as having been transported at the base of the ice

 

Groundwater: The mass of water in the ground below the phreatic zone, occupying the total pore space in the rock and moving slowly downhill where permeability allows

 

Gully: A small steep-sided valley or erosional channel from 1 meter to about 10 meters across

 

Guyot: A flat-topped submerged mountain or seamount found in the ocean

 

Gyre: The circular rotation of the waters of each major sea, driven by prevailing winds and the Coriolis effect

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Half-life: The time required for half of a homogeneous sample of radioactive material to decay

 

Hanging valley: A former glacial tributary valley that enters a larger glacial valley above its base, high up on the valley wall

 

Hard water: Water that contains sufficient dissolved calcium and magnesium to cause a carbonate scale to form when the water is boiled or to prevent the sudsing of soap.

 

Heat conduction: The transfer of the rapid vibrational energy of atoms and molecules, which constitutes heat energy, through the mechanism of atomic or molecular impact.

 

Heat engine: A device that transfers heat from a place of high temperature to a place of lower temperature and does mechanical work in the process.

 

Hill: A natural land elevation, usually less than 1000 feet above its surroundings, with a rounded outline. The distinction between hill and mountain depends on the locality.

 

Hogback: A formation similar to a Cuesta in that it is a ridge formed by slower erosion of hard strata, but having two steep, equally inclined slopes.

 

Hooke's Law: The principle that the stress within a solid is proportional to the strain. It holds only for strains of a few percent or less.

 

Hornfels: A high-temperature, low-pressure metamorphic rock of uniform grain size showing no foliation. Usually formed by contact metamorphism.

 

Horst: An elongate, elevated block of crust forming a ridge or plateau, typically bounded by parallel, outward-dipping normal faults.

 

Hot spring: A spring whose waters are above both human body and soil temperature as a result of plutonism at depth.

 

Humus: The decayed part of the organic matter in a soil

 

Hydration: A chemical reaction, usually in weathering, which adds water or OH to a mineral structure

 

Hydraulic conductivity: A measure of the permeability of a rock or soil: the volume of flow through a unit surface in unit time with unit hydraulic pressure difference as the driving force.

 

Hydrocarbon: An organic chemical compound made up of carbon and hydrogen atoms arranged in chains or rings

 

Hydrologic cycle: The cyclical movement of water from the ocean to the atmosphere, through rain to the surface, through runoff and groundwater to streams, and back to the sea.

 

Hydrology: The science of that part of the hydrologic cycle between rain and return to the sea; the study of water on and within the land.

 

Hydrothermal activity: Any process involving high-temperature groundwaters, especially the alteration and emplacement of minerals and the formation of hot springs and geysers.

 

Hydrothermal vein: A cluster of minerals precipitated by hydrothermal activity in a rock cavity

 

Hypocenter: The point below the epicenter at which an earthquake actually begins; the focus

 

Hypsometric diagram: A graph that shows in any way the relative amounts of the Earth's surface at different elevations with regard to sea level.

 

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Igneous rock: A rock formed by congealing rapidly or slowly from a molten state

 

Ignimbrite: An igneous rock formed by the lithification of volcanic ash and volcanic breccia

 

Inclination: The angle between a line in the Earth's magnetic field and the horizontal plane; also a synonym for dip

 

Index of refraction: The ratio of the speed of light in a vacuum to the speed in a material; this ratio determines the amount that light is refracted as it passes into a crystal.

 

Infiltration: The movement of groundwater or hydrothermal water into rock or soil through joints and pores

 

Interfacial angle: The angle between two crystal faces of a crystal, characteristic of a mineral's symmetry

 

Interior drainage: A system of streams that converge in a closed basin and evaporate without reaching the sea

 

Intermontane basi n: A basin between mountain ranges, often formed over a graben

 

Intrusion: An igneous rock body that has forced its way in a molten state into surrounding country rock

 

Intrusive rock: Igneous rock that is interpreted as a former intrusion from its cross-cutting contacts, chilled margins, or other field relations.

 

Ion: An atom or group of atoms that has gained or lost electrons and so has a net electric charge

 

Ionic bond: A bond formed between atoms by electrostatic attraction between oppositely charged ions

 

Iron formation: A sedimentary rock containing much iron, usually more than 15 percent as sulfide, oxide, hydroxide, or carbonate; a low-grade ore of iron.

 

Isograd: A line or curved surface connecting rocks that have undergone an equivalent degree of metamorphism

 

Isostasy: The mechanism whereby areas of the crust rise or subside until the mass of their topography is buoyantly supported or compensated by the thickness of crust below, which "floats" on the denser mantle. The theory that continents and mountains are supported by low-density crustal "roots."

 

Isotop e: One o f several forms of one element, all having the same number of protons in the nucleus, but differing in their number of neutrons and thus atomic weight.

 

Isotope geology: The study of the relative abundances of isotopes in rocks to determine their ages (see geo-chronology) or conditions of formation.

 

Isotropic substance: One in which the magnitude of a physical property, such as transmission of light is independent of crystallographic direction

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Joint: A large and relatively planar fracture in a rock across which there is no relative displacement of the two sides

 

Juvenile gas: Gases that come to the surface for the first time from the deep interior

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Kerogen: A mixture of organic substances found in many fine-grained sedimentary rocks and a major constituent of oil shale

 

Kett l e: A s mall hollow or depression formed in glacial deposits when outwash was deposited around a residual block of ice that later melted.

 

Kilobar: A unit of pressure equal to 1000 bars

 

Kimberlite: A peridotite containing garnet and olivine and found in volcanic pipes, through which it may come from the upper Mantle

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Laccolith: A sill-like igneous intrusion that forces apart two strata and forms a round, lens-shaped body many times wider than it is thick.

 

Lahar: A mudflow of unconsolidated volcanic ash, dust, breccia, and boulders mixed with rain or the water of a lake displaced by a lava flow.

 

Laminar flow: A flow regime in which particle paths are straight or gently curved and parallel

 

Landslide: The rapid downslope movement of soil and rock material, often lubricated by groundwater, over a basal shear zone; also the tongue of stationary material deposited by such an event.

 

Lapilli: A fragment of volcanic rock formed when magma is ejected into the air by expanding gases. The size of the fragments ranges from sand- to cobble-size.

 

Lateral moraine: A moraine formed along the side of a valley glacier and composed of rock scraped off or fallen from the valley sides.

 

Lava: Magma or molten rock that has reached the surface

 

Lava tube: A sinuous, hollow tunnel formed when the outside of a lava flow cools and solidifies and the molten material passing through it is drained away.

 

Leaching: The removal of elements from a soil by dissolution in water moving downward in the ground

 

Left-lateral fault: A strike-slip fault on which the displacement of the far block is to the left when viewed from either side

 

Levee: A low ridge along a stream bank, formed by deposits left when floodwater decelerates on leaving the channel; also an artificial barrier to floods built in the same form.

 

Limb (fold ): The relat ively planar part of a fold or of two adjacent folds (for example, the steeply dipping part of a stratum between an anticline and syncline).

 

Limestone : A sedime ntary rock composed principally of calcium carbonate (CaCO2), usually as the mineral calcite

 

Lineation: Any linear arrangement of features found in a rock

 

Lithification: The processes that convert a sediment into a sedimentary rock

 

Lithology: The systematic description of rocks, in terms of mineral composition and texture

 

Lithosphere: The outer, rigid shell of the Earth, situated above the asthenosphere and containing the crust, continents, and plates.

 

Lode: An unusually large vein or set of veins containing ore minerals

 

Longitudinal dune: A long dune parallel to the direction of the prevailing wind

 

Longitudinal profile: A cross section of a stream from its mouth to its head, showing elevation versus distance to the mouth

 

Longshore current: A current that moves parallel to a shore and is formed from the momentum of breaking waves that approach the shore obliquely.

 

Longshore drift: The movement of sediment along a beach by swash and backwash of waves that approach the shore obliquely

 

Lopolith: A large laccolith that is bowl-shaped and depressed in the center, possibly by subsidence of an emptied magma chamber beneath the intrusion.

 

Lowland: Land of general low relief at the lower levels of regional elevation

 

Low-velocity zone: A region in the Earth, especially a planar layer that has lower seismic-wave velocities than the region immediately above it.

 

Luster: The general textural impression of a mineral surface, given by the light reflected from it. Terms such as metallic, submetallic are standardized but subjective.

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Maar volcano: A volcanic crater without a cone, believed to have been formed by an explosive eruption of trapped gases

 

Mafic mineral: A dark-colored mineral rich in iron and magnesium, especially a pyroxene, amphibole, or olivine

 

Magma : Molt en rock material that forms igneous rocks upon cooling. Magma that reaches the surface is referred to as lava

 

Magma chamber: A magma-filled cavity within the lithosphere

 

Magmatic water: Water that is dissolved in a magma or that is derived from such water

 

Magnetic anomaly: The value of the local magnetic field remaining after the subtraction of the dipole portion of the Earth's field

 

Magnetic coupling: The transfer of momentum between celestial bodies, especially dust and gas clouds, through magnetic forces

 

Magnetic north pole: (1) The point where the Earth's surface intersects the axis of the dipole that best approximates the Earth's field. (2) The point where the Earth's magnetic field dips vertically downward.

 

Magnetic stratigraphy: The study and correlation of polarity epochs and events in the history of the Earth's magnetic field as contained in magnetic rocks.

 

Magnetometer: An instrument for measuring either one orthogonal component or the entire intensity of the Earth's magnetic field at various points.

 

Magnitude: A measure of earthquake size, determined by taking the common logarithm base 10) of the largest ground motion observed during the arrival of a P-wave or seismic surface wave and applying a standard correction for distance to the epicenter.

 

Manganese nodule: A small, rounded concretion found on the deep ocean floor that may contain as much as 20 percent manganese and smaller amounts of iron, copper, and nickel oxides and hydroxides.

 

Mantle: The main bulk of the Earth, between the crust and core, ranging from depths of about 40 to 3480 kilometers. It is composed of dense mafic silicates and divided into concentric layers by phase changes that are caused by the increase in pressure with depth.

 

Massive rock: A rock that is little or not at all broken by joints, cracks, foliation, or bedding, tending to present a homogeneous appearance.

 

Mass movement: A downhill movement of soil or fractured rock under the force of gravity

 

Mass spectrometer: An instrument for separating ions of different mass but equal charge (mainly isotopes in geology) and measuring their relative quantities.

 

Maturity: A stage in the geomorphic cycle in which maximum relief and well-developed drainage are both present

 

Meander: Broad, semicircular curves in a stream that develop as the stream erodes the outer bank of a curve and deposits sediment against the inner bank.

 

Mechanical weathering: The set of all physical processes by which an outcrop is broken up into small particles

 

Medial moraine: A long stripe of rock debris carried on or within a glacier resulting from the convergence of lateral moraines where two glaciers join.

 

Medical geology: The application of geologic science to problems of health, especially those relating to mineral sources of toxic or nutritious elements and natural dispersal of toxic pollutants.

 

Mesophere: The lower mantle

 

Metamorphism: The changes of mineralogy and texture imposed on a rock by pressure and temperature in the Earth's interior.

 

Meteoric water: Rainwater, snow, hail, and sleet

 

Meteorite: A stony or metallic object from inter-planetary space that penetrates the atmosphere to impact on the surface

 

Micrometeorite: A meteorite less than 1 millimeter in diameter

 

Microseism: A weak vibration of the ground that can be detected by seismographs and which is caused by waves, wind, or human activity, but not by an earthquake.

 

Migmatite: A rock with both igneous and metamorphic characteristics that shows large crystals and laminar flow structures. Probably formed metamorphically in the presence of water and without melting.

 

Mineral: A naturally occurring element or compound with a precise chemical formula and a regular internal lattice structure. Organic products are usually not included.

 

Mineralogy: The study of mineral composition, structure, appearance, stability, occurrence, and associations

 

Miogeosyncline: A Geosyncline that is situated near a craton and receives chemical and well-sorted elastic sediments from the continent.

 

Mohorovic discontinuity: The boundary between crust and mantle, marked by a rapid increase in seismic wave velocity to more than 8 kilometers per second. Depth: 5 to 45 kilometers. Abbreviated "Moho" or "M-discontinuity."

 

Mohs scale of hardness: An empirical, ascending scale of mineral hardness with talc as 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, and diamond 10.

 

Monadnock: An isolated hill or mountain rising above a peneplain

 

Monocline: The S-shaped fold connecting two horizontal parts of the same stratum at different elevations. Its central limb is usually not overturned.

 

Moraine: A glacial deposit of till left at the margin of an ice sheet. See specifically by name, ground moraine, longitudinal moraine, medial moraine, and terminal moraine.

 

Mountain: A steep-sided topographic elevation larger than a hill; also a single prominence forming part of a ridge or mountain range.

 

Mudflow: A mass movement of material finer than sand, lubricated with large amounts of water. Mudstone: The citified equivalent of mud, a fine grained sedimentary rock similar to shale but more massive.

 

My.: Abbreviation for "million years."

 

Mylonite: A very fine lithified fault breccia commonly found in major thrust faults and produced by shearing and rolling during fault movement.

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Native metal: A natural deposit of a metallic element in pure metallic form, neither oxidized nor combined with sulfur or other elements.

 

Neap tide: A tide cycle of unusually small amplitude, which occurs twice monthly when the lunar and solar tides are opposed-that is, when the gravitational pull of the Sun is at right angles to that of the Moon.

 

Nebula: An immense, diffuse body of interstellar gas and dust that has not condensed into a star

 

Nebular hypothesis: A theory of the formation of the planets that states that a rotating nebula contracted and was then torn into fragments by centrifugal forces, with planets condensing from the fragments.

 

Neutron: An electrically neutral elementary particle in the atomic nucleus having the mass of one proton

 

Neutron-activation analysis: A method of identifying isotopes of an element by bombarding them with neutrons and observing the characteristic radioactive decay products emitted.

 

Normal fault: A dip-slip fault in which the block above the fault has moved downward relative to the block below

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Oblique-slip fault: A fault that combines some strike slip motion with some dip-slip motion

 

Obsidian : Dark vo lcanic glass of felsic composition

 

Octahedral coordination: The packing of six ions around an ion of opposite charge to form an octahedron

 

Oil field: An underground accumulation of oil and gas concentrated beneath an impermeable trap, preventing its escape upward

 

Oil shale: A dark-colored shale containing organic material that can be crushed and heated to liberate gaseous hydrocarbons

 

Old age: A stage in the geomorphic cycle, characterized by formation of a peneplain near sea level

 

Oolite: A sedimentary carbonate particle composed of spherical grains precipitated from warm ocean water on carbonate platforms. Also a rock composed of such particles

 

Opaque mineral: A mineral which transmits no light through a thin section under a microscope. Usually a native metal, sulfide, or metallic oxide mineral

 

Ophiolite suite: An assemblage of mafic and ultra-mafic igneous rocks with deep-sea sediments supposedly associated with divergence zones and the sea-floor environment

 

Orbit: The elliptical or hyperbolic path traced by a planet or meteorite or satellite in the presence of a more massive body

 

Ore: A natural deposit in which a valuable metallic element occurs in high enough concentration to make mining economically feasible

 

Ore mineral: The mineral of an ore that contains the useful element

 

Original Horizontality, Principle of: The proposition of Steno, that all sedimentary bedding is horizontal at the time of deposition

 

Orogenic belt: A linear region, often a former geo-syncline, that has been subjected to folding, and other deformation in a mountain-building episode

 

Orogeny: The tectonic process in which large areas are folded, thrust-faulted, metamorphosed, and subjected to plutonism. The cycle ends with uplift and the formation of mountains

 

Oscillation ripple : A ripple with a symmetrical cross section and a sharp peak formed by waves

 

Outcrop : A segm ent of bedrock exposed to the atmosphere

 

Outgassing: The release of juvenile gases to the atmosphere and oceans by volcanism

 

Outwash: A glaciofluvial sediment that is deposited by meltwater streams emanating from a glacier. Overturned fold: A fold in which a limb has tilted past vertical so that the older strata are uppermost. Oxbow lake: A long, broad, crescent-shaped lake formed when a stream abandons a meander and takes a new course

 

Oxidation: A chemical reaction in which electrons are lost from an atom and its charge becomes more positive

 

Oxidized element: An element occurring in the more positively charged of two common ionic forms

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Pahoehoe: A basaltic lava flow with a glassy, smooth, and undulating, or ropy, surface

 

Paleoclimate: The average state or typical conditions of climate during some past geologic period

 

Paleocurrent map: A map of depositional currents that have been inferred from cross-bedding, ripples, or other sedimentary structures

 

Paleogeographic map: A map showing the surface landforms and coastline of an area at some time in the geologic past

 

Paleomagnetism: The science of the reconstruction of the Earth's ancient magnetic field and the positions of the continents from the evidence of remnant magnetization in ancient rocks

 

Paleontology: The science of fossils, of ancient life-forms, and their evolution

 

Paleowind: A prevailing wind direction in an area, inferred from dune structure or the distribution of volcanic ash for one particular time in geologic history

 

Pangaea: According to some theories, a great proto-continent from which all present continents have broken off by the mechanism of sea-floor spreading and continental drift

 

Panthalassa: A hypothetical primeval ocean covering two-thirds of the world except for the continent of Pangaea

 

Parent element: An element that is transformed by radioactive decay to a different (daughter) element

 

Peat: A marsh or swamp deposit of water-soaked plant remains containing more than 50 percent carbon

 

Pedalfer: A common soil type in humid regions, characterized by an abundance of iron oxides and clay minerals deposited in the B-horizon by leaching

 

Pediment: A planar, sloping rock surface forming a ramp up to the front of a mountain range in an arid region. It may be covered locally by thin alluvium

 

Pedocal: A common soil type of arid regions, characterized by accumulation of calcium carbonate in the A-horizon

 

Pegmatite: An igneous rock with extremely large grains, more than a centimeter in diameter. It may be of any composition but most frequently is granitic

 

Pelagic sediment: Deep-sea sediments composed of fine-grained detritus that slowly settles from surface waters. Common constituents are clay, radiolarian ooze, and foraminiferal ooze.

 

Peneplain: A hypothetical extensive area of low elevation and relief reduced to near sea level by a long period of erosion and representing the end product of the ideal geomorphic cycle

 

Perched groundwater: An isolated body of ground-water that is perched above and separated from the main water table by an aquiclude

 

Peridotite: A coarse-grained mafic igneous rock composed of olivine with accessory amounts of pyroxene and amphibole but little or no feldspar

 

Potable water: Water that is agreeable to the taste and not dangerous to the health

 

Pothole: A semispherical hole in the bedrock of a stream bed, formed by abrasion of small pebbles and cobbles in a strong current

 

Ppm: Abbreviation for "parts per million."

 

Pratt isostatic compensation: The mechanism in which variations in crustal density act to counterbalance the varying weight of topographic features. The crust is here assumed to be of approximately uniform thickness, thus a mountain range would be underlain by lighter rocks

 

Preferred orientation: Any deviation from randomness in the distribution of the crystallographic or grain shape axes of minerals of a rock (including flow cleavage and foliation), produced by deformation and non-uniform stress during crystallization in metamorphic rocks or by depositional currents in sediments

 

Proto-sun: A large cloud of dust and gas gradually coalescing into a star under the force of gravity

 

Proven reserves: Deposits of fossil fuels whose location and extent are known, as opposed to potential but unproved ('*discovered") deposits

 

Pumice: A form of volcanic glass, usually of silicic composition, so filled with vesicles that it resembles a sponge and is very light

 

P-wave: The primary or fastest wave traveling away from a seismic event through the solid rock, and consisting of a train of compressions and dilations of the material

 

Pyroclastic rock: A rock formed by the accumulation of fragments of volcanic rock scattered by volcanic explosions

 

Pyroclastic texture: The unsorted, angular, and un-rounded texture of the fragments in a pyroclastic rock

 

Pyroxene granulite: A coarse-grained contact metamorphic rock containing pyroxene, formed at high temperatures and low pressures

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Quartz arenite: A sandstone containing very little except pure quartz grains and cement

 

Quartzite: (1) A very hard, clean, white metamorphic rock formed from a quartz arenite sandstone. (2) A quartz arenite containing so much cement that it resembles ( 1 )

 

Quartzose sandstone: (1) A quartz arenite. (2) A clean quartz sandstone, less pure than a quartz arenite, that may contain a moderate amount of other detrital minerals and/or calcite cement

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Radial drainage: A system of streams running in a radial pattern away from the center of a circular elevation, such as a volcano or dome

 

Radiative transfer: One mechanism for the movement of heat, in which it takes the form of long-wavelength infrared radiation

 

Radiolarian: A class of one-celled marine animals with siliceous skeletons that have existed in the ocean throughout the Phanerozoic Eon

 

Radiolarian ooze: A siliceous deep-sea sediment composed largely of the skeletons of radiolaria. Radiolarite: The lithified sedimentary rock formed from radiolarian ooze

 

Ray: A linear landform of the lunar surface emanating from a large crater and extending as much as 100 kilometers outward, probably consisting of fine ejecta thrown out by the impact of a meteorite

 

Reaction series: A series of chemical reactions occurring in a cooling magma by which a mineral formed at high temperature becomes unstable in the melt and reacts to form another mineral (see also Discontinuous reaction series)

 

Recharge : In hydr ology, the replenishment of ground-water by infiltration of meteoric water through the soil

 

Recrystallization: The growth of new mineral grains in a rock at the expense of old grains, which supply the material

 

Rectangular drainage: A system of streams in which each straight segment of each stream takes one of two characteristic perpendicular directions, with right-angle bends between. The streams are usually following two perpendicular sets of joints

 

Recumbent fold: An overturned fold with both limbs nearly horizontal

 

Refraction (wave): The departure of a wave from its original direction of travel at the interface with a material of different index of refraction (light) or seismic wave velocity (see also Seismic refraction)

 

Regional metamorphism: Metamorphism occurring over a wide area and caused by deep burial and high internal temperatures of the Earth

 

Regolith: Any solid material lying on top of bedrock. Includes soil, alluvium, and rock fragments weathered from the bedrock

 

Regression: A drop in sea level that causes an area of the Earth to be uncovered by seawater, ending marine deposition

 

Relief: The maximum regional difference in elevation

 

Remote sensing: The study of Earth surface conditions and materials from airplanes and satellites by means of photography, spectroscopy, or radar

 

Replacement deposit: A deposit of ore minerals by hydrothermal solutions that have first dissolved the original mineral to form a small cavity

 

Respiration: The chemical reaction by which carbohydrates are oxidized and by which all animals and plants convert their food into energy. Carbon dioxide is released and oxygen used up

 

Reversible reaction: A chemical reaction which can proceed in either direction, depending on the concentration of reacting materials

 

Rheidity: (1) The ability of a substance to yield to viscous flow under large strains. (2) One thousand times the time required for a substance to stop changing shape when stress is no longer applied

 

Rhyolite: The fine-grained volcanic or extrusive equivalent of granite, light brown to gray and compact. Richter magnitude scale: See Magnitude

 

Ridge (mid-ocean): A major linear elevated landform of the ocean floor, from 200 to 20,000 kilometers in extent. It is not a single ridge, but resembles a mountain range and may have a central rift valley

 

Rift valley: A fault trough formed in a divergence zone or other area of tension

 

Right-lateral fault: A strike-slip fault on which the displacement of the far block is to the right when viewed from either side

 

Ring dike: A dike in the form of a segment of a cone or cylinder, having an arcuare outcrop

 

Rip current: A current that flows strongly away from the sea shore through gaps in the surf zone at intervals along the shoreline

 

Ripple: A very small dune of sand or silt whose long dimension is formed at right angles to the current. River order: See Stream order

 

Rock cycl e: The geol ogic cycle, with emphasis on the rocks produced; sedimentary rocks are metamorphosed to metamorphic rocks, or melted to create igneous rocks, and all rocks may be uplifted and eroded to make sediments, which lithify to sedimentary rocks

 

Rock flour: A glacial sediment of extremely fine (silt-and clay-size) ground rock formed by abrasion of rocks at the base of the glacier

 

Rock glacier: A glacier-like mass of rock fragments or talus with interstitial ice that moves downhill under the force of gravity

 

Rockslide: A landslide involving mainly large blocks of detached bedrock with little or no soil or sand. Rounding: The degree to which the edges and corners of a particle become worn and rounded as a result of abrasion during transportation. Expressed as angular, subrounded, well-rounded, etc

 

Runoff: The amount of rain water directly leaving an area in surface drainage, as opposed to the amount that seeps out as groundwater

 

Rupture strength: The greatest stress that a material can sustain without fracturing at one atmosphere pressure

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Saltation: The movement of sand or fine sediment by short jumps above the ground or stream bed under the influence of a current too weak to keep it permanently suspended

 

Sandblasting: A physical weathering process in which rock is eroded by the impact of sand grains carried by the wind, frequently leading to ventifact formation of pebbles and cobbles

 

Sandstone: A detrital sedimentary rock composed of grains from 1/16 to 2 millimeters in diameter, dominated in most sandstones by quartz, feldspar, and rock fragments, bound together by a cement of silica, carbonate, or other minerals or a matrix of clay minerals

 

Schist: A metamorphic rock characterized by strong foliation or schistosity

 

Schistosity: The parallel arrangement of shaly or prismatic minerals like micas and amphiboles resulting from nonhydrostatic stress in metamorphism

 

Scoria: Congealed lava, usually of mafic composition, with a large number of vesicles formed by gases coming out of solution

 

Sea-floor spreading: The mechanism by which new sea floor crust is created at ridges in divergence zones and adjacent plates are moved apart to make room. This process may continue at 0.5 to 10 centimeters/year through many geologic periods

 

Seamount: An isolated tall mountain on the sea floor that may extend more than 1 kilometer from base to peak (see also Guyot

 

Secular variatio n: Slow changes in the orientation of the Earth's magnetic field that appear to be long lasting and internal in origin as opposed to rapid fluctuations, which are external in origin

 

Sedimentary rock: A rock formed by the accumulation and cementation of mineral grains transported by wind, water, or ice to the site of deposition or chemically precipitated at the depositional site

 

Sedimentary structure: Any structure of a sedimentary or weakly metamorphosed rock that was formed at the time of deposition; includes bedding, cross-bedding, graded bedding, ripples, scour marks, mud-cracks

 

Sedimentation: The process of deposition of mineral grains or precipitates in beds or other accumulations. Seif dune: A longitudinal dune that shows the sculpturing effect of cross-winds not parallel to its axis

 

Seismic discontinuity: A surface within the Earth across which P-wave or S-wave velocities change rapidly, usually by more than +~0.2 kilometer/second

 

Seismicity: The world-wide or local distribution of earthquakes in space and time; a general term for the number of earthquakes in a unit of time

 

Seismic profile: The data collected from a set of seismographs arranged in a straight line with an artificial seismic source, especially the times of P-wave arrivals

 

Seismic reflectio n: A mode of seismi c prospecting in which the seismic profile is examined for waves that have reflected from near-horizontal strata below the surface

 

Seismic refraction: A mode of seismic prospecting in which the seismic profile is examined for waves that have been refracted upward from seismic discontinuities below the profile. Greater depths may be reached than through seismic reflection

 

Seismic surface wave: A seismic wave that follows the earth's surface only, with a speed less than that of S-waves. There are Raleigh waves (forward and vertical vibrations) and Love waves (transverse vibrations)

 

Seismic transition zone: A seismic discontinuity, found in all parts of the Earth, at which the velocity increases rapidly with depth; especially the one at 300 to 600 kilometers

 

Stratification: A structure of sedimentary rocks, which have recognizable parallel beds of considerable lateral extent

 

Stratigraphic sequence: A set of beds deposited that reflects the geologic history of a region

 

Stratigraphy: The science of the description, correlation, and classification of strata in sedimentary rocks, including the interpretation of the depositional environments of those strata

 

Stratovolcano: A volcanic cone consisting of both lava and pyroclastic rocks, often conical

 

Strea k: The fine deposit of mineral dust left on an abrasive surface when a mineral is scraped across it; especially the characteristic color of the dust

 

Streak plat e: A ceramic abrasive surface for streak tests

 

Streaming flow: A tranquil flow slower than shooting flow

 

Streamline: A curved line representing the successive positions of a particle in a flow as time passes

 

Stream order: The hierarchical number of a stream segment in dendritic drainage: the smallest tributary streams have order one and at each junction of streams of equal order the order of the subsequent segment is one higher

 

Stress: A quantity describing the forces acting on each part of a body in units of force per unit area. Striation: See Glacial striation

 

Strike: The angle between true North and the horizontal line contained in any planar feature (inclined bed, dike, fault plane, etc.); also the geographic direction of this horizontal line

 

Strike-slip fault: A fault whose relative displacement is purely horizontal

 

Stromatolite: A fossil form representing the growth habit of an algal mat: concentric spherules, stacked hemispheres, or flat sheets of calcium carbonate and trapped silt encountered in limestones

 

Subduction zone: A dipping planar zone descending away from a trench and defined by high seismicity, interpreted as the shear zone between a sinking oceanic plate and an overriding plate

 

Sublimation: A phase change from the solid to the gaseous state, without passing through the liquid state

 

Submarine canyon: An underwater canyon in the continental shelf

 

Subsidence: A gentle epeirogenic movement where a broad area of the crust sinks without appreciable deformation

 

Superposed stream: A stream that flows through resistant formations because its course was established at a higher level on uniform rocks before down-cutting began

 

Superposition, Principle of: The principle stated by Steno that, except in extremely deformed strata, a bed that overlies another bed is always the younger

 

Supersaturation: The unstable state of a solution that contains more solute than its solubility allows

 

Surf: The breaking or tumbling forward of water waves as they approach the shore

 

Surf zone: An offshore belt along which the waves collapse into breakers as they approach the shore

 

Suspended load: The fine sediment kept suspended in a stream because the settling velocity is lower than the upward velocity of eddies

 

Swash: The landward rush of water from a breaking wave up the slope of the beach

S-wave: The secondary seismic wave, traveling slower than the P-wave, and consisting of elastic vibrations transverse to the direction of travel. It cannot penetrate a liquid

Swell: An oceanic water wave with a wavelength on the order of 30 meters or more and a height of perhaps 2 meters or less that may travel great distances from its source

Symbiosis: The interaction of two mutually supporting species that do not compete with or prey upon each other

 

Syncline: A large fold whose limbs are higher than its center; a fold with the youngest strata in the center

 

System (stratigraphy): A stratigraphic unit larger than a series, consisting of all the rocks deposited in one period of an era

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