Lg wave ENDTERM BEGDEFA surface wave which travels through the continental crust.
macroturbulence ENDTERM BEGDEFFlood flows of great depth and high velocity are characterized by special turbulent flow phenomena that are exceedingly powerful. Macroturbulence phenomena include powerful upward vortices called kolks"intences shocks resulting from sudden pressure change known as cavitation and powerful roller vortices oriented parallel to flow. These features provide enormous shear stresses and stream power to flood flows."
"Original Horizontality, Principle of" ENDTERM BEGDEF" The proposition of Steno, that all sedimentary bedding is horizontal at the time of deposition. "
"Superposition, Principle of" ENDTERM BEGDEF" The principle stated by Steno that, except in extremely deformed strata, a bed that overlies another bed is always the younger. "
"Uniformitarianism, Principle of" ENDTERM BEGDEF The concept that the processes that have shaped the Earth through geologic time are the same as those observable today.
670-km discontinuity ENDTERM BEGDEF"Boundary that separates the upper mantle (35-670 km) from the lower mantle (670-2900 km). It could be a natural barrier to whole-mantle convection."
A-horizon ENDTERM BEGDEF" The uppermost layer of a soil, containing organic material and leached minerals. "
A-type earthquake ENDTERM BEGDEFEvents with clear P-waves and S-waves occurring under volcanoes at depths of 1-10 km.
Aa ENDTERM BEGDEF A blocky and fragmented form of lava occurring in flows with fissured and angular surfaces.
Aa ENDTERM BEGDEFHawaiian term used to describe a lava flow whose surface is broken into rough angular fragments. Aa flows commonly develop from pahoehoe flows as they cool and lose gas.
Ablation (glacial) ENDTERM BEGDEF"All processes which include melting evaporation (sublimation) wind erosion and calving (breaking off of ice masses) that remove snow or ice from a glacier or snowfield. The term also refers to the amount of snow orice removed by these processes.
Absolute date ENDTERM BEGDEFAn estimate of the true age of a mineral or rock based on the rate of decay of radioactive minerals.
Abyssal plain ENDTERM BEGDEFThe broad expanse of sea floor lying between about 3 and 6 kilometers (-2 to 4 miles) below sea level.
Accelerometer ENDTERM BEGDEFA seismograph for measuring ground acceleration as a function of time.
Accretionary wedge ENDTERM BEGDEFAccumulation on the ocean floor of sedimentary material scraped off the subducted plate by the upper plate.
Accumulation (glacial) ENDTERM BEGDEFAll processes including snowfall condensation avalanching snow transport by wind and freezing of liquid water that add snow or ice to a glacier floating ice or snow cover. The term also includes the amount of snow or other solid precipitation added to a glacier or snowfield by these processes.
Accumulative minerals ENDTERM BEGDEFMinerals that make up the extract assemblage during fractional crystallization of a magma.
Active fault ENDTERM BEGDEFA fault along which slip has occurred in historical (or Holocene) time or earthquake foci are located.
Active lobe ENDTERM BEGDEF of a delta. The site on a delta where functioning distributary channels cause the delta to grow seaward.
Active volcano ENDTERM BEGDEFA volcano that is erupting; or one that while not erupting at the present has erupted within geologically recent time and is considered likely to do so in the geologically near future.
Actualism ENDTERM BEGDEFThe interpretation of ancient rocks by applying the results of analyses of modern-day geologic processes in accordance with the principle of uniformitarianism.
Adaptation ENDTERM BEGDEFA feature of an organism that serves one or more functions useful to the organism.
Adaptive breakthrough ENDTERM BEGDEFAn evolutionary innovation that affords a group of organisms a special ecologic opportunity and often leads to the adaptive radiation of that group.
Adaptive radiation ENDTERM BEGDEF"Evolutionary diversification of a taxon's morphology, ecology, physiology, behavior and other characteristics over a geologically short time interval leading to the appearance of a number of new taxa."
Adaptive radiation ENDTERM BEGDEFThe rapid origins of many new species or higher taxa from a single ancestral group.
Aeolian deposits ENDTERM BEGDEFSedimentary deposits consisting of windblown dust.
AFMAG ENDTERM BEGDEFAudio Frequency Magnetic technique in which natural electomagnetic fields in the audio-frequency range are used to study lateral changes in earth resistivity.
Aftershock ENDTERM BEGDEF"An earthquake which follows a larger earthquake or main shock and originates in or near the rupture zone of the larger earthquake. Generally, major earthquakes are followed by a larger number of aftershocks decreasing in frequency with time. "
Age ENDTERM BEGDEF geologic. The division of geologic time smaller than an epoch.
Aggregation ENDTERM BEGDEFThe process of combination of very small volcanic ash or dust particles in volcanic plumes in the atmosphere to form larger particles which settle out rapidly to the earth's surface due to their large diameter.
Agung ENDTERM BEGDEF"Active volcano, elevation 3141 m (10380 ft) in Bali, Indonesia (8.342ø S 115.508ø E). Agung is known to have erupted three times,  the last in 1964."
Albedo ENDTERM BEGDEFThe percentage of solar radiation reflected from the earth's surface. This percentage is higher for ice than for land or water and usually higher for land than for water.
Algal mat ENDTERM BEGDEF A layered communal growth of algae observed in fossils an in present day tidal zones associated with carbonate sedimentation.
Algal ridge ENDTERM BEGDEFThe durable structure formed by coralline algae that buttresses the front of a modern coral reef.
Alkali metal ENDTERM BEGDEF A strongly basic metal like potassium or sodium.
Alkane ENDTERM BEGDEF"One of the group of hydrocarbons of the paraffin series, for example methane, ethane and propane."
Allelochemicals ENDTERM BEGDEFChemicals produced by one soecies that enter the environment and influence other species.
Allochthonous clay ENDTERM BEGDEFClay originating in an environment different from the final sedimentary environment (rocky substrate soil or upstream sediment subjected to erosion).
Alluvial fan ENDTERM BEGDEF" A low, cone shaped deposit of terrestrial sediment formed where a stream undergoes an abrupt reduction of slope. "
Alluvium ENDTERM BEGDEF" An unconsolidated terrestrial sediment composed of sorted or unsorted sand, gravel, and clay that had been deposited by water. "
Alluvium ENDTERM BEGDEF"Unconsolidated detrital material deposited during comparatively recent geologic time by a stream or river. Alluvial deposits include mid channel bars, point bars, floodplain deposits,alluvial fans, and fluvial terraces."
Alluvlal fan ENDTERM BEGDEF"A low cone-shaped structure that forms where an abrupt reduction in slope -- for example, the transition from a highland area to a broad valley -- causes a stream to slow down."
Amniote egg ENDTERM BEGDEFThe type of egg laid by reptiles and birds having a nutritious yolk and a hard outer shell to protect the embryo from the dry environment. The amniote egg is named for the amnion a sac that contains the embryo.
Amplitude ENDTERM BEGDEFThe maximum height of a wave crest or depth of a trough.
AMT ENDTERM BEGDEF"Audio MagnetoTelluric method in which magnetotelluric measurements are made in the audio frequency range (10-10,000 Hz) for medium to shallow depth investigation."
Andesite ENDTERM BEGDEF A find-grained extrusive igneous rock intermediate in composition between rhyolite which is felsic and basalt which is mafic.
Andesite ENDTERM BEGDEF"Intermediate volcanic rocks containing 54 to 62 percent silica and moderate amounts of iron and magnesium. Andesite minerals commonly include plagioclase and hornblende with lesser amounts of mica, pyroxene and various accessory minerals. Andesites are aphanitic in texture and are usually medium dark in color. They occur with composite volcanic cones associated with convergent plate margins. "
Angle of repose ENDTERM BEGDEF The steepest slope angle in which a particular sediment will lie without cascading down.
Angstrom ENDTERM BEGDEF A length of 10 to the minus tenth meter or one hundred millionth of a centimeter.
Angular unconformity ENDTERM BEGDEF An unconformity in which the bedding planes of the rocks above and below are not parallel.
Angular unconformity ENDTERM BEGDEFAn unconformity separating horiiontal strata above from older strata that had been tilted and eroded.
Anhydrite ENDTERM BEGDEFThe mineral that consists of calcium sulfate (CaS04) or the rock composed of this mineral.
Anomaly ENDTERM BEGDEF magnetic. A local increase or decrease in the strength of the earth's magnetic field caused by the magnetism of nearby sediments or rocks.
Anthracite ENDTERM BEGDEF" The most highly metamorphosed form of coal, containing 92 to 98 percent of fixed carbon. It is black, hard, and glassy. "
Anticline ENDTERM BEGDEFA fold that is concave in a downward direction -- that is the vertex is the lowest point.
Aquiclude ENDTERM BEGDEFAn impermeable geologic formation or stratum which will not hold or transmit fluid.
Aquifer ENDTERM BEGDEF A permeable formation that stores and transmits groundwater in sufficient quantity to supply wells.
Aquifer ENDTERM BEGDEFSaturated permeable geologic unit that can store and transmit significant quantities of groundwater.
Aquitard ENDTERM BEGDEFA geologic formation or stratum that significantly retards fluid movement.
Arc tholeiite ENDTERM BEGDEFBasalt with a characteristic chemical composition found in an island or continental volcanic arc (also a high-K tholeiite).
Arkose ENDTERM BEGDEF" A variety of sandstone containing abundant feldspar and quartz, frequently in angular, poorly sorted grains. "
Array ENDTERM BEGDEFAn ordered arrangement of seismometers or geophones the data from which feeds into a central receiver.
Arrival time ENDTERM BEGDEFThe time at which a particular wave phase arrives at a detector.
Arrival ENDTERM BEGDEFThe appearance of seismic energy on a seismic record.
Arroyo ENDTERM BEGDEF" A steep-sided and flat-bottomed gulley in an arid region that is occupied by a stream only intermittently, after rains. "
Artesian well ENDTERM BEGDEF 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.
Artesian well ENDTERM BEGDEFA well in an aquifer where the groundwater is confined under pressure and the water level will rise above the top of the confined aquifer.
Artificial recharge ENDTERM BEGDEF"The unnatural addition of surface waters to groundwater. Recharge could result from reservoirs storage basins, leaky canals, direct injection of water into an aquifer, or by spreading water over a large land surface. "
Aseismic ENDTERM BEGDEFNot associated with an earthquake as in aseismic slip. Also used to indicate an area with no record of earthquakes; an aseismic zone.
Ash flow ENDTERM BEGDEFA turbulent mixture of gas and rock fragments most of which are ash-sized particles ejected violently from a crater or fissure. The mass of pyroclastics is normally of very high temperature and moves rapidly down the slopes or even along a level surface.
Ash ENDTERM BEGDEFFine particles of rock material ejected during an explosive volcanic eruption (commonly intermediate to felsic events). Ash may be either solid or molten when first erupted and generally measures less than 0.10 inch in size (larger particles have other names).
Ashfall (subaerial) ENDTERM BEGDEFVolcanic ash that has fallen through the air. The resulting deposit is usually well sorted and exhibits a finely layered structure.
Asthenosphere ENDTERM BEGDEF"The layer below the lithosphere which is marked by low seismic wave velocities and high seismic wave attenuation and probably partially molten. The layer or shell of Earth below the lithosphere which is weak and in which large-scale isostatic adjustments take place, magmas may be generated,and seismic waves are strongly attenuated."
Astrobleme ENDTERM BEGDEF A circular erosional feature that has been ascribed to the impact of a meteorite or comet.
Atmosphere (unit) ENDTERM BEGDEF" A unit of pressure equal to 101,325 newtons per square meter, or about 14.7 pounds per square inch. "
Atoll ENDTERM BEGDEF A continuous or broken circle of coral reef and low coral islands surrounding a central lagoon.
Atoll ENDTERM BEGDEFA circular or horseshoe-shaped organic reef growing on a submerged volcano.
Authigenesis ENDTERM BEGDEFProcess by which new minerals form in place within a sediment during deposition or in the early stages of diagenesis.
Autochthonous clay ENDTERM BEGDEF"Clay originating in the soft sediment or sedimentary rock where it is identified and where it formed either by neoformation (i.e., from dissolved elements) or by transformation (i.e., from mineral growth on preexisting sedimentary substrate)."
Avalanche ENDTERM BEGDEF"A large mass of material falling or sliding rapidly due to the force of gravity. In many cases, water acts as a catalyst and/or lubricant. Avalanches often are classified by what is moving, such as a snow, ice, soil, or rock avalanche. A mixture of these materials is commonly called a debris flow. "
Axial plane ENDTERM BEGDEFAn imaginary plane that cuts through a fold dividing it as symmetrically as possible.
B-horizon ENDTERM BEGDEF" The intermediate layer in a soil, situated below the A-horizon and consisting of clays and oxides. Also called the zone of accumulation. "
B-type earthquake ENDTERM BEGDEFEvents with weak P-waves and no S-waves a low-frequency content and occurring under volcanoes at depths < 1 km.
Backwash ENDTERM BEGDEF The return flow of water down a beach after a wave has broken.
Banded iron formation ENDTERM BEGDEFAn iron formation that consists of alternating iron-rich and iron-poor layers. Most rocks of this type are older than about 2 billion years.
Banded iron ore ENDTERM BEGDEF A sediment consisting of layers of chert alternating with bands of ferric iron oxides (hematite and limonite) in valuable concentrations.
Bankfull stage ENDTERM BEGDEF The height of water in a stream that just corresponds to the level of the surrounding floodplain.
Bar-finger sand ENDTERM BEGDEF 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.
Bar (stream) ENDTERM BEGDEF" An accumulation of sediment, usually sandy, which forms at the borders or in the channels of streams or offshore from a beach. "
Bar ENDTERM BEGDEF A unit of pressure equal to 10 to the sixth dynes/square centimeter; approximately one atmosphere.
Barchan ENDTERM BEGDEF A crescent-shaped sand dune moving across a clean surface with its convex face upwind and its concave slip face downwind.
Barrier island ENDTERM BEGDEF" A long, narrow island parallel to the shore, composed of sand and built by wave action. "
Barrier island ENDTERM BEGDEFAn elongate island composed of sand heaped up by waves that lies approximately parallel to the shoreline of an ocean.
Barrier reef ENDTERM BEGDEF"An elongate organic reef that parallels a coastline and is large enough to dissipate ocean waves, leaving a quiet-water lagoon on its landward side."
Basal sliding ENDTERM BEGDEFThe movement or speed of movement of a glacier on its bed. Also called basal slip.
Basalt ENDTERM BEGDEF" A fine-grained, dark, mafic igneous rock composed largely of plagioclase feldspar and pyroxene. "
Basalt ENDTERM BEGDEF"Volcanic rock (or magma) that is generally dark in color, contains 45 to 54 percent silica, and is rich in iron and magnesium. An eruption of basaltic magma is generally quiet and results in flows (both vesicular and non-vesicular) and breccias. Undersea eruptions commonly result in the formation of pillow lavas. Basalt represents the initial differentiated material erupted by the earth at spreading centers. "
Base-level ENDTERM BEGDEF The level below which a stream cannot erode; usually sea level sometimes locally th level of a lake or resistant formation.
Basement rocks ENDTERM BEGDEFRocks beneath a large geologic feature (such as the fold-and-thrust belt of a mountain system) which are genetically unrelated to the overlying feature.
Basement ENDTERM BEGDEF" 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 ENDTERM BEGDEF" Any igneous rock containing mafic minerals rich in iron and magnesium, but containing no quartz and little sodium rich plagioclase feldspar. "
Basin ENDTERM BEGDEF" In tectonics, a circular, syncline-like depression of strata. In sedimentology, the site of accumulation of a large thickness of sediments. "
Batholith ENDTERM BEGDEF" 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 ENDTERM BEGDEF The study and mapping of sea-floor topography.
Bauxite ENDTERM BEGDEF A rock composed primarily of hydrous aluminum oxides and formed by weathering in tropical areas with good drainage; a major ore of aluminum.
Bed-load ENDTERM BEGDEF The sediment that a stream moves alon the bottom of its channel by rolling and bouncing.
Bed ENDTERM BEGDEFA distinct sedimentary layer (stratum) thicker than 1 centimeter.
Bedding ENDTERM BEGDEF 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.
Belemnite ENDTERM BEGDEFExtinct invertebrate animal (cephalopod) known from cigar-shaped fossils.
Benioff zone ENDTERM BEGDEFA narrow zone defined by earthquake foci and tens of kilometers thick dipping from the surface under the Earth's crust. Zone of earthquake foci produced near the surface of and within the subducted lithosphere.
Benthic foraminifera ENDTERM BEGDEF"Benthic protozoans (i.e., live on the seafloor)which may form carbonate tests. The tests are preserved in sediments and form an important basis for paleoceanographic reconstructions."
Bentonite ENDTERM BEGDEFA clay material composed principally of the mineral montmorillonite. It has a great affinity for fresh water and when hydrated will increase its volume more than seven times. Water/bentoninte suspensions are essentially impermeable. Commonly used as a sealant for ponds.
Beta-particle ENDTERM BEGDEF An electron emitted with high energy and velocity from a nucleus undergoing radioactive decay.
Bimodal distribution ENDTERM BEGDEFA size or mass spectrum that has two peaks of concentration.
Biochemical precipitate ENDTERM BEGDEF" A sediment, especially of limestone or iron, formed from elements extracted from sea water by living organisms. "
Biogeochemistry ENDTERM BEGDEF"The chemical interactions among the atmosphere, biosphere, hydrosphereand lithosphere. Study of the cycling transformations and transport of chemicals in and between landscapes and ecosystems."
Biogeography ENDTERM BEGDEFThe study of the distribution of organisms on a geographic scale.
Biostratigraphic unlt ENDTERM BEGDEFA body of rock such as a zone defined on the basis of its fossil content and having approximately time-parallel upper and lower boundaries.
Biostratigraphy ENDTERM BEGDEF"Relative (not absolute, i.e. number of years) age dating and time correlation of stratified rocks (mostly sedimentary plus some volcanics) by means of fossils contained in those rocks based on our knowledge of their time sequence locally regionally and globally -- all of this ultimately being a function of evolutionary change."
Biostratigraphy ENDTERM BEGDEFThe study and classification of rocks and their history based on their fossil content.
Bioturbation ENDTERM BEGDEF"Disturbance of oceanic sediments near the water interface by bottom-dwelling (benthic), organisms, for example, burrowing worms."
Bird's-eye limestone ENDTERM BEGDEFLayered limestone that is full of holes some of which may have been secondarily filled by cementing minerals. The holes were produced by burrowing animals or by gas bubbles. Most of these limestones are of intertidal or supratidal origin.
Bituminous coal ENDTERM BEGDEF A soft coal formed by an intermediate degree of metamorphism and containing 15 to 20 percent volatiles. The most common grade of coal.
Black smokers ENDTERM BEGDEFSeafloor hydrothermal fluids jetting from chimneys typically about 10 cm in radius. Flow rates are ~ 1-5 m/s; temperature is 350ø-400ø C. Black coloration is due to sulfide mineral precipitates.
Block fault ENDTERM BEGDEF A structure formed when the crust is divided into blocks of different elevation by a set of normal faults.
Blowout ENDTERM BEGDEF A shallow circular or elliptical depression in sand or dry soil formed by wind erosion.
Body wave ENDTERM BEGDEFA seismic wave that can travel through the interior of the earth. P-waves and S-waves are body waves.
Bolson ENDTERM BEGDEF" In arid regions, a basin filled with alluvium and intermittent playa lakes and having no outlet. "
Bomb ENDTERM BEGDEFFragments of molten or semi-molten rock several inches to several feet in diameter which are blown out during an explosive volcanic eruption. Because of their semi-plastic condition bombs are often modified in shape during their flight or upon impact.
Bond ENDTERM BEGDEF 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.
Borehole geophysics ENDTERM BEGDEFScience of (seismic or electrical) physical measurement between boreholes or between a borehole and the surface.
Boulder ENDTERM BEGDEFA piece of gravel larger than 256 millimeters (~10 inches) in size.
Brackish water ENDTERM BEGDEFWater that is lower in salinity than normal sea water and higher in salinity than freshwater ranging from 30 to 0.5 parts salt per 1000 parts water.
Braided stream ENDTERM BEGDEF"A stream that has many intertwining channels separated by bars of coarse sediment. Braided streams develop where sediment is supplied to the stream system at a very high rate on an alluvial fan, for example, or in front of a melting glacier."
Breccia ENDTERM BEGDEF"A rock that resembles conglomerate in consisting of clasts of gravel surrounded by sand, but in breccia the clasts are angular, whereas in conglomerate they are rounded."
Brittle-ductile transition zone ENDTERM BEGDEFThe location at depth within the earth's crust where the temperature and pressure have risen to such a high level that directed stress results in plastic deformation as opposed to fracturing and faulting.
Brownian coagulation ENDTERM BEGDEFThe process by which a particle grows by the rapid condensation of gases and fine particles.
Butte ENDTERM BEGDEF 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.
C-horizon ENDTERM BEGDEF" The lowest layer of soil, consisting of fragments of rock and their chemically weathered products. "
Calcrete ENDTERM BEGDEFA surficial gravel and sand conglomerate cemented by calcium carbonate.
Caldera ENDTERM BEGDEF" A large, circular depression in a volcanic terrane, typically originating in collapse, explosion, or erosion. "
Caldera ENDTERM BEGDEF"From the Spanish for cauldron. A basin-shaped volcanic depression; by definition, at least a mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes. Crater Lake occupies the best-known caldera in the Cascades. "
Caliche ENDTERM BEGDEFNodular calcium carbonate (opaque reddish-brown-to-white) that accumulates in the B-horizon ot soils in warm climates that are dry part of the year. Also called calcrete.
Carbonaceous chondrite ENDTERM BEGDEFA stony meteorite that contains carbon compounds.
Carbonate ion ENDTERM BEGDEF The anion group CO3 with a charge of minus two.
Carbonate mineral ENDTERM BEGDEF"A mineral in which the basic building block is a carbon atom linked to three oxygen atoms. Calcite, aragonite and dolomite are the most abundant examples found in sediments and sedimentary rocks."
Carbonate platform ENDTERM BEGDEF A submarine or intertidal shelf whose elevation is maintained by active shallow water carbonate deposition.
Carbonate rock ENDTERM BEGDEF" A rock composed of carbonate minerals, especially limestone and dolomite. "
Carbonate rock ENDTERM BEGDEF"A sedimentary rock that consists primarily of carbonate minerals. The dominant mineral is nearly always either calcite, in which case the rock is limestone, or dolomite, in which case the rock is dolomite."
Carbonate rocks ENDTERM BEGDEFGeneral term for limestones and dolostones.
Carbonate sediment ENDTERM BEGDEF"Unconsolidated sediment that consisls primarily of carbonate minerals, usually aragonite or calcite."
Carbonic acid ENDTERM BEGDEF The weak acid H2CO3 formed by the dissolution of CO2 in water.
Carbonization ENDTERM BEGDEF"The mode of fossilization in which liquids and gases escape, leaving a residue of carbon on the surface of an impression of the organism."
Cataclastic rock ENDTERM BEGDEF A breccia of powdered rock formed by crushing and shearing during tectonic movements.
Cataclastic rock ENDTERM BEGDEF"Metamorphic rocks that resemble breccias or poorly sorted sandstones and that form by dynamic metamorphism, which breaks and reorients grains."
Catastrophism ENDTERM BEGDEFThe doctrine that sudden violent and widespread events caused by supernatural forces formed most of the rocks that are visible at the earth's surface.
Catena ENDTERM BEGDEFSequence of proximately located soils of about the same age and derived from similar parent material but having different characteristics due to variation in relief and/or drainage.
Cation exchange capacity ENDTERM BEGDEFSum total of exchangeable cations retained by negatively charged soil particles. Units of milliequivalents per 100 grams of soil. Abbreviated CEC.
Cation ENDTERM BEGDEF Any ion with a positive electric charge.
Cementation ENDTERM BEGDEFThe lithification of sediment by the precipitation of minerals from watery solutions percolating through the sediment.
Central angle ENDTERM BEGDEFAn angle with the vertex at the center of the Earth with one ray passing through the hypocenter (and also the epicenter) and the other ray passing through the recording station.
Central vent ENDTERM BEGDEF" The largest vent of a volcano, situated at the center of its cone. "
Chemical remanent magnetization ENDTERM BEGDEFMagnetization acquired isothermally through chemical change at temperatures below the Curie temperature either by grain growth or by alteration of a magnetic parent to a magnetic daaghter.
Chemical sediment ENDTERM BEGDEF" One that is formed at or near its place of deposition by chemical precipitation, usually from sea water. "
Chemical sediment ENDTERM BEGDEFA sediment created by precipitation of one or more minerals from natural waters.
Chemical weathering ENDTERM BEGDEF The total set of all chemical reactions that act on rock exposed to water and atmosphere and so change it minerals to stable forms.
Chemosynthesis ENDTERM BEGDEFThe breakdown of simple chemical compounds within a cell for the production of energy. Sulfate-reducing bacleria exemplify this process.
Chert ENDTERM BEGDEF" A sedimetary form of amorphous or extremely fine-grained silic, partially hydrous, found in concretions and beds. "
Chert ENDTERM BEGDEF"An impure rock, often gray in color, that consists primarily of extremely small quartz crystals precipitated from water solutions."
Chloroplast ENDTERM BEGDEFA body within a plant cell or plantlike cell that serves as the site of photosynthesis within the cell. Chloroplasts are apparently evolutionary descendants of blue-green algae that became trapped in other single-celled organisms.
Cinder cone ENDTERM BEGDEF" A steep, conical hill built up about a volcanic vent and composed of coarse pyroclasts expelled from the vent by escaping gases. "
Cinder cone ENDTERM BEGDEFA volcanic cone built entirely of loose fragmented material (pyroclastics).
Circumpolar current ENDTERM BEGDEFThe circular flow of water around Antarctica resulting from the juncture of the west-wind drifls of the Atlantic Pacific and Indian Oceans.
Cirque ENDTERM BEGDEF" 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. "
Clast ENDTERM BEGDEFA solid product of erosion. Clasts are sometimes referred to as detritus or detrital material.
Clastic rock ENDTERM BEGDEF A sedimentary rock formed from mineral particles (clasts) that were mechanically transported.
Clastic rock ENDTERM BEGDEFA rock that is an aggregate of detrital material or clasts.
Clastic ENDTERM BEGDEF"Pertaining to a rock or sediment composed principally of fragmental detrital materials.
Clay-sized sediment ENDTERM BEGDEFSediment in which particles are smaller than 1/256 millimeter. Most sediment of this size belongs to the clay mineral family.
Clay ENDTERM BEGDEF" 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. "
Clay ENDTERM BEGDEF"Particles of size less than two microns basically made up of clay minerals which are hydrated aluminosilicates.
Claystone ENDTERM BEGDEFA sedimentary rock that consists primarily of clay but that is not fissile like shale.
Closed hydrochemical system ENDTERM BEGDEFPhysical portion of the earth in which the mass of a given immobile chemical element remains constant throughout a chemical and physical transformation process and is enriched only by removal of mobile elements.
Coal ENDTERM BEGDEF The metamorphic product of stratified plant remains. It contains more than 50 percent carbon compounds and burns readily.
Coal ENDTERM BEGDEFA metamorphic rock formed from stratified plant remains. It contains more than 50 percent carbon and burns readily.
Coastal plain ENDTERM BEGDEF A low plain of little relief adjacent to the ocean and covered with gently dipping sediments.
Cobble ENDTERM BEGDEFA piece of gravel between 8 and 256 millimeters in size.
Coccolith ENDTERM BEGDEFOne of the many armorlike plates that surround single-celled marine algae known as coccolithophores.
Cogenetic (comagmatic) ENDTERM BEGDEF"Igneous rocks related by differenntiation to a common parental source.
Components principle ENDTERM BEGDEFThe principle stating that a body of rock is younger than any other body of rock from which any of its components are derived.
Composite cone ENDTERM BEGDEF" The volcanic cone of a stratovolcano, composed of both cinders and lava flows. "
Composite volcano ENDTERM BEGDEFA steep volcanic cone built by both lava flows and pyroclastic eruptions.
Compositional maturity ENDTERM BEGDEF"Extent to which a clastic sediment approaches the compositional end product of chemical weathering; the extent to which the most stable components of a sediment have been enriched over unstable components. In sandstone, compositional maturity can be conveniently expressed in terms of the abundance of quartz the most stable common phase in most sandstones."
Concretion ENDTERM BEGDEFA hard nodular structure formed in sediment or in a sedimentary rock by diagenesis.
Condensation ENDTERM BEGDEFThe change of state of water from the vapor to the liquid phase. Results in liberation of 80 calories per cubic centimeter.
Conduction ENDTERM BEGDEFTransfer of heat from one place to another by molecular agitation and without movement of the medium.
Conglomerate ENDTERM BEGDEFA rock consisting of rounded clasts of gravel surrounded by sand.
Connate water ENDTERM BEGDEF"Water included in the groundwater which is derived from the rock itself, as opposed to water which has percolated down from the surface. "
Contact metamorphism ENDTERM BEGDEF Mineralogical and textural changes and deformation of rock resulting from the head and pressure of an igneous intrusion in the near vicinity.
Contact metamorphism ENDTERM BEGDEFLocal meta morphism caused by igneous intrusion that bakes nearby rocks.
Continental accretion ENDTERM BEGDEFThe marginal growth of a continent along a subduction zone by mountain building or by addition of a microplate.
Continental crust ENDTERM BEGDEF"Solid outer layers of the earth, including the rocks of the continents. "
Continental crust ENDTERM BEGDEFThe layer of the Earth that lies under continents and the continental shelves. It ranges in thickness from 35 to 60 km. Its upper layer has a density of ~2.7 g/cm3 and is composed of rocks that are rich in silica and alumina.
Continental drift ENDTERM BEGDEFThe theory that horizontal movement of the earth's surface causes slow relative movements of the continents toward or away from one another.
Continental margin ENDTERM BEGDEF"The submarine edge of the continental shelf, from which the continental slope descends. In the modern world this margin generally lies about 200 meters (~600 feet) below sea level."
Continental rise ENDTERM BEGDEFA more gently sloping region along the base of the continental slope. The continental rise is a depositional feature formed of sediment transported down the slope often by turbidity currents.
Continental shelf ENDTERM BEGDEFThat part of the continent that is covered by water. Continental shelves are more than 322 km wide and about 122 m deep. At the edges of the shelves the continental slopes drop rapidly from 100 to 200 m to 3000 to 3700 m.
Convection ENDTERM BEGDEFRotational flow of a fluid resulting from imbalances in density. This often occurs because the fluid below is heated and becomes less dense than the fluid above or because the fluid above is cooled and becomes more dense than the fluid below.
Core ENDTERM BEGDEFThe innermost layers of the Earth. The inner core is solid and has a radius of about 1300 kilometers. (The radius of the Earth is about 6371 kilometers.) The outer core is fluid and is about 2300 kilometers thick. S-waves cannot travel through the outer core.
Cosmogenic isotope ENDTERM BEGDEFIsotope produced by nuclear reactions induced by cosmic-ray particles.
Cotectic ENDTERM BEGDEFLine curve or surface on a phase diagram depicting crystal-melt equilibria along which two crytals co-precipitate.
Crater ENDTERM BEGDEF"A steep-sided usually circular depression formed by either explosion or collapse at a volcanic vent or surface impact of a meteorite, asteroid, or comet."
Crust ENDTERM BEGDEFThe outermost rock shell. Outer part of the earth composed essentially of crystalline rocks. Crustal thickness is approximately 50 km and outer radius is approximately 6371 km.
CSAMT ENDTERM BEGDEFControled Source Audio-MagnetoTelluric method in which an artificial source generally a long cable is used to simulate the audio-frequency magnetotelluric fields.
Curie temperature ENDTERM BEGDEFTemperature above which there is no spontaneous magnetization.
Dacite ENDTERM BEGDEFVolcanic rock (or lava) that characteristically is light in color and contains 62 to 69 percent silica and moderate amounts of sodium and potassium.
Datum plane ENDTERM BEGDEF" An artificially established, well surveyed horizontal plane against which elevations, depths, tides, etc. are measured (for example mean sea-level). "
Daughter element ENDTERM BEGDEF Also daughter product. An element that occurs in a rock as end product of the radioactive decay of another element.
Debris avalanche ENDTERM BEGDEF A fast downhill mass movement of soil and rock.
Debris avalanche ENDTERM BEGDEF"A flow of unsorted masses of rock and other material downslope under the influence of gravity. Water is commonly involved as a catalyst and/or lubricant. For example, a rapid mass movement that included fragmented cold and hot volcanic rock, water, snow, glacial ice, trees and other debris and hot pyroclastic material was associated with the May 181980 eruption of Mt. St. Helens. Most of the deposits in the upper valley of the North Fork Toutle River and in the vicinity of Spirit Lake are from the debris avalanche resulting from the eruption. "
Declination ENDTERM BEGDEF" At any place on Earth, the angle between the magnetic and rotational poles. "
Deflation ENDTERM BEGDEF The removal of clay and dust from dry soil by strong winds.
Delta kame ENDTERM BEGDEF" A deposit having the form of a steep, flat topped hill, left at the front of a retreating continental glacier. "
Delta ENDTERM BEGDEF A body of sediment deposited in an ocean or lake at the mouth of a stream.
Dendritic crystals ENDTERM BEGDEFCrystals that grow in a branching form resembling trees.
Dendritic drainage ENDTERM BEGDEF A stream system that branches irregularly and resembles a branching tree in plan.
Dendrogeomorphology ENDTERM BEGDEFApplication of tree-ring studies (dendrochronology) to the interpretation of geomorphologic processes. Often supplemented by related botanical observations.
Density current ENDTERM BEGDEF A subaqueous current that flows on the bottom of a sea or lake because entering water is more dense due to temperature or suspended sediments.
Density ENDTERM BEGDEF" The mass per unit volume of a substance, commonly expressed in grams/ cubic centimeter. "
Density ENDTERM BEGDEFThe mass per unit volume of a substance commonly expressed in grams per cubic centimeter.
Deposition ENDTERM BEGDEF A general term for the accumualtion of sediments by either physical or chemical sedimentation.
Depostion remanent magnetization ENDTERM BEGDEF A weak magnetization created in sedimentary rocks by the rotation of magnetic crystals into line with the ambient field during settling.
Desert pavement ENDTERM BEGDEF" A residual deposit produced by continued deflation, which removes the fine grains of a soil and leaves a surface covered with closely packed cobbles. "
Detachment plane ENDTERM BEGDEFThe surface along which a landslide disconnects from its original position.
Detrital sediment ENDTERM BEGDEF A sediment deposited by a physical process.
Detritus ENDTERM BEGDEFLoose rock or mineral material that is dislodged from bedrock by mechanical means and transported from its place of origin.
Dew point ENDTERM BEGDEFThe temperature (elevation) where adiabatic cooling results in the initiation of condensation of water vapor into cloud droplets.
Di-polar ENDTERM BEGDEFThe arrangement of the hydrogen atoms of a water molecule at 105 deg. across the oxygen results in a slight electrical charge to the molecule.
Diagenesis ENDTERM BEGDEF" The physical and chemical changes undergone by a sediment during lithification and compcation, excluding erosion and metamorphism. "
Diagenesis ENDTERM BEGDEF"Chemical and physical changes undergone by a sediment after its initial deposition, exclusive of surficial alteration (chemical weathering) and metamorphism (pervasive recrystallization). Includes processes like compaction, cementation and replacement."
Diatom ooze ENDTERM BEGDEF A fine muddy sediment consisting of the hard parts of diatoms.
Diatom ENDTERM BEGDEF A one celled plant that has a siliceous framework and grows in oceans and lakes.
Diatomite ENDTERM BEGDEF A siliceous chert-like sediment formed from the hard parts of diatoms.
Diatreme ENDTERM BEGDEF A volcanic vent filled with breccia by the explosive escape of gases.
Differentiated planet ENDTERM BEGDEF One that is chemically zoned because heavy materials have sunk to the center and light materials have accumulated in a crust.
Differentiation (fractionation) ENDTERM BEGDEF"Process by which one magma gives rise to another of different composition. Differentiated or fractionated magmas are said to be evolved from a more primitive parent. Processes by which magmas evolve from basaltic to more silicic compositions; includes crystal fractionation, magma mixing, and crustal contamination."
Diffusion of magnetic field lines ENDTERM BEGDEFMotion of magnetic field lines through an electrically conducting fluid by virtue of the fluid's finite electrical resistivity.
Dilatancy ENDTERM BEGDEFInelastic volume increase caused by the occurrence of small cracks in rock or soil under stress.
Dip-slip fault ENDTERM BEGDEFA fault in which the relative displacement is along the direction of dip of the fault plane; the offset is either normal or reverse.
Dip ENDTERM BEGDEF 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.
Dip ENDTERM BEGDEFThe angle by which a rock layer or fault plane deviates from the horizontal. The angle is measured in a plane perpendicular to the strike.
Discharge area ENDTERM BEGDEFRegion in which there is a net loss of water from the groundwater system (discharge) to surface water bodies or by evapotranspiration.
Dispersion (wave) ENDTERM BEGDEFThe spreading out of a wave train due to each wavelength traveling with its own velocity.
Displacement ENDTERM BEGDEFVector defining offset on a fault that may represent slip in a single event or the accumulation of slip over a much longer time.
Dissolved load ENDTERM BEGDEF"Part of the total stream load consisting of substances in dissolved form such as Ca2+, HCO-3, H4SiO4."
Divide ENDTERM BEGDEF A ridge of high ground separating two drainage basins emptied by different streams.
Dome ENDTERM BEGDEF" In structural geology, a round or elliptical upwarp of strata resembling a short anticline. "
Dome ENDTERM BEGDEF"A steep-sided mass of viscous (doughy) lava extruded from a volcanic vent often circular in plane view and spiny, rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome. "
Dormant volcano ENDTERM BEGDEF"A volcano that is presently inactive but which may erupt again. The major volcanic cones of the Cascade Range in Washington, Oregon, and California, for example, are believed to be dormant rather than extinct. "
Drainage basin ENDTERM BEGDEF A region of land surrounded by divides and crossed by streams that eventually converge to one river or lake.
Drift (glacial) ENDTERM BEGDEF" A collective term for all te rock, sand, and clay that is transported and deposited by a glacier either as till or as outwash. "
Drift (glacial) ENDTERM BEGDEFGeneral term for material deposited by a glacier.
Drumlin ENDTERM BEGDEF" A smooth, streamlined hill composed of till. "
Dry wash ENDTERM BEGDEF An intermittent stream bed in an arroyo or canyon that carries water only briefly after a rain.
Dune ENDTERM BEGDEF An elongated mound of sand formed by wind or water.
Duricrust ENDTERM BEGDEFIndurated soil crust or hardcap occurring on or near an eroded planation surface. May be indicative of climatic change from tropical to drier conditions with alternating wet and dry seasons. Contains tubular voids acting as avenues of translocation.
Dust veil index ENDTERM BEGDEF"A quantitative method developed by H. H. Lamb for comparing the magnitude of volcanic eruptions. The formulae use observations either of the depletion of the solar beam, temperature lowering in the middle latitudes, or the quantity of solid matter dispersed as dust. The reference dust veil index is 1000, assigned to the Krakatoa 1883 eruption and the index is calculated using all three methods, where the information is available for statistical comparison purposes. Abbreviated D.V.I."
Dynamo action ENDTERM BEGDEF"Process whereby motions within an electrcally conducting fluid, interacting with a magnetic field, convert their kinetic energy into magnetic energy, thereby sustaining the magnetic field."
Earthflow ENDTERM BEGDEF 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 swarm ENDTERM BEGDEF"A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time. "
Earthquake ENDTERM BEGDEF The violent oscillatory motion of the ground caused by the passage of seismic waves radiating froma fault along which sudden movement has taken place.
Earthquake ENDTERM BEGDEFThe vibration of the Earth caused by the passage of seismic waves radiating from some source of elastic energy.
Ebb tide ENDTERM BEGDEF The part of the tide cycle during which the water level is falling.
Eccentricity ENDTERM BEGDEF"Measures how elliptical the earth-sun orbit is, with consequent effects on solar radiation received by the earth. If e is the eccentricity then (1 + e)/(1 - e) is the ratio of the farthest and closest earth-sun distance."
Echo-sounder ENDTERM BEGDEF An oceanographic instrument that emits sound pulses into the water and measures its depth by the time elapesd before they return.
Ecliptic ENDTERM BEGDEF The palne that contains the Earth's orbit around the Sun.
Eclogite ENDTERM BEGDEF An extremely high pressure metamorphic rock containing garnet and pyroxene.
Ecology ENDTERM BEGDEF" The science of the life cycles, populations, and interactions of vairous biological species as controlled by their physical environment, including also the effect of life forms upon the environment. "
Ecosystem function ENDTERM BEGDEF"Ecological processes such as primary production, decomposition, and nutrient cycling that occur at the scale of a system and are affected by a variety of organisms and the environment's microclimate."
Effective diffusion coefficient ENDTERM BEGDEFMeasure of the rate of diffusion of a substance in a heterogeneous medium like a sediment-water system. Tortuosity resulting from the presence of solid particles is accounted for in this type of coefficient.
Ejecta blanket ENDTERM BEGDEFCircular apron of debris surrounding an impact crater.
Ejecta ENDTERM BEGDEF"Material that is thrown out by a volcano, including pyroclastic material (tephra) and,  from some volcanoes. lava bombs. "
El Chichon ENDTERM BEGDEF"Active volcano in Mexico (17.360ø N, 93.228ø W), elevation 2225 m (7300 ft). El Chichon is known to have erupted five times, most recently in 1983."
Elastic limit ENDTERM BEGDEF The maximum stress that can be applied to a body without resulting in permanent strain.
Elastic rebound theory ENDTERM BEGDEF" 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. "
Elastic rebound theory ENDTERM BEGDEF"The theory of earthquake generation proposing that faults remain locked while strain energy slowly accumulates in the surrounding rockand then suddenly slip, releasing energy."
Elastic wave ENDTERM BEGDEF"A wave that is propagated by some kind of elastic deformation, that is, a change in shape that disappears when the forces are removed. A seismic wave is a type of elastic wave. "
Electron ENDTERM BEGDEF A negatively charged particle with negligible mass orbiting around the nucleus of an atom.
Electronegativity ENDTERM BEGDEFRelative measure (on a scale of 4) of an atom's attraction for the electrons in its outer shell.
Elevation ENDTERM BEGDEF" The vertical height of one point on the Earth above a given datum plane, usually sea level. "
Elliptical orbit ENDTERM BEGDEF" An orbit with the shape of a geometrical ellipse. All orbits are elliptical or hyperbolic, with the Sun occupying one focus. "
Enrichment factor ENDTERM BEGDEFRatio of chemical concentration of an element in a weathered material to that in its fresh parent material.
Eolian ENDTERM BEGDEF Pertaining to or deposited by wind.
Eon ENDTERM BEGDEF" 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 ENDTERM BEGDEF The point on the Earth's surface directly above the focus or hypocenter of an Earthquake.
Epicenter ENDTERM BEGDEFThe point on the Earth's surface directly above the focus (or hypocenter) of an earthquake.
Epoch ENDTERM BEGDEF" One subdivision of a geologic period, often chosen to correspond to a stratigraphic series. Also used fo a division of time corresponding to a paleomagnetic interval. "
Equilibrium line ENDTERM BEGDEFThe level on a glacier where accumulation equals ablation and the net balance equals zero.
Era ENDTERM BEGDEF" A time period including several periods, but smaller than an eon. Commonly recognized eras are Precambrian, Paleozoic, Mesozoic, and Cenozoic. "
Erosion ENDTERM BEGDEF The set of all processes by which soil and rock are loosened and moved downhill or downwind.
Eruption column ENDTERM BEGDEF"A mixture of hot volcanic ash, gases and entrained atmospheric air that rises vertically above an erupting volcano to heights of several tens of kilometers in the earth's atmosphere. The rise of the column is in part due to kinetic energy of the jet of gases and ash issuing from the volcano, but primarily due to the buoyant rise of the low-density mixture in the cooler atmosphere."
Eruption plume ENDTERM BEGDEF"Dispersla of an eruption column downwind from the volcano, due to the effects of prevailing atmospheric circulation in the atratosphere and troposphere."
Eskar ENDTERM BEGDEF" A glacial deposit in the form of a continuous, winding ridge, formed from the deposits of a stream flowing beneath the ice. "
Eugeosyncline ENDTERM BEGDEF The seaward part of a geosyncline; characterized by clastic sediments and volcanism.
Euler pole ENDTERM BEGDEF"Pole passing through the center of the earth about which a plate can rotate as a rigid body from any initial to any final position on the earth's surface.
Eustatic change ENDTERM BEGDEF Sea level changes that affect the whole Earth.
Eutrophication ENDTERM BEGDEF" A superabundance of algal life in a body of water; caused by an unusual influx of nitrate, phosphate, or other nutrients. "
Evaporite ENDTERM BEGDEF" A chemical sedimentary rock consisting of minerals precipitated by evaporating waters, especially sal and gypsum. "
Evapotranspiration ENDTERM BEGDEFWater used by plants and animals and subsequently returned directly to the atmosphere.
Evergreen ENDTERM BEGDEF"Coniferous, needle-leaved or sclerophyllous plant that does not echibit a marked seasonal leaf fall."
Exfoliation ENDTERM BEGDEF A physical weathering process in which sheets of rock are fractured and detached from an outcrop.
Exobiology ENDTERM BEGDEF The study of life outside the Earth.
Exogenic cycle ENDTERM BEGDEF"Portion of the earth's system involved in the movement and mass transfer of substances through the cycle of weatherin, erosion,transportation, deposition, burial,diagenesis and uplift. Reservoirs in the cycle generally include the atmosphere, ocean biota, sediments and shallow crust."
Exogenic system ENDTERM BEGDEF"Earth's interacting envelopes: the hydrosphere (mostly the ocean), atmosphere, biosphere,reactive lithosphere (that part of the rocky crust within reach of the processes of weathering). The processes of deep seawater circulation at spreading ridges overlap with endogenic (within the earth) processes. Also relevant are extraterrestrial processes. Refractory problems such as mass extinction usually become arenas for competing theories drawn from the three sources."
Explosion earthquake ENDTERM BEGDEF"Events with ground waves (including P-waves and S-waves) and often air waves, which travel through the air and are transmitted back into the ground in the vicinity of the seismometer."
Extinct volcano ENDTERM BEGDEFA volcano that is not presently erupting and is not likely to do so for a very long time in the future.
Extinction angle ENDTERM BEGDEF" 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. "
Extract assemblage ENDTERM BEGDEFAssemblage of accumulative minerals extracted from a parent magma duting fractional crystallization to produce a daughter magma.
Facies ENDTERM BEGDEF The set of all characteristics of a sedimetary rock that indicates its particular environment of deposition and which distinguish it from other facies in the same rock.
Fallout ENDTERM BEGDEFSettling and deposition of particulate matter such as tephra and volcanic aerosols out of an eruption plume and onto the earth's surface.
Fault-block mountain ENDTERM BEGDEF A mountain or range formed as a horst when it was elevated between parallel normal faults.
Fault plane ENDTERM BEGDEF The plane that best approximates the fracture surface of a fault.
Fault plane ENDTERM BEGDEF"The plane that most closely coincides with the rupture surface of a fault
Fault ENDTERM BEGDEF A planar or gently curved fracture in the Earth's crust across which there has been relative displacement.
Fault ENDTERM BEGDEF"A fracture or zone of fractures in rock along which the two sides have been displaced relative to each other parallel to the fracture. The total fault offset may range from centimeters to kilometers. Large fracture in the ground with two sides displaced relative to each other in drections parallel to the fracture.
Faunal succession ENDTERM BEGDEF"The evolutionary sequence of life forms, especially as recorded by the fossil remains in a stratigraphic sequence. "
Felsic ENDTERM BEGDEF" An adjective used to describe a light-colored igneous rock poor in iron and magnesium content, abundant in feldspars and quartz. "
FEM ENDTERM BEGDEF"Frequency Electromagnetic Method, in which the primary field is sinusoidal in character.
Ferro-magnesian elements ENDTERM BEGDEF"Elements that are readily substituted into silicate minerals that have Fe and Mg as major constituents. Examples include Cr, Ni, V, Sc, Co."
Fiord ENDTERM BEGDEF A former glacial valley with steep walls an a U-shaped profile now occupied by the sea.
Firn ENDTERM BEGDEF"Material that is transitional between snow and glacier ice. It is formed from snow after passing through one summer melt season and becomes glacier ice after its permeability to liquid water falls to zero.
Fissures ENDTERM BEGDEF"Elongated fractures or cracks on the slopes of a volcano. Fissure eruptions typically produce liquid flows,but pyroclastics may also be ejected. "
Flood basalt ENDTERM BEGDEF" A plateau basalt extending many kilometers in flat, layered flows originating in fissure eruptions. "
Flood basalt ENDTERM BEGDEF"Laterally extensive deposits of basaltic lava flows resulting from outpouring of vast volumes of magmas during fissure eruptions.
Flood plain ENDTERM BEGDEF 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 ENDTERM BEGDEF The part of the tide cycle during which the water is rising or leveling off at high water.
Flow cleavage ENDTERM BEGDEF" In a metamorphic rock, the parallel arrangement of all planar or linear crystals as a result of rock flowage during metamorphism. "
Flow rheology ENDTERM BEGDEF"Deformational character of the flow when stress is applied. Rheology varies due to differences in fluid strength, viscosity, density and sediment concentration."
Fluid inclusion ENDTERM BEGDEF A small body of fluid that is entrapped in a crystal and has the same composition as the fluid from which the crystal formed.
Fluid inclusion ENDTERM BEGDEF"Bubble of liquid or gas trapped inside a solid mineral-phase, usually formed during crystallization of the mineral in the presence of a fluid phase. Tiny cavities, 1.0-100 microns in diameter, containing liquid and/or gas,formed by the entrapment of fluid in crystal irregularities. Fluid inclusions provide information on the temperature, pressure,and chemical composition of the fluids and/or gases trapped at the time of mineral formation."
Flume ENDTERM BEGDEF A laboratory model of stream flow and sedimentation consisting of a rectangular channel filled with sediment and running water.
Fluorescence ENDTERM BEGDEF"Emission of a certain wavelength light resulting from and occurring only during the absorption of light of another wavelength.
Focus (earthquake) ENDTERM BEGDEF The point at which the rupture occurs; synonomous with hypocenter.
Fold ENDTERM BEGDEF" A planar feature, such as a bedding plane, that has been strongly warped, presumably by deformation. "
Foliation ENDTERM BEGDEF" Any planar set of minerals or banding of mineral concentrations including cleavage, found in a metamorphic rock. "
Foraminifer ENDTERM BEGDEF A class of oceanic protozoa most of which have shells composed of calcite.
Foraminiferal ooze ENDTERM BEGDEF A calcareous sediment composed of the shells of dead Foraminifera.
Foreshocks ENDTERM BEGDEF"Smaller earthquakes preceding the largest earthquake of a series concentrated in a restricted crustal volume.
Formation ENDTERM BEGDEF The basic unit for the naming of rocks in stratigraphy
Forset bed ENDTERM BEGDEF One of the inclined beds found in crossbedding; also an inclined bed deposited on the outer front of a delta.
Fossil fuel ENDTERM BEGDEF" 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. "
Fossil ENDTERM BEGDEF" 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. "
Free oscillation ENDTERM BEGDEF The ringing or periodic deformation of the whole Earth at characteristic low frequencies after a major earthquake.
Friction breccia ENDTERM BEGDEF A breccia formed in a fault zone or volcanic pipe by the relative motion of two rock bodies.
Fringing reef ENDTERM BEGDEF A coral reef that is directly attached to a landmass not made of coral.
Fumarole ENDTERM BEGDEF" A small vent in the ground from which volcanic gases and heated groundwater emerge, but not lava. "
Fumarole ENDTERM BEGDEF"A vent or opening through which issue steam, hydrogen sulfide or other gases. The craters of many dormant volcanoes contain active fumaroles. "
Fumarolic emission ENDTERM BEGDEF"Low-level and generally steady output of low-temperature gases such as sulfur, halogens,water vapor and carbon dioxide from dormant volcanoes and geothermal fields in volcanic regions."
Gabbro ENDTERM BEGDEF" A black, coarse-grained, intrusive igneous rock, composed of calcic feldspars and pryoxene. The intrusive equivalent of basalt. "
Galvanic interaction ENDTERM BEGDEF"Redox reaction between two mineral entities with different rest potential, where the mineral with lower rest potential acts as reductant and the mineral with the higher rest potential acts as oxidant/reductant in conveying electrons to oxygen or some other suitable acceptor.
Gas-to-particle reaction ENDTERM BEGDEF"The formation of particles by the rapid condensation of a gas.BEGDEF"
Gaussian coefficients ENDTERM BEGDEF"Coefficients of a spherical harmonic expansion of the potential function of the geomagnetic field.
Geochronology ENDTERM BEGDEF" 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. "
Geodetic ENDTERM BEGDEF"Pertaining to measurement of the shape and dimensions of the earth
Geodimeter ENDTERM BEGDEF"A surveying instrument to measure the distance between two points on the Earth's surface
Geoid ENDTERM BEGDEF"Level (or equipotential) surface at mean sea level. Surface of constant gravitational potential that is chosen to define the earth's shape. At sea the geoid corresponds to the time-averaged surface of the ocean (to an accuracy of 1-2 m).
Geologic cycle ENDTERM BEGDEF" 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. "
Geomagnetic field ENDTERM BEGDEF"Main magnetic field of the earth -- the part generated within the earth.
Geomagnetic polarity time scale ENDTERM BEGDEF"Reference for the polarity history of the geomagnetic field, including current correlations of polarity intervals to biozonations, geologic stages and numerical ages.
Geomorphic cycle ENDTERM BEGDEF" 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 ENDTERM BEGDEF The science of surface landforms and their interpretation on the basis of geology and climate.
Geomorphology ENDTERM BEGDEF"The study of present-day landforms, including their classification, description, nature, origin development, and relationships to underlying structures. Also the history of geologic changes as recorded by these surface features. The term is sometimes restricted to features produced only by erosion and deposition.
Geophysical noise ENDTERM BEGDEF"Fluctuations in a geophysical parameter that are of a quasirandom nature and cannot be attributed to fluctuations in known causative phenomena.
Geopotential model ENDTERM BEGDEF"Set of coefficients of a series representation of the earth's gravitational field in terms of spherical harmonics.
Geopressured systems ENDTERM BEGDEF"Hydrothermal systems contained in sedimentary rock in regions of normal geothermal gradient. They represent a special case in which the pore fluids are under pressure exceeding the hydrostatic pressure.
Geosyncline ENDTERM BEGDEF" 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 ENDTERM BEGDEF A curving surface within Earth along which the temperature is constant.
Geothermal power ENDTERM BEGDEFPower generated by using the heat energy of the earth.
Geyser ENDTERM BEGDEF 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 abrasion ENDTERM BEGDEFA common mechanical weathering process where rock and debris frozen into the sides and bottom of a glacier act like sandpaper and wear down the bedrock the glacier is mowing across.
Glacial maximum ENDTERM BEGDEF"The position or time of the greatest advance of a glacier (e.g., the greatest equatorward advance of Pleistocene glaciation).
Glacial rebound ENDTERM BEGDEF" 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 rebound ENDTERM BEGDEF"The isostatic adjustment of previously glaciated areas after glacial retreat (e.g., the uplift of Scandinavia after the most recent glaciation.
Glacial striations ENDTERM BEGDEF" Scratches left on bedrock and boulders by overriding ice, and showing the direction of motion. "
Glacial valley ENDTERM BEGDEF" A valley occupied or formerly occupied by a glacier, typically with a U-shaped profile. "
Glacier surge ENDTERM BEGDEF" A period of unusually rapid movement of one glacier, sometimes lasting more than a year. "
Glacier ENDTERM BEGDEF" 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 ENDTERM BEGDEF"A mass of land ice that is formed by the cumulative recrystallization of firn. A glacier flows slowly from an accumulation area to an ablation area. Some well-known glaciers are: the Zermatt, Stechelberg, Grinelwald, and Les Diablerets in Switzerland; the Nigards, Gaupne, Fanarak, and Lom in Norway; the Wright Taylor and Wilson Piedmont glaciers in Antarctica; Grinnell glacier in Glacier National Park, Montana, U.S., and the Teton glacier in Teton National Park, Wyoming, U.S.
Glaclolacustrine ENDTERM BEGDEF"Used to describe sediments deposited in a lake in contact with or receiving most of its water and sediments from a glacier, and also landforms resulting from such deposition.
Glass ENDTERM BEGDEF A rock formed when magma is too rapidly cooled (quenched) to allow crystal growth.
Glassiness ENDTERM BEGDEF The content of extent of glass in an igneous rock.
Gneiss ENDTERM BEGDEF A coarse-grained regional metamorphic rock that shows compositional banding and parallel alignment of minerals.
Graben ENDTERM BEGDEF A downthrown block between two normal faults of parallel strike but converging dips; hence a tensional feature. See also horst.
Graben ENDTERM BEGDEFA relatively depressed crustal unit bounded by faults on its long sides. Grabens may range in scale from meters to many kilometers. Rift valley becomes the preferred term when dimensions approach the thickness of the brittle crust (>30 km).
Graded bedding ENDTERM BEGDEF" 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 ENDTERM BEGDEF" A ~;tream 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 ENDTERM BEGDEF" A coarse-grained, intrusive igneous rock composed of quartz, orthoclase feldspar, sodic plagioclase feldspar, and micas. Also sometimes a metamorphic product. "
Granitization ENDTERM BEGDEF The formation of metamorphic granite from other rocks by recrystallization with or without complete melting.
Granular snow ENDTERM BEGDEF Snow that has been metamorphosed into small granules of ice.
Granulite ENDTERM BEGDEF A metamorphic rock with coarse interlocking grains and little or no foliation.
Gravel ENDTERM BEGDEF" The coarsest of alluvial sediments, containing mostly particles larger than 2 mm in size and including cobbles and boulders. "
Gravity anomaly ENDTERM BEGDEF" 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 ENDTERM BEGDEF The measurement of gravity at regularly-spaced grid points with repetitions to control instrument drift.
Great earthquake ENDTERM BEGDEFAn earthquake having a magnitude of 8 or greater on the Richter scale.
Greenhouse effect ENDTERM BEGDEF 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.
Greenhouse or icehouse ENDTERM BEGDEF"Useful polarized concepts of the state of the planet. Greenhouse collects together global warmth, flattened thermal gradients, increased humidity, high sea level and reduced vigor of oceanic circulation with a tendency toward enhanced anoxia in some parts of the ocean. Icehouse is essentially the reverse state in that assemblage of generalizations. It is important to add not-so-icecap-prone and icecap-prone, respectively, without implying necessarily that the world was ever ice-free.
Greenschist ENDTERM BEGDEF" A metamorphic schist containing chlorite and epidote (which are green) and formed by low-temperature, low-pressure metamorphism. "
Grenz ENDTERM BEGDEF"A soil horizo, frequently marked by a bed of clay that results from a temporary halt in the accumulation of vegetal material.
Ground moraine ENDTERM BEGDEF" A glacial deposit of till with no marked relief, interpreted as having been transported at the base of the ice. "
Groundwater ENDTERM BEGDEF" 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. "
Groundwater ENDTERM BEGDEF"The supply of fresh water found beneath the surface of the Earth (usually in aquifers) that often supplies wells and springs.
Gully ENDTERM BEGDEF A small steep-sided valley or erosional channel from 1 meter to about 10 meters across.
Guyot ENDTERM BEGDEF A flat-topped submerged mountain or sea-mount found in the ocean.
Gyre ENDTERM BEGDEF" The circular rotation of the waters of each major sea, driven by prevailing winds and the Coriolis effect. "
Half-life ENDTERM BEGDEF The time required for half of a homogeneous sample of radioactive material to decay.
Hanging valley ENDTERM BEGDEF" A former glacial tributary valley that enters a larger glacial valley above its base, high up on the valley wall. "
Hard water ENDTERM BEGDEF Water that contains sufiqcient dissolved calcium and magnesium to cause a carbonate scale to form when the water is boiled or to prevent the sudsing of soap.
Harmonic tremor ENDTERM BEGDEFA continuous release of seismic energy typically associated with the underground movement of magma. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault. The seismic signal is characterized by a nearly monotonic sinusoidal appearance and low frequency (0.5-5 Hz).
Heat conduction ENDTERM BEGDEF" 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 ENDTERM BEGDEF A device that transfers heat from a place of high temperature to a place of lower temperature and does mechanical work in the process.
Heat sink ENDTERM BEGDEF"That portion of a thermodynamic system that absorbs unused heat.
Hertz ENDTERM BEGDEF"The unit of frequency equal to one cycle per second.
Hill ENDTERM BEGDEF" 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. "
Historical biogeography ENDTERM BEGDEF"Discipline that deals with the ever-changing distribution of organisms, marine and non-marine, over the earth's surface through geologic time; i.e., the historical counterpart of present-day biogeography, which discusses the modern distribution of all living organisms.
Hogback ENDTERM BEGDEF" 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. "
Holocene ENDTERM BEGDEF"The most recent epoch of the Quaternary period covering approximately the last 10,000 years.
Homopycnal ENDTERM BEGDEF"Describes water entering a lake that has the same density as the lake water.
Hooke's Law ENDTERM BEGDEF 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 ENDTERM BEGDEF" A high-temperature, low-pressure metamorphic rock of uniform grain size showing no folia-tion. Usually formed by contact metamorphism. "
Horst ENDTERM BEGDEF" An elongate, elevated block of crust forming a ridge or plateau, typically bounded by parallel, outward-dipping normal faults. "
Hot spring ENDTERM BEGDEF A spring whose waters are above both human body and soil temperature as a result ofpluto-nism at depth.
Hotspot ENDTERM BEGDEF"Name commonly given to the surface expression (uplift, volcanism, high-heat flow) of an upwelling mantle plume.
Humic acids ENDTERM BEGDEF"Suite of organic acids (RCOOH) of indefinite composition and molecular structure produced by decomposition and condensation reactions. Operationally defined as being solubilized from peat, humus, soil, sediment or other material by dilute-alkali extraction.
Humus ENDTERM BEGDEF The decayed part of the organic matter in a soil.
Hydration ENDTERM BEGDEF" A chemical reaction, usually in weathering, which adds water or OH to a mineral structure. "
Hydraulic conductivity ENDTERM BEGDEF A measure of the permeability of a rock or soil
Hydrocarbon ENDTERM BEGDEF An organic chemical compound made up of carbon and hydrogen atoms arranged in chains or rings.
Hydrologic cycle ENDTERM BEGDEF" 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 ENDTERM BEGDEF 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.
Hydrosphere ENDTERM BEGDEF"The aqueous envelope of the Earth, including the oceans, freshwater lakes, rivers, saline lakes and inland seas, soil moisture and vadose water, groundwaters and atmospheric vapor.
Hydrothermal activity ENDTERM BEGDEF" Any process involving high-temperature groundwaters, especially the alteration and emplacement of minerals and the formation of hot springs and geysers. "
Hydrothermal activity ENDTERM BEGDEF"Interaction of an aqueous solution with rock at high temperature and pressure, leading to alteration of the rock and dissolution of some of the rock minerals and formation of others.
Hydrothermal vein ENDTERM BEGDEF A cluster of minerals precipitated by hydrothermal activity in a rock cavity.
Hyperpycnal ENDTERM BEGDEF"Describes water that is denser than the water in the lake it enters.
Hypocenter ENDTERM BEGDEF The point below the epicenter at which an earthquake actually begins; the focus.
Hypocenter ENDTERM BEGDEFThe calculated location of the focus of an earthquake.
Hypsometric diagram ENDTERM BEGDEF A graph that shows in any way the relative amounts of the Earth's surface at different elevations with regard to sea level.
Ice age ENDTERM BEGDEF"A glacial epoch or time of extensive glacial activity. Also, as Ice Age, which refers to the latest glacial epoch, the Pleistocene Epoch. Periods characterized by very low temperature worldwide and advancing glaciers.
Ice core ENDTERM BEGDEF"Deep drill into the earth's permanent glaciers revealing the history of the atmospheric gas and dust content, enabling deduction of former atmospheric temperatures.
Ice sheet ENDTERM BEGDEF"A glacier of considerable thickness and more than 50,000 sq. km in area. It forms a continuous cover of ice and snow over a land surface. An ice sheet is not confined by the underlying topography but spreads outward in all directions. During the Pleistocene Epoch ice sheets covered large parts of North America and northern Europe but they are now confined to polar regions (e.g., Greenland and Antarctica). Also called continental glacier.
Ice shelf ENDTERM BEGDEF"A sheet of very thick ice with a level or gently undulating surface. It is attached to the land on one side, but most of it is floating. On the seaward side it is bounded by a steep cliff (ice front) two to 50 m or more above sea level. Ice shelves have formed along polar coasts (e.g., Antarctica and Greenland); they are very wide, with some extending several hundreds of kilometers toward the sea from the coastline. They increase in size from annual snow accumulation and seaward extension of land glaciers. They decrease in size from warming, melting and calving.
Ice stream ENDTERM BEGDEF"Large, fast glaciers embedded in slow-moving ice. Flow velocities are a few hundred meters to kilometers per year.
Iceberg ENDTERM BEGDEF"A floating ice mass up to 100 km long and 200-300 m thick that has broken off (calved) from ice shelves, glaciers or coastal ice cliffs into the ocean.
Igneous rock ENDTERM BEGDEF A rock formed by congealing rapidly or slowly from a molten state.
Ignimbrite ENDTERM BEGDEF An igneous rock formed by the lithification of volcanic ash and volcanic breccia.
Impact crater ENDTERM BEGDEF"Circular, rimmed depression produced by the impact of a solid body traveling at more than a few kilometers per second.
Impedance ENDTERM BEGDEF"Apparent resistance to flow of alternating current; it is analagous to electrical resistance in direct current method and is a complex quantity. For plane wave electromagnetic sources. the impedance of the ground is equal to the ratio of the orthogonal horizontal electric and magnetic fields.
Inclination ENDTERM BEGDEF The angle between a line in the Earth's magnetic field and the horizontal plane; also a synonym for dip.
Index of refraction ENDTERM BEGDEF 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.
Induction ENDTERM BEGDEF"Process by which a body becomes magnetized or electrified by merely placing it in a magnetic or electric field. Also refers to the process under which electric currents are initiated in a conductor by merely placing it in an electromagnetic field.
Infiltration ENDTERM BEGDEF The movement of groundwater or hydrothermal water into rock or soil through joints and pores.
Infrared radiation ENDTERM BEGDEF"Electromagnetic radiation lying in the wavelength interval from 0.7 microns to 1000 microns. Its lower limit is bounded by visible radiation and its upper limit is bounded by microwave radiation. Most of the energy emitted by the Earth and its atmosphere is at infrared wavelengths. Infrared radiation is generated almost entirely by large-scale intramolecular processes. The tri-atomic gases, such as water vapor, carbon dioxide, and ozone absorb infrared radiation and play important roles in the propagation of infrared radiation in the atmosphere. Abbreviated IR; also called longwave radiation
Integrated time scale (Eocene) ENDTERM BEGDEF"The Eocene rock record initially was and still is recognized by its characteristic fossils; fossils are cross-checked against the geomagnetic reversal scale. But rates of change and other quantifications demand numerical calibration -- time in millions of years -- which is founded ultimately in isotopic decay in the potassium-argon and other decay series. The geomagnetic scale consists of chrons, of which Chron C24 and Chron C19 are important here. Some fossil successions have their own names; thus, e.g., the Wasatchian North American land mammal age."""
Intensity ENDTERM BEGDEF"A measure of the effects of an earthquake at a particular place on humans, structures and (or) the land itself. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the point and the local geology at that point. "
Interfacial angle ENDTERM BEGDEF" The angle between two crystal faces of a crystal, characteristic of a mineral's symmetry. "
Interior drainage ENDTERM BEGDEF A system of streams that converge in a closed basin and evaporate without reaching the sea.
Intermontane basin ENDTERM BEGDEF" A basin between mountain ranges, often formed over a graben. "
Intrusion ENDTERM BEGDEF An igneous rock body that has forced its way in a molten state into surrounding country rock.
Intrusive rock ENDTERM BEGDEF" Igneous rock that is interpreted as a former intrusion from its cross-cutting contacts, chilled margins, or other field relations. "
Ion ENDTERM BEGDEF An atom or group of atoms that has gained or lost electrons and so has a net electric charge.
Ion ENDTERM BEGDEF"An atom or molecule that has an electric charge.
Ionic bond ENDTERM BEGDEF A bond formed between atoms by electrostatic attraction between oppositely charged ions.
Iron formation ENDTERM BEGDEF" A sedimentary rock containing much iron, usually more than 15 percent as sulfide, oxide, hydroxide, or carbonate; a low-grade ore of iron. "
Isograd ENDTERM BEGDEF A line or curved surface connecting rocks that have undergone an equivalent degree of meta-morphism.
Isoseismal line ENDTERM BEGDEFA line connecting points on the Earth's surface at which earthquake intensity is the same. It is usually a closed curve around the epicenter.
Isostasy ENDTERM BEGDEF" 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 denset mantle. The theory that continents and mountains are supported by low-density crustal roots. "
Isostasy ENDTERM BEGDEFThe vertical readjustment of the surface of the earth due to the addition or removal of weight. Commonly associated with the advance and retreat of glacial ice.
Isostatic adjustment ENDTERM BEGDEF"The process whereby lateral transport at the Earth's surface from erosion or deposition is compensated for by movements in a subcrustal layer to maintain equilibrium among units of varying masses and densities. Also called isostatic compensation.
Isotherm ENDTERM BEGDEF"A line on a chart that connects all points of equal or constant temperature.
Isotope geology ENDTERM BEGDEF The study of the relative abundances of isotopes in rocks to determine their ages (see geo-chronology) or conditions of formation.
Isotope ENDTERM BEGDEF" One of 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 ENDTERM BEGDEF"One of two or more atoms that have the same atomic number (i.e., the same number of protons in their nuclei) but have different mass numbers. Forms of the same element that have the same number of protons but differing numbers of neutrons. This mass difference gives rise to slightly differing chemical properties for each isotope. Of interest here are the stble isotopes of carbon: carbon-12 (or 12C) and carbon-13 (or 13C), which are stable over time and carbon-14, which is a radioisotope and decays with time to an isotope of nitrogen.
Isotopes in paleoceanography ENDTERM BEGDEF"Stable isotopes of some light elements occuring in nature in given ratios fractionated by the biosphere and thereby providing informative signals about the latter. Most useful ratios are 16O/18O; 12C/13C. Marine organisms biomineralize using oxygen in their calcite, etc., and in doing so record the temperature of the reservoir (i.e., the ocean) in the 16O/18O ratio, which is expressed as delta-18O, a departure from an agreed standard. But evaporation removes a bit more 16O -- a lighter fraction -- from the reservoir. That does not matter at geological time scales until that very light fraction is locked up as an icecap, whereupon the reservoir is distorted in the heavier direction. Many believe that that ice effect is not a great distorter in the Eocene; it becomes assertive in the Oligocene. Carbon behaves differently. Photosynthesis removes 12C preferentially, so that calcite skeletons are enriched in 13C. Thus high productivity is signaled as an increased difference between the planktonic record (from the photic zone) and the benthic record (where the light fraction is returned to the system). But when both signals go positive through time that shift signals a removal of organically fixed carbon from the reservoir altogether -- into oceanic sediments, becoming a potential source of oil, or into increased terrestrial plant biomass and thence into coals.
Isotropic substance ENDTERM BEGDEF" One in which the magnitude of a physical property, such as transmission of light is independent of crystallographic direction. "
Joint ENDTERM BEGDEF A large and relatively planar fracture in a rock across which there is no relative displacement of the two sides.
J”kulhlaup ENDTERM BEGDEF"Sudden, often catastrophic, flood discharge resulting from the failure of part of a glacier-ice or glacial-drift barrier that forms part of the basin of a glacial lake.
Juvenile gas ENDTERM BEGDEF Gases that come to the surface for the first time from the deep interior.
Kerogen ENDTERM BEGDEF A mixture of organic substances found in many fine-grained sedimentary rocks and a major constituent of oil shale.
Kerogen ENDTERM BEGDEF"Solid bituminous mineraloid substance in oil shales.
Kettle ENDTERM BEGDEF A small hollow or depression formed in glacial deposits when outwash was deposited around a residual block of ice that later melted.
Kilobar ENDTERM BEGDEF A unit of pressure equal to 1000 bars.
Kimberlite ENDTERM BEGDEF" A peridotite containing garnet and olivine and found in volcanic pipes, through which it may come from the upper Mantle. "
Klippe ENDTERM BEGDEFA remmant of a nappe.
Komatilte ENDTERM BEGDEF"Volcanic rock containing more than 18 perceny by weight MgO.
Krakatoa ENDTERM BEGDEF"Active volcano, elevation 813 m (2667 ft), which forms an island in West Indonesia between Java and Sumatra (6.102 S, 105.423 E). Its eruption in 1883, which was one of the most violent in modern times, scattered debris and darkened skies over vast areas. Additional eruptions occurred in late 1927 and in the 1960s.
Laccolith ENDTERM BEGDEF" 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 ENDTERM BEGDEF" A mudflow of unconsolidated volcanic ash, dust, breccia, and boulders mixed with rain or the water of a lake displaced by a lava flow. "
Lake Missoula ENDTERM BEGDEFGlacial lake whose floods when its ice damn broke created the Grand Coulee. Glaciers impounded Clark Fork River.
Laminar flow ENDTERM BEGDEF A flow regime in which particle paths are straight or gently curved and parallel.
Landslide ENDTERM BEGDEF" 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. "
Landslide ENDTERM BEGDEFAn abrupt movement of soil and bedrock downhill in response to gravity. Landslides can be triggered by an earthquake or other natural causes. Undersea landslides can cause tsunamis.
Langley ENDTERM BEGDEFThe unit of solar energy relating to the amount which reaches a specific area of the earth's surface.
Lapilli ENDTERM BEGDEF 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.
Lapilli ENDTERM BEGDEF"Literally little stones; round to angular rock fragments measuring 1/10 inch to 2 1/2 inches in diameter, which may be ejected in either a solid or molten state. "
Large-ion lithophile elements ENDTERM BEGDEF"Elements with an affinity for oxygen-containing compounds, particularly silicate minerals, and possessing ionic radii larger than most common rock-forming elements. Examples include K, Cs, Th, U, La, Ba
Lateral moraine ENDTERM BEGDEF A moraine formed along the side of a valley glacier and composed of rock scraped off or fallen from the valley sides.
Lateritization ENDTERM BEGDEF"Soil-forming process typical of warm humid climates where in mature landscapes, primary minerals are essentially completely weathered away to form red soils (laterites) of high aggregate stability composed primarily of sesquioxides (oxides and hydroxides of iron and aluminum).
Latitude ENDTERM BEGDEFThe location of a point north or south of the equator. Latitude is shown on a map or globe as east-west lines parallel to the equator.
Lava flow ENDTERM BEGDEFAn outpouring of lava onto the land surface from a vent or fissure. Also a solidified tongue-like or shee- like body formed by outpouring lava.
Lava tube ENDTERM BEGDEF" 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. "
Lava tube ENDTERM BEGDEF"A tunnel formed when the surface of a lava flow cools and solidifies, while the still-molten interior flows through and drains away. "
Lava ENDTERM BEGDEF Magma or molten rock that has reached the surface.
Lava ENDTERM BEGDEFMagma which has reached the surface through a volcanic eruption. The term is most commonly applied to streams of liquid rock that flow from a crater or fissure. It also refers to cooled and solidified rock.
Leaching ENDTERM BEGDEF The removal of elements from a soil by dissolution in water moving downward in the ground.
Leaking mode ENDTERM BEGDEF"A surface seismic wave which is imperfectly trapped so that its energy leaks or escapes across a layer boundary, causing some attenuation or loss of energy. "
Left-lateral fault ENDTERM BEGDEF A strike-slip fault on which the displacement of the far block is to the left when viewed from either side.
Levee ENDTERM BEGDEF" 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) ENDTERM BEGDEF" The relatively planar part of a fold or of two adjacent folds (for example, the steeply dipping part of a stratum between an anticline and syncline). "
Limb ENDTERM BEGDEF"The edge of a celestial body as it appears to an observer; the edge of the sun's disk, for example
Limestone ENDTERM BEGDEF" A sedimentary rock composed principally of calcium carbonate (CaCO2), usually as the mineral calcite. "
Lineation ENDTERM BEGDEF Any linear arrangement of features found in a rock.
Liquefaction ENDTERM BEGDEFThe process in which a solid (soil) takes on the characteristics of a liquid as a result of an increase in pore pressure and a reduction in stress.
Liquid of descent ENDTERM BEGDEF"In a graph of chemical composition, a line connecting magmas related by differentiation to a common parent. The chemical expression of magmatic evolution.
Lithification ENDTERM BEGDEF The processes that convert a sediment into a sedimentary rock.
Lithology ENDTERM BEGDEF" The systematic description of rocks, in terms of mineral composition and texture. "
Lithosphere ENDTERM BEGDEF" The outer, rigid shell of the Earth, situated above the asthenosphere and containing the crust, continents, and plates. "
Lithosphere ENDTERM BEGDEF"The component of the Earth's surface comprising the rock, soil and sediments. It is a relatively passive component of the climate system and its physical characteristics are treated as fixed elements in the determination of climate. Semirigid upper platelike layer of the earth. Its thickness is near zero at oceanic ridge axes, but it increases with age to about 100 km. Oceanic lithosphere consists of a crustal layer ~5 km thick of basaltic composition underlain by mantle rocks. Continental lithosphere consists of a layer of crustal rock 30-60 km thick of low density and variable composition underlain by mantle rocks.
Lode ENDTERM BEGDEF An unusually large vein or set of veins containing ore minerals.
Loess ENDTERM BEGDEF"A buff-colored, wind-blown deposit of fine silt, which is frequently exposed in bluffs with steep faces. The thickness can range from 6 to 30 m. The loess of the United States and Europe is thought to be the fine materials first transported and deposited by the waters of melting ice sheets during the glacial period. It was later blown considerable distances with in some cases deposition in lakes. The origin of Asiatic loess however is apparently wind-blown dust from central Asian deserts.
Long-period event ENDTERM BEGDEF"Discrete events with nearly monotonic, low-frequency (1-5 Hz) waveforms that resonate for many cycles.
Longitude ENDTERM BEGDEF"The location of a point east or west of the prime meridian. Longitude is shown on a map or globe as north-south lines left and right of the prime meridian, which passes through Greenwich, England. "
Longitudinal dune ENDTERM BEGDEF A long dune parallel to the direction of the prevailing wind.
Longitudinal profile ENDTERM BEGDEF" A cross section of a stream from its mouth to its head, showing elevation versus distance to the mouth. "
Longshore current ENDTERM BEGDEF A current that moves parallel to a shore and is formed from the momentum of breaking waves that approach the shore obliquely.
Longshore drift ENDTERM BEGDEF The movement of sediment along a beach by swash and backwash of waves that approach the shore obliquely.
Lopolith ENDTERM BEGDEF" A large laccolith that is bowl-shaped and depressed in the center, possibly by subsidence of an emptied magma chamber beneath the intrusion. "
Love wave ENDTERM BEGDEF"A major type of surface wave having a horizontal motion that is shear or transverse to the direction of propagation (travel). It is named after A.E.H. Love, the English mathematician who discovered it. "
Low-frequency event ENDTERM BEGDEF"General term for discrete events with either enriched low frequencies or deficient high frequencies compared to typical earthquakes.
Low-velocity zone ENDTERM BEGDEF" A region in the Earth, especially a planar layer, that has lower seismic-wave velocities than the region immediately above it. "
Low-velocity zone ENDTERM BEGDEFAny layer in the Earth in which seismic wave velocities are lower than in the layers above and below.
Lowland ENDTERM BEGDEF Land of general low relief at the lower levels of regional elevation.
Luster ENDTERM BEGDEF" The general textural impression of a mineral surface, given by the light reflected from it. Terms such as metallic, submetallic are standardized but subjective. "
M.y. ENDTERM BEGDEF Abbreviation for million years.
Maar volcano ENDTERM BEGDEF" A volcanic crater without a cone, believed to have been formed by an explosive eruption of trapped gases. "
Mafic mineral ENDTERM BEGDEF" A dark-colored mineral rich in iron and magnesium, especially a pyroxene, amphibole, or olivine. "
Magma chamber ENDTERM BEGDEF A magma-filled cavity within the lithosphere.
Magma chamber ENDTERM BEGDEFThe subterranean cavity containing the gas-rich liquid magma which feeds a volcano.
Magma ENDTERM BEGDEF Molten rock material that forms igneous rocks upon cooling. Magma that reaches the surface is referred to as lava.
Magma ENDTERM BEGDEF"Naturally occurring liquid rock often containing volatiles, such as water or carbon dioxide. Igneous rocks form by solidification of magma, usually by freezing. Molten silicate rock with dissolved gases. If erupted and degassed it is termed lava.
Magmatic water ENDTERM BEGDEF Water that is dissolved in a magma or that is derived from such water.
Magnetic anomaly ENDTERM BEGDEF The value of the local magnetic field remaining after the subtraction of the dipole portion of the Earth's field.
Magnetic coupling ENDTERM BEGDEF" The transfer of momentum between celestial bodies, especially dust and gas clouds, through magnetic forces. "
Magnetic diffusivity ENDTERM BEGDEF"Constitutive pressure associated with the diffusive manner in which magnetic field lines slip through electrically conducting fluid as a result of the action of dissipation caused by finite electrical reisitance.
Magnetic field lines ENDTERM BEGDEF"Mathematical construction: a family of curves in space giving at each point the direction (by the tangent of the line) and the strength (by the local density of lines) of a magnetic field.
Magnetic jerk ENDTERM BEGDEF"Change in the slope of the geomagnetic secular variation. The change occurs within a few years and is global in extent.
Magnetic north pole ENDTERM BEGDEF (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 ENDTERM BEGDEF The study and correlation of polarity epochs and events in the history of the Earth's magnetic field as contained in magnetic rocks.
Magnetometer ENDTERM BEGDEF An instrument for measuring either one orthogonal component or the entire intensity of the Earth's magnetic field at various points.
Magnitude ENDTERM BEGDEF" 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. "
Magnitude ENDTERM BEGDEF"A measure of the strength of an earthquake or strain energy released by it as determined by seismographic observations. This is a logarithmic value originally defined by Charles Richter (1935). An increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released. In other words a magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake -- or it takes about 900 magnitude 4.7 earthquakes to equal the energy released in a single 6.7 earthquake! There is no beginning nor end to this scale. However, rock mechanics seems to preclude earthquakes smaller than about -1 or larger than about 9.5. A magnitude -1.0 event releases about 900 times less energy than a magnitude 1.0 quake. Except in special circumstances, earthquakes below magnitude 2.5 are generally not felt by humans. "
Major earthquake ENDTERM BEGDEFAn earthquake having a magnitude of 7 to 7.99 on the Richter scale.
Manganese nodule ENDTERM BEGDEF" 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 ENDTERM BEGDEF" 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. "
Mantle ENDTERM BEGDEFThe layer of rock that lies between the crust and the outer core of the Earth. It is approximately 2900 kilometers thick and is the largest of the Earth's major layers.
Mass balance ENDTERM BEGDEF"The application of the principle of the conservation of matter. For example, the mass of a glacier is not destroyed or created; the mass of a glacier and all its constitutive components remains the same despite alterations in their physical states. The mass balance of a glacier is calculated with the input/output relationships of ice, firn and snow, usually measured in water equivalent. Output includes all ablative processes of surface melting basal melting evaporation wind deflation calving and internal melting. Input includes direct precipitation avalanching and the growth of superimposed ice.
Mass movement ENDTERM BEGDEF A downhill movement of soil or fractured rock under the force of gravity.
Mass spectrometer ENDTERM BEGDEF An instrument for separating ions of different mass but equal charge (mainly isotopes in geology) and measuring their relative quantities.
Massif ENDTERM BEGDEF"An isolated block within a group of mountains. It indicates that a mountain is a segment of crystalline basement, or a pluton which survived the erosional processes that wore down the rock that originally surrounded it."
Massive rock ENDTERM BEGDEF" A rock that is little or not at all broken by joints, cracks, foliation, or bedding, tending to present a homogeneous appearance. "
Massive sulfide ore deposit ENDTERM BEGDEF"Occurrence of a concentrated mass of sulfide minerals such as pyrite, sphalerite or chalcopyrite in one place, as opposed to their being disseminated or occurring in veins.
Maturity ENDTERM BEGDEF A stage in the geomorphic cycle in which maximum relief and well-developed drainage are oth present.
Mauna Loa ENDTERM BEGDEF"An intermittently active volcano, elevation 4170 m (13,680 ft) on the island of Hawaii (19.475 N, 155.608 W); the last eruption was in 1984. Also see Mauna Loa record.
Mean sea level ENDTERM BEGDEF"The average height of the sea surface based upon hourly observation of the tide height on the open coast or in adjacent water that have free access to the sea. In the United States, it is defined as the average height of the sea surface for all stages of the tide over a nineteen year period. Mean sea level, commonly abbreviated MSL and referred to simply as sea level, serves as the reference surface for all altitudes in upper atmospheric studies.
Meander ENDTERM BEGDEF" 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 ENDTERM BEGDEF The set of all physical processes by which an outcrop is broken up into small particles.
Medial moraine ENDTERM BEGDEF 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 ENDTERM BEGDEF" 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 ENDTERM BEGDEF The lower mantle.
Metamorphic ENDTERM BEGDEFFrom the Greek meta (change) and morph (form). Commonly occurs to rocks which are subjected to increased heat and/or pressure. Also applies to the conversion of snow into glacial ice.
Metamorphism ENDTERM BEGDEF The changes of mineralogy and texture imposed on a rock by pressure and temperature in the Earth's interior. Meteoric water
Meteorite ENDTERM BEGDEF A stoney or metallic object from inter-planetary space that penetrates the atmosphere to impact on the surface.
Microearthquake ENDTERM BEGDEFAn earthquake having a magnitude of 2 or less on the Richter scale.
Micrometeorite ENDTERM BEGDEF A meteorite less than 1 millimeter in diameter.
Micromorphology ENDTERM BEGDEF"Texture of a soil as viewed in petrographic thin section.
Microseism ENDTERM BEGDEF" 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. "
Microseism ENDTERM BEGDEFA more or less continuous motion in the Earth that is unrelated to an earthquake and that has a period of 1.0 to 9.0 seconds. It is caused by a variety of natural and artificial agents.
Migmatite ENDTERM BEGDEF 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.
Milankovitch theory ENDTERM BEGDEF"An astronomical theory formulated by the Yugoslav mathematician Milutin Milankovitch that associates climate change with fluctuations in the seasonal and geographic distribution of insolation determined by periodic variation of the Earth's eccentricity and obliquity and the longitude of perihelion.
Mineral resource analysis ENDTERM BEGDEF"Probabilistic modeling of occurrences and sizes of undiscovered deposits for mineral and energy resources evaluation and exploration.
Mineral ENDTERM BEGDEF A naturally occurring element or compound with a precise chemical formula and a regular internal lattice structure. Organic products are usually not included.
Mineral ENDTERM BEGDEF"A naturally occurring, inorganic, crystalline solid with a definite internal structure and chemical composition. "
Mineralogy ENDTERM BEGDEF" The study of mineral composition, structure, appearance, stability, occurrence, and associations. "
Miogeosyncline ENDTERM BEGDEF A Geosyncline that is situated near a craton and receives chemical and well-sorted elastic sediments from the continent.
Model age ENDTERM BEGDEF"Age of a rock or mineral that is determined by isotopic methods in which the initial isotopic ratio of the sample assumed.
Modeling ENDTERM BEGDEF"An investigative technique that uses a mathematical or physical representation of a system or theory that accounts for all or some of its known properties. Models are often used to test the effects of changes of system components on the overall performance of the system
Modified Mercalli intensity scale ENDTERM BEGDEFProposed in 1902 by Giuseppi Mercalli of Italy; updated in 1931 by H.O. Wood and F. Neumann as Modified Mercalli; further revised in 1956 by C. F. Richter and presented here in abridged form. Earthquake intensity on the MM scale is given as Roman numerals to prevent confusion with Richter magnitudes and refers to the earthquakes maximum intensity.
Mohorovic discontinuity ENDTERM BEGDEF" The boundary between crust and mantle, marked by a rapid increase in seismic wave velocity to more than 8 kilometers per second. Depth"
Mohorovicic discontinuity ENDTERM BEGDEF"The boundary surface or sharp seismic-velocity discontinuity (pronounced Mo-ho-ro-vi-chich) that separates the Earth's crust from the underlying mantle. Named for Andrija Mohorovicic, the Croatian seismologist who first suggested its existence. "
Mohs scale of hardness ENDTERM BEGDEF" 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 ENDTERM BEGDEF An isolated hill or mountain rising above a peneplain.
Monocline ENDTERM BEGDEF The S-shaped fold connecting two horizontal parts of the same stratum at different elevations. Its central limb is usually not overturned.
Moraine ENDTERM BEGDEF" 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 ENDTERM BEGDEF A steep-sided topographic elevation larger than a hill; also a single prominence forming part of a ridge or mountain range.
Mudflow ENDTERM BEGDEF" A mass movement of material finer than sand, lubricated with large amounts of water. Mudstone"
Mudflow ENDTERM BEGDEFA flow of water-saturated earth material possessing a high degree of fluidity during movement. A less-saturated flowing mass is often called a debris flow. A mudflow originating on the flank of a volcano is properly called a lahar.
Multivariate analysis ENDTERM BEGDEF"Simultaneous statistical analysis with respect to more than two variables; includes regression analysis for expressing a dependent variable in terms of explanatory variables with diagnostics for influence of observations and variables.
Mylonite ENDTERM BEGDEF A very fine lithified fault breccia commonly found in major thrust faults and produced by shearing and rolling during fault movement.
Nappe ENDTERM BEGDEF"A large body of rock that has been moved by gravity, thrust faulting, or any other mechansm some distance from its place of origin."
Native metal ENDTERM BEGDEF" A natural deposit of a metallic element in pure metallic form, neither oxidized nor combined with sulfur or other elements. "
Neap tide ENDTERM BEGDEF" 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 ENDTERM BEGDEF" An immense, diffuse body of interstellar gas and dust that has not condensed into a star. "
Nebular hypothesis ENDTERM BEGDEF" 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-activation analysis ENDTERM BEGDEF A method of identifying isotopes of an element by bombarding them with neutrons and observing the characteristic radioactive decay products emitted.
Neutron ENDTERM BEGDEF An electrically neutral elementary particle in the atomic nucleus having the mass of one proton.
Norite ENDTERM BEGDEFAn igneous rock with mafic composition.
Normal fault ENDTERM BEGDEF A dip-slip fault in which the block above the fault has moved downward relative to the block below.
Normal modes of vibration ENDTERM BEGDEF"Natural resonances on the coupled periodic displacements of atoms in a material about their equilibrium positions.
Normal polarity ENDTERM BEGDEF"Geomagnetic polarity that is the same as the present polarity.
Nu‚e ardente ENDTERM BEGDEFA French term applied to a highly heated mass of gas-charged ash which is expelled with explosive force down the mountainside; a significant volcanic hazard.
Nunatak ENDTERM BEGDEFIslands of mountain tops that pertrude above glaciers.
Oblique-slip fault ENDTERM BEGDEF A fault that combines some strike slip motion with some dip-slip motion.
Obsidian ENDTERM BEGDEF Dark volcanic glass of felsic composition.
Obsidian ENDTERM BEGDEFA black or dark-colored volcanic glass usually of rhyolitic (felsic) composition.
Oceanic magnetic anomalies ENDTERM BEGDEF"Systematic fluctuations in magnetic field intensity that are a result of the juxtaposition of ocean floor formed (and magnetized) during intervals of normal and reversed geomagnetic polarity.
Octahedral coordination ENDTERM BEGDEF The packing of six ions around an ion of opposite charge to form an octa-hedron.
Oil field ENDTERM BEGDEF" An underground accumulation of oil and gas concentrated beneath an impermeable trap, preventing its escape upward. "
Oil shale ENDTERM BEGDEF A dark-colored shale containing organic material that can be crushed and heated to liberate gaseous hydrocarbons.
Old age ENDTERM BEGDEF" A stage in the geomorphic cycle, characterized by formation of a peneplain near sea level. "
Olivine ENDTERM BEGDEF"Mineral of (MgFe)2SiO4 composition. It is the main component of the upper mantle.
Oncolites ENDTERM BEGDEF"Small accumulations of algae that formed in warm shallow water and were rolled back and forth by currents to become shaped liked little balls. (Found in 350Myr Lower Paleozoic Mississippian Leadville Limestone at Horseshoe Cirque, Fairplay Co)"
Oolite ENDTERM BEGDEF 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 ENDTERM BEGDEF" A mineral which transmits no light through a thin section under a microscope. Usually a native metal, sulfide, or metallic oxide mineral. "
Open hydrochemical system ENDTERM BEGDEF"Portion in which the mass of a chemical element is not conserved locally and migrates across the boundaries of the system in solution as a solute or as a suspended mineral particle.
Ophiolite suite ENDTERM BEGDEF An assemblage of mafic and ultra-mafic igneous rocks with deep-sea sediments supposedly associated with divergence zones and the sea-floor environment.
Ophiolite ENDTERM BEGDEF"Assemblage of rock units including pillow basalts, diabase sheeted dikes, gabbros and serpentized periodites lying uncomformably on continental lithosphere, which likely represent sections of oceanic lithosphere accreted to the continent during plate collision. Body of rock composed of some or all of the following principal rock units: basaltic lava; sheeted diabase dikes; cumulate gabbro, norite, pyroxenite, and periodite; metamorphic-textured herzolite,harzburgite and dunite. Layered suite of basic and ultrabasic silicate rocks consisting of pillow basalts, basaltic dikes, gabbros and periodites that are thought to represent segments of oceanic lithosphere that have been thrust onto a convergent plate margins."
Orbicular rocks ENDTERM BEGDEFEllipsoidal-shaped masses of rock consisting of successive shells of dark minerals (biotite) and light minerals (feldspar). 200 known localities WW
Orbit ENDTERM BEGDEF The elliptical or hyperbolic path traced by a planet or meteorite or satellite in the presence of a more massive body.
Order of magnitude ENDTERM BEGDEF"Ten times; thus an increase of two orders of magnitude is 10 x 10 or 100 times.
Ore mineral ENDTERM BEGDEF The mineral of an ore that contains the useful element.
Ore ENDTERM BEGDEF A natural deposit in which a valuable metallic element occurs in high enough concentration to make mining economically feasible.
Orogenic belt ENDTERM BEGDEF" A linear region, often a former geo-syncline, that has been subjected to folding, and other deformation in a mountain-building episode. "
Orogenic zone ENDTERM BEGDEF"Linear region of the crust, where the lithosphere is relatively mobile and deformed into high-standing mountain belts. Most commonly forms in association with subduction zones or continental collisions.
Orogeny ENDTERM BEGDEF" 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 ENDTERM BEGDEF A ripple with a symmetrical cross section and a sharp peak formed by waves.
Outcrop ENDTERM BEGDEF A segment of bedrock exposed to the atmosphere.
Outgassing ENDTERM BEGDEF The release of juvenile gases to the atmosphere and oceans by volcanism.
Outwash ENDTERM BEGDEF A glaciofluvial sediment that is deposited by meltwater streams emanating from a glacier. Overturned fold
Oxidation ENDTERM BEGDEF A chemical reaction in which electrons are lost from an atom and its charge becomes more positive.
Oxidized element ENDTERM BEGDEF An element occurring in the more positively charged of two common ionic forms.
P-wave ENDTERM BEGDEF" 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. "
P wave ENDTERM BEGDEF"Primary, longitudinal, irrotational, push, pressure, dilatational, compressional or push-pull wave. P waves are the fastest body waves and arrive at stations before the S waves or secondary waves. The waves carry energy through the Earth as longitudinal waves, moving particles in the same line as the direction of the wave. P waves can travel through all layers of the Earth. P waves are generally felt by humans as a bang or thump."
Pahoehoe ENDTERM BEGDEF" A basaltic lava flow with a glassy, smooth, and undulating, or ropy, surface. "
Pahoehoe ENDTERM BEGDEFHawaiian term for a fluid volcanic eruption resulting in broad basaltic shield volcanoes. The highly fluid magma flows readily and hardens into ropey forms as it cools.
Paleoclimate ENDTERM BEGDEF The average state or typical conditions of climate during some past geologic period.
Paleocompetence ENDTERM BEGDEF"Competence is a measure of a stream's abiility to entrain sediment particles, measured by the maximum grain size that can be transported. Empirical relationships between grain size and flow velocity or stream power are applied to paleoflood deposits to derive estimates of these same paleoflow parameters.
Paleocurrent map ENDTERM BEGDEF" A map of depositional currents that have been inferred from cross-bedding, ripples, or other sedimentary structures. "
Paleogeographic map ENDTERM BEGDEF A map showing the surface landforms and coastline of an area at some time in the geologic past.
Paleogeography ENDTERM BEGDEF"Working out by means of geologic criteria (chiefly chemically sensitive sedimentary rocks and environmentally sensitive rocks and structures, such as nonmarine shoreline or deep-water types) and structural geologic evidence for the displacement of rock bodies that originally were continuous geophysical criteria (chiefly the evidence of remanent magnetism, which provides data regarding the original latitudinal position of rock bodies,  although now whether that latitude is north or south) and paleontologic criteria (chiefly the evidence of historical biogeography that helps to establish which rock bodies contain fossils suggesting original reproductive communication between same) of the ever-changing geographies of the past.
Paleomagnetism ENDTERM BEGDEF The science of the reconstruction of the Earth's ancient magnetic field and the positions of the continents from the evidence of remanent magnetization in ancient rocks.
Paleomagnetism ENDTERM BEGDEF"The natural magnetic traces that reveal the intensity and direction of Earth's magnetic field in the geologic past. Also, the study of these magnetic traces. "
Paleontology ENDTERM BEGDEF" The science of fossils, of ancient life-forms, and their evolution. "
Paleorift ENDTERM BEGDEF"Dead or dormant rift structure with no remaining transient or thermal phenomena (recent or contemporary magmatism or enhanced heat flow)
Paleoseismology ENDTERM BEGDEF"Study of prehistoric earthquakes, often by excavation across an active fault to examine evidence of past surface-rupturing events.
Paleowind ENDTERM BEGDEF" 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 ENDTERM BEGDEF" 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 ENDTERM BEGDEF A hypothetical primaeval ocean covering two-thirds of the world except for the continent of Pangaea.
Parent element ENDTERM BEGDEF An element that is transformed by radioactive decay to a different (daughter) element.
Partitioning ENDTERM BEGDEF"Process by which a given element distributes itself to different extents in two ohases in equilibrium with each other. The ratio of the concentration of the element in one phase limits and is known as the Nernst partition coefficient.
Peat ENDTERM BEGDEF A marsh or swamp deposit of water-soaked plant remains containing more than 50 percent carbon.
Pedalfer ENDTERM BEGDEF" 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 ENDTERM BEGDEF" 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 ENDTERM BEGDEF" A common soil type of arid regions, characterized by accumulation of calcium carbonate in the A-horizon. "
Pegmatite ENDTERM BEGDEF" An igneous rock with extremely large grains, more than a centimeter in diameter. It may be of any composition but most frequently is granitic. "
Pegmatold ENDTERM BEGDEF"Name given to a coarse-grained rock in which minerals have a crystalline outline and range from one to several centimeters in size, as in pegmatite, but which lacks the large proportions of quartz and orthoclase that characterize pegmatite.
Pelagic sediment ENDTERM BEGDEF" Deep-sea sediments composed of fine-grained detritus that slowly settles from surface waters. Common constituents are clay, radiolarian ooze, and foraminiferal ooze. "
Peneplain ENDTERM BEGDEF 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 ENDTERM BEGDEF An isolated body of ground-water that is perched above and separated from the main water table by an aquiclude.
Percolation ENDTERM BEGDEF"The movement of water downward and radially through the subsurface soil layers, usually continuing downward to the groundwater.
Peridotite ENDTERM BEGDEF A coarse-grained mafic igneous rock composed of olivine with accessory amounts of pyroxene and amphibole but little or no feldspar.
Period ENDTERM BEGDEFThe time between two successive wave crests.
Permafrost ENDTERM BEGDEF"Perenially frozen ground that occurs whereever the ground temperatures remain continuously below 0ø C for two or more years. Discontinuous or patchy permafrost occurs north of the 0ø C mean annual air temperature isotherm; continuous permafrost up to several hundred meters in thickness is widespread in Siberia and northern Alaska and Canada.
Perovskite ENDTERM BEGDEF"Structure of the main mineral phase of the lower mantle. (MgFe)SiO3 perovskite represents approximately 40% by volume of the entire earth
Phase ENDTERM BEGDEFThe onset of a displacement or oscillation on a seismogram indicating the arrival of a different type of seismic wave.
Phenocryst ENDTERM BEGDEF"Large conspicuous crystal set in a fine-grained matrix.
Phreatic eruption (explosion) ENDTERM BEGDEFAn explosive volcanic eruption caused when water and heated volcanic rocks interact to produce a violent expulsion of steam and pulverized rocks. Magma is not involved.
Pisolites ENDTERM BEGDEF"Concretionary spherical nodules formed by transformation of duricrust by accretionary growth and dilation in the uppermost laterite zone.
Planetesimal ENDTERM BEGDEF"Bodies in the solar nebul, formed from accumulation of dust grains. They vary greatly in size from a few microns up to the size of small planets.
Planktonic foraminifera ENDTERM BEGDEF"Planktonic protozoans which may form carbonate tests. The tests are preserved in sediments and form an important basis for paleoceanographic reconstructions.
Plate boundary ENDTERM BEGDEFThe place where two or more plates in the Earth's crust meet.
Plate tectonics ENDTERM BEGDEF"The theory that the Earth's crust and upper mantle (the lithosphere) is broken into a number of more or less rigid, but constantly moving, segments or plates. "
Plate tectonics ENDTERM BEGDEF"The theory that the earth's crust is broken into many large fragments (plates), which move in relation to one another, shifting continents, forming new ocean crust, and causing volcanic eruptions. "
Plate ENDTERM BEGDEFOne of the huge sections which make up the Earth's crust. The plates are continuously moving.
Pleistocene ENDTERM BEGDEF"The earlier of the two periods of the Quaternary period, starting 2 to 3 million years before the present and ending about 10,000 years ago. It was a time of glacial activity. A vast amount of evidence has been collected that indicates dramatic oscillations in climatic conditions of global proportion throughout the Pleistocene. It has been well documented that global ice mass has undergone huge fluctuations on the order of 10E19 kg (equivalent to more than 100 meters of sea level) during the last million years. In addition, it has been demonstrated that the concentrations of atmospheric CO2 vary by at least 30% through a glacial cycle, with glacial periods characterized by low pCO2 (~190 ppm) and interglacials by high pCO2 (~280 ppm). Ocean temperature and circulation have also undergone glacial-interglacial change."
Plug dome ENDTERM BEGDEF"The steep-sided, rounded mound formed when viscous lava wells up into a crater and is too stiff to flow away. It piles up as a dome-shaped mass, often completely filling the vent from which it emerged. "
Plug ENDTERM BEGDEFSolidified lava that fills the conduit of a volcano. Plugs (also called volcanic necks) are usually more resistant to erosion than the material making up the surrounding cone and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away.
Plume ENDTERM BEGDEF"Volume of contaminated groundwater that occupies a continuous region of an aquifer and emanates from a single source.
Pluton ENDTERM BEGDEFA large igneous intrusion formed at great depth in the crust.
Podzolization ENDTERM BEGDEF"Soil-forming process typified by formation of humic acids and leaching of acids and sesquioxides. Continuum of soils developed range from peats, podzol soils (upper mineral soil bleached nearly white by organic acids) to podzolic soils where leaching of sesquioxides is insufficient to bleach mineral soil.
Polar motion ENDTERM BEGDEF"Path of the earth's spin axis relative to an earth fixed-coordinate system. Movement of the rotation axis relative to geographic (earth-fixed) coordinates, consisting mainly of a slow drift, a 14-month Chandler wobble and a 12-month annual wobble.
Polarity chron ENDTERM BEGDEF"Time interval of constant geomagnetic polarity.
Polarity zone ENDTERM BEGDEF"Stratigraphic interval in which the rocks or sediments carry a magnetization indicating formation in a field of constant polarity.
Positive feedback ENDTERM BEGDEF"An interaction that amplifies the response of the system in which it is incorporated.
Potable water ENDTERM BEGDEF Water that is agreeable to the taste and not dangerous to the health.
Potential energy (gravitational) ENDTERM BEGDEFThe stored energy of a substance. Water has a lot of this if there is an elevation difference. Potential energy can be converted to kinetic energy if the water (or other substance) is allowed to move.
Potentiometric surface ENDTERM BEGDEF"Imaginary surface defined by the levels to which water will rise in wells that are open at the same elevation. The slope of the potentiometric surface determines the horizontal direction of groundwater flow.
Pothole ENDTERM BEGDEF" A semispherical hole in the bedrock of a stream bed, formed by abrasion of small pebbles and cobbles in a strong current. "
Ppm ENDTERM BEGDEF Abbreviation for parts per million.
Pratt isostatic compensation ENDTERM BEGDEF" 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. "
Precipitation ENDTERM BEGDEF"Any condensed water falling from the atmosphere to the surface of the earth. Common types include rain, snow, sleet and hail. "
Preferred orientation ENDTERM BEGDEF" Any deviation from random-hess in the distribution of the crystallographic or grain shape axes of minerals of a rock (including flow cleavage and foliation), produced by deformation and nonuniform stress during crystallization in metamorphic rocks or by depositional currents in sediments. "
Pressure gradient ENDTERM BEGDEF"The rate of decrease in atmospheric pressure per unit of horizontal distance; used to indicate relative pressures on the earth's surface and measured in the direction of most rapid decrease.
Primary magma ENDTERM BEGDEF"Magma produced by melting a solid precursor. Magma only slightly differentiated in composition from primary magma is called primitive.
Primary phase field ENDTERM BEGDEF"Area or volume on a phase diagram depicting crystal-melt equilibria in which one crystalline phase precipitates.
Principle of opposition ENDTERM BEGDEF"Graphical technique used to model fractional crystallization of a magma that determines the composition of the extract assemblage.
Proto-sun ENDTERM BEGDEF A large cloud of dust and gas gradually coalescing into a star under the force of gravity.
Proven reserves ENDTERM BEGDEF" Deposits of fossil fuels whose location and extent are known, as opposed to potential but unproved ('*discovered) deposits. "
Proxy climate indicators ENDTERM BEGDEF"Dateable evidence of a biological or geological phenomenon whose condition, at least in part is attibutable to climatic conditions at the time of its formation. Proxy data are any material that provides an indirect measure of climate and include documentary evidence of crop yields, harvest dates, glacier movements, tree rings, varves, glaciers and snow lines, insect remains marine microfauna, isotope measurements: 18O in ice sheets, 18O, 2H, and 13C in tree rings; CaCO3 in sediments; and speleothems. There are three main problems in using proxy data: (1) dating, (2) lag and response time, and (3) meteorological interpretation. Tree rings,  pollen deposits from varved lakes, and ice cores are the most promising proxy data sources for reconstructing the climate of the last five millennia because the dating are precise on an annual basis while other proxy data sources may only be precise to +/- 100 years.
Pumice ENDTERM BEGDEF" A form of volcanic glass, usually of silicic composition, so filled with vesicles that it resembles a sponge and is very light. "
Pumice ENDTERM BEGDEF"Light-colored, frothy volcanic rock,usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava. Commonly seen as lumps or fragments of pea-size and larger, but can also occur abundantly as ash-sized particles. "
Pyroclastic flow ENDTERM BEGDEF"Lateral flowage of a turbulent mixture of hot gases and unsorted pyroclastic material (volcanic fragments, crystals, ash, pumice and glass shards) that can move at high speed (50 to 100 miles an hour.) The term also can refer to the deposit so formed. "
Pyroclastic rock ENDTERM BEGDEF A rock formed by the accumulation of fragments of volcanic rock scattered by volcanic explosions.
Pyroclastic texture ENDTERM BEGDEF" The unsorted, angular, and un-rounded texture of the fragments in a pyroclastic rock. "
Pyroclastic ENDTERM BEGDEF"Igneous material erupted explosively as finely pulverized lava (bombs, blocks, pumice, ash and hot gases).
Pyroxene granulite ENDTERM BEGDEF" A coarse-grained contact metamorphic rock containing pyroxene, formed at high temperatures and low pressures. "
Quantitative stratigraphy ENDTERM BEGDEF"Use of computer-based mathematical methods to calibrate stratigraphic models that with a minimum of information provide maximum predictive potency and include formulation of confidence limits.
Quartz arenite ENDTERM BEGDEF A sandstone containing very little except pure quartz grains and cement.
Quartzite ENDTERM BEGDEF" (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 ENDTERM BEGDEF" (1) A quartz arenite. (2) A clean quartz sandstone, less pure than a quartz arehire, that may contain a moderate amount of other detrital minerals and/or calcite cement. "
Quaternary period ENDTERM BEGDEF"The latest period of geologic time, covering the most-recent two million years of the Earth's history. It is divided into two epochs: the Pleistocene -- two million years ago to approximately 10,000 years ago -- and the Holocene -- the period from approximately 10,000 years ago to the present. The Quaternary period is the artificial division of time separating prehuman and human periods. It contains five ice ages and four interglaqcial ages, and temperature indicators seem to show sharp abrupt changes by several degrees.
Radial drainage ENDTERM BEGDEF" 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 ENDTERM BEGDEF" One mechanism for the movement of heat, in which it takes the form of long-wavelength infrared radiation. "
Radiolarian ooze ENDTERM BEGDEF A siliceous deep-sea sediment composed largely of the skeletons of radiolaria. Radiolarite
Radiolarian ENDTERM BEGDEF A class of one-celled marine animals with siliceous skeletons that have existed in the ocean throughout the Phanerozoic Eon.
Ray ENDTERM BEGDEF" 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. "
Rayleigh wave ENDTERM BEGDEF"A type of surface wave having a retrograde elliptical motion at the Earth's surface similar to the waves caused when a stone is dropped into a pond. These are the slowest but often the largest and most destructive of the wave types caused by an earthquake. They are usually felt as a rolling or rocking motion and in the case of major earthquakes can be seen as they approach. Named after Lord Rayleigh, the English physicist who predicted its existence. "
Reaction series ENDTERM BEGDEF 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 area ENDTERM BEGDEF"Region in which there is a net addition of water to the groundwater system (recharge) as a result of infiltration from surface water bodies or an excess of precipitation over, evapotranspiration and runoff.
Recharge ENDTERM BEGDEF" In hydrology, the replenishment of ground-water by infiltration of meteoric water through the soil. "
Recharge ENDTERM BEGDEF"The process by which water is added to a reservoir or zone of saturation, often by runoff or percolation from the soil surface.
Recrystallization ENDTERM BEGDEF" The growth of new mineral grains in a rock at the expense of old grains, which supply the material. "
Rectangular drainage ENDTERM BEGDEF" 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 ENDTERM BEGDEF An overturned fold with both limbs nearly horizontal.
Recurrence Interval ENDTERM BEGDEFThe approximate length of time between earthquakes in a specific seismically active area.
Reflectivity ENDTERM BEGDEF"The ratio of the energy carried by a wave that is reflected from a surface to the energy of a wave incident on the surface.
Refraction (wave) ENDTERM BEGDEF 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 ENDTERM BEGDEF Metamorphism occurring over a wide area and caused by deep burial and high internal temperatures of the Earth.
Regolith reduction ENDTERM BEGDEF"Diminishment in volume and mass of parent material to a smaller volume and mass of weathered soil residue as mobile constituents are carried away, in solution or by aeolian transport.
Regolith ENDTERM BEGDEF" Any solid material lying on top of bedrock. Includes soil, alluvium, and rock fragments weathered from the bedrock. "
Regression ENDTERM BEGDEF" A drop in sea level that causes an area of the Earth to be uncovered by sea water, ending marine deposition. "
Relief ENDTERM BEGDEF The maximum regional difference in elevation.
Remote sensing ENDTERM BEGDEF" The study of Earth surface conditions and materials from airplanes and satellites by means of photography, spectroscopy, or radar. "
Replacement deposit ENDTERM BEGDEF A deposit of ore minerals by hydrothermal solutions that have first dissolved the original mineral to form a small cavity.
Reservoir ENDTERM BEGDEF"Any natural or artificial holding area used to store, regulate, or control a substance.
Residual enrichment ENDTERM BEGDEF"Increase in the enrichment factor of a relatively immobile element by virtue of hydrochemical leaching of mobile species.
Respiration ENDTERM BEGDEF 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.
Reverse fault ENDTERM BEGDEF"General term for a fault dipping between 30ø and vertical, and with the hanging wall moving up with respect to the foot wall.
Reverse weathering ENDTERM BEGDEF"Collection of processes that lead to return of protons consumed in the process of chemical weathering to earth's surface system.
Reversed polarity ENDTERM BEGDEF"Geomagnetic polarity that is opposite to the present polarity.
Reversible reaction ENDTERM BEGDEF" A chemical reaction which can proceed in either direction, depending on the concentration of reacting materials. "
Rheidity ENDTERM BEGDEF (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.
Rheology ENDTERM BEGDEF"Science of the deformation and flow of matter.
Rhyolite ENDTERM BEGDEF" The fine-grained volcanic or extrusive equivalent of granite, light brown to gray and compact. Richter magnitude scale"
Rhyolite ENDTERM BEGDEF"Volcanic rock (or lava) that characteristically is light in color, contains 69 percent silica or more, and is rich in potassium and sodium. It is fine grained, which although different in texture has the same composition as granite."
Richter scale ENDTERM BEGDEFThe system used to measure the strength of an earthquake. Developed by Charles Richter in 1935 as a means of categorizing local earthquakes. It is a collection of mathematical formulas; it is not a physical device.
Ridge (mid-ocean) ENDTERM BEGDEF" 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 system ENDTERM BEGDEF"The oceanic ridges formed where tectonic plates are separating and a new crust is being created; also, their land counterparts, such as the East African Rift. "
Rift valley ENDTERM BEGDEF A fault trough formed in a divergence zone or other area of tension.
Right-lateral fault ENDTERM BEGDEF A strike-slip fault on which the displacement of the far block is to the right when viewed from either side.
Ring current ENDTERM BEGDEF"Electrical current at about three earth radii from the center of the earth. The current results from charged particles trapped in the geomagnetic field.
Ring dike ENDTERM BEGDEF" A dike in the form of a segment of a cone or cylinder, having an arcuare outcrop. "
Ring of fire ENDTERM BEGDEFThe regions of mountain-building earthquakes and volcanoes that surround the Pacific basin.
Rip current ENDTERM BEGDEF A current that flows strongly away from the sea shore through gaps in the surf zone at intervals along the shoreline.
Ripple ENDTERM BEGDEF A very small dune of sand or silt whose long dimension is formed at right angles to the current. River order
Roche Moutonnee ENDTERM BEGDEFA small symetrical shaped hill formed by glacial erosion. The upper sides are rounded and smoothed whilw the lower sides are rough due to quarrying by the glacier.
Rock cycle ENDTERM BEGDEF" The geologic cycle, with emphasis on the rocks produced; sedimentary rocks are metamor-phosed 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 ENDTERM BEGDEF 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 ENDTERM BEGDEF A glacier-like mass of rock fragments or talus with interstitial ice that moves downhill under the force of gravity.
Rockslide ENDTERM BEGDEF A landslide involving mainly large blocks of detached bedrock with little or no soil or sand. Rounding
Runoff ENDTERM BEGDEF" The amount of rain water directly leaving an area in surface drainage, as opposed to the amount that seeps out as groundwater. "
Rupture strength ENDTERM BEGDEF The greatest stress that a material can sustain without fracturing at one atmosphere pressure.
Rupture zone ENDTERM BEGDEF"The area of the Earth through which faulting occurred during an earthquake. For very small earthquakes, this zone could be the size of a pinhead, but in the case of a great earthquakethe rupture zone may extend several hundred kilometers in length and tens of kilometers in width. "
S-wave ENDTERM BEGDEF" 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. "
S wave ENDTERM BEGDEF"Secondary wave, a shear or transverse elastic wave, usually the most prominent wave on a seismogram; travels more slowly than P-waves and thus arrives later on seismograms. Shear, secondary, rotational, tangential, equivoluminal, distortional, transverse or shake wave. These waves carry energy through the Earth in very complex patterns of transverse (crosswise) waves. These waves move more slowly than P waves, but in an earthquake they are usually bigger. S waves cannot travel through the outer core because these waves cannot exist in fluids, such as air, water or molten rock."
Salt water intrusion ENDTERM BEGDEF"The invasion of fresh surface or groundwater by salt water.
Saltation ENDTERM BEGDEF 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 ENDTERM BEGDEF" 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 ENDTERM BEGDEF" 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. "
Saprolite ENDTERM BEGDEF"Fine-grained weathering zone with microscopic or submicroscopic porosity occurring immediately above parent material bedrock. Saprolite is not contaminated by invasion of detritus from above and may or may not be isovolumetric.
Sapropel ENDTERM BEGDEF"Lithified organic-rich marine sediment deposited under anoxic conditions. If the organic content is very high, the sediments lithify to become black shale; if dilution by clays or other materials occurs, the sediments lithify to become marlstone.
Schist ENDTERM BEGDEF A metamorphic rock characterized by strong foliation or schistosity.
Schistosity ENDTERM BEGDEF The parallel arrangement of sheely or prismatic minerals like micas and amphiboles resulting from nonhydrostatic stress in meramorphism.
Scoria ENDTERM BEGDEF" Congealed lava, usually of mafic composition, with a large number of vesicles formed by gases coming out of solution. "
ScS ENDTERM BEGDEF"Seismic phase consisting of a shear wave traveling down through the mantle that is reflected at the core-mantle boundary and returns to the earth's surface as a shear wave. ScSn denotes a multiple ScS wave that is further reflected n-1 times at the earth's free surface.
Sea-floor spreading ENDTERM BEGDEF 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.
Sea aerosol ENDTERM BEGDEF"Colloidal-size particles of substances derived from the sea surface.
Seafloor spreading ENDTERM BEGDEFThe mechanism by which new seafloor crust is created at oceanic ridges and slowly spreads away as tectonic plates separate.
Seamount ENDTERM BEGDEF An isolated tall mountain on the sea floor that may extend more than 1 kilometer from base to peak (see also Guyot).
Secular variation ENDTERM BEGDEF" 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. "
Secular variation ENDTERM BEGDEF"That portion of the temporal variation of the geomagnetic field due to time-varying external current systems are excluded form this definition.
Sediment ENDTERM BEGDEFRock debris commonly produced by mechanical or chemical weathering processes.
Sedimentary petrology ENDTERM BEGDEF"Study of the composition, characteristics and origin of sediments and sedimentary rocks.
Sedimentary rock ENDTERM BEGDEF" 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 ENDTERM BEGDEF" 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 ENDTERM BEGDEF The process of deposition of mineral grains or precipitates in beds or other accumulations. Seif dune
Seiche ENDTERM BEGDEF"A free or standing wave oscillation of the surface of water in an enclosed basin that is initiated by local atmospheric changes, tidal currents or earthquakes. Similar to water sloshing in a bathtub. "
Seismic belt ENDTERM BEGDEF"An elongated earthquake zone, for example, circum-Pacific, Mediterranean, Rocky Mountain. About 60% of the world's earthquakes occur in the circum-Pacific seismic belt. "
Seismic constant ENDTERM BEGDEF"In building codes dealing with earthquake hazards, an arbitrarily-set acceleration value (in units of gravity) that a building must withstand. "
Seismic discontinuity ENDTERM BEGDEF" A surface within the Earth across which P-wave or S-wave velocities change rapidly, usually by more than +~0.2 kilometer/second. "
Seismic discontinuity ENDTERM BEGDEF"Regionally extensive, thin zones in the crust or mantle where seismic velocities change abruptly. May be the result of changes in composition, mineral phases or structural character.
Seismic profile ENDTERM BEGDEF" The data collected from a set of seis-toographs arranged in a straight line with an artificial seismic source, especially the times of P-wave arrivals. "
Seismic reflection ENDTERM BEGDEF A mode of seismic prospecting in which the seismic profile is examined for waves that have reflected from near-horizontal strata below the surface.
Seismic refraction ENDTERM BEGDEF 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 sea wave ENDTERM BEGDEFA tsunami generated by an undersea earthquake.
Seismic surface wave ENDTERM BEGDEF" 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 ENDTERM BEGDEF" 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. "
Seismic zone ENDTERM BEGDEFA region in which earthquakes are known to occur.
Seismic ENDTERM BEGDEF"Pertaining to earthquake activity.
Seismicity ENDTERM BEGDEF 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.
Seismicity ENDTERM BEGDEFEarthquake activity.
Seismogram ENDTERM BEGDEFA written record of an earthquake recorded by a seismograph.
Seismograph ENDTERM BEGDEFAn instrument that records seismic waves; that is vibrations of the earth. Used to record and measure earthquakes.
Sferics ENDTERM BEGDEF"Natural atmospheric fluctuations of the electromagnetic fields, caused mainly by thunderstorm activity in the range of 1-10E5 Hz.
Shearing ENDTERM BEGDEFThe motion of surfaces sliding past one another.
Shield volcano ENDTERM BEGDEFA gently sloping volcano in the shape of a flattened dome built almost exclusively of mafic lava flows. The Hawaiian Islands are a good example.
Shield ENDTERM BEGDEF"Tectonically stable, low-lying Precambrian nucleus of continents. Commonly contains the metamorphosed roots of ancient orogenic activity.
Shock metamorphism ENDTERM BEGDEF"Characteristic changes in rock mineralogy caused by the compression of a passing shock wave.
Shock wave ENDTERM BEGDEF"Moving wave of strong compression that usually travels faster than sound in the medium.
Signal-to-noise ratio ENDTERM BEGDEF"A quantitative measure of the statistical detectability of a signal, expressed as a ratio of the magnitude of the signal relative to the variability. For first detection of a CO2-induced climate change, the model signal is the mean change or anomaly in some climatic variable,usually surface air temperatures, attributed by a numerical model to increased concentrations of carbon dioxide. Observed noise is the standards deviation or natural variability computed from observations of that variable and agjusted for sample size, autocorrelation and time averaging.
Skin depth ENDTERM BEGDEF"Measure of the effective depth of penetration of electromagnetic fields. It is defined as the depth at which the amplitude of an electromagnetic field drops to 37% of its surface value.
Snowline ENDTERM BEGDEFThe lower limit of any year's permanent snowfall. Separates the Zone of Accumulation from the Zone of Ablation.
Soft mode ENDTERM BEGDEF"Molecular vibrational mode for which the atomic displacements track the structural changes followed during a displacive phase transition. The frequency of the soft mode goes to zero at the transition temperature or pressure.
Soil carbonate ENDTERM BEGDEF"In regions that receive about 1.3 m of rain or less, calcium carbonate will often accumulate in the lower portion of a soil profile. In fine-grained soils, such as under the prairies of the mid-western United States, the carbonate often takes the form of discrete nodules up to several centimeters in diameter. In gravelly soils such as are common in the western U.S., carbonate will begin by coating clasts, and with time it will entirely plug soil voids, forming what is commonly called a caliche or more formally a calcic horizon.
Soil profile ENDTERM BEGDEF"Vertical section of the soil through all of its distinct layers (horizons) produced by the soil-forming process and extending into the parent material.
Soil ENDTERM BEGDEF"In a geologic sense of the term, a weathered horizon at the earth's surface. During periods of landscape stability, various physical and chemical processes modify the upper few meters of the surface. Typically, new mineral assemblages such as clays are introduced,  as is organic matter, and calcium carbonate if the climate is dry. Once a soil is buried by other sediments, it is then referred to as a paleosol.
Solid state ENDTERM BEGDEF"In metamorphism, indicates the change of mineral identity without melting. All ion migration occurs while the rock (or pre-glacial ice) is still solid. "
Solute ENDTERM BEGDEF"Organic or inorganic species that is dissolved in groundwater.
Soret effect thermal diffusivity) ENDTERM BEGDEF"Solute flux due to a temperature gradient.
Spasmodic tremor ENDTERM BEGDEF"Continuous seismic signal with pulsating high-frequency appearance; can be composed of discrete earthquakes occurring closely spaced in time or can be phaseless.
Speleothem ENDTERM BEGDEF"A mineral deposit formed in a cave by the action of water.
Spines ENDTERM BEGDEFHorn-like projections formed upon a lava dome.
Spontaneous magnetization ENDTERM BEGDEF"Magnetization resulting from cooperative behavior of unpaired spins within a crystal lattice, creating a magnetic moment that is independent of the external field.
Spread ENDTERM BEGDEFThe layout of seismometer or geophone groups from which data from a single shot (the explosive charge) are recorded simultaneously.
Spreading center ENDTERM BEGDEFAn elongated region where two plates are being pulled away from each other. New crust is formed as molten rock is forced upward into the gap. Examples of spreading centers include the Mid-Atlantic Ridge and the East African Rift.
Spring ENDTERM BEGDEFA surface flow of groundwater which occurs any time the water table intersects the surface.
Stochastic inversion ENDTERM BEGDEF"Method for computing Gaussian coefficients (or equivalent parameters) that is based on a priori estimates of parameter and noise covariances.
Strain ENDTERM BEGDEF"Change in a volume of a system induced by deformation; it is positive for dilation and negative for contraction and is distinct from regolith or volume reduction, which may be an isovolumetric removal of soluble constituents in solution without deformation.
Stratification ENDTERM BEGDEF" A structure of sedimentary rocks, which have recognizable parallel beds of considerable lateral extent. "
Stratification ENDTERM BEGDEF"Separating into layers.
Stratigraphic sequence ENDTERM BEGDEF A set of beds deposited that reflects the geologic history of a region.
Stratigraphy ENDTERM BEGDEF" The science of the description, correlation, and classification of strata in sedimentary rocks, including the interpretation of the depositional environments of those strata. "
Stratovolcano ENDTERM BEGDEF" A volcanic cone consisting of both lava and pyroclastic rocks, often conical. "
Stratovolcano ENDTERM BEGDEF"A volcano composed of both lava flows and pyroclastic material. Also called Composite volcanoes. Common at convergent boundaries. Excellent examples in the U.S. include Mt. St. Helens, Mt. Shasta, and the other peaks of the Cascade Range of California, Oregon and Washington. "
Streak plate ENDTERM BEGDEF A ceramic abrasive surface for streak tests.
Streak ENDTERM BEGDEF 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.
Stream order ENDTERM BEGDEF The hierarchical number of a stream segment in dendritic drainage
Streaming flow ENDTERM BEGDEF A tranquil flow slower than shooting flow.
Streamline ENDTERM BEGDEF A curved line representing the successive positions of a particle in a flow as time passes.
Stress probing ENDTERM BEGDEF"Measurement of in situ stresses in a dedicated small-diamter hole drilled ahead of the primary drill bit.
Stress ENDTERM BEGDEF A quantity describing the forces acting on each part of a body in units of force per unit area. Striation
Striations (glacial) ENDTERM BEGDEFGrooves eroded into bedrock by rock debris frozen into the base of a glacier.
Strike-slip fault ENDTERM BEGDEF A fault whose relative displacement is purely horizontal.
Strike-slip fault ENDTERM BEGDEF"General term for a vertical fault with horizontal displacement vector producing lateral relative motion of the rock on either side.
Strike ENDTERM BEGDEF" 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. "
Stromatolite ENDTERM BEGDEF A fossil form representing the growth habit of an algal mat
Subduction zone ENDTERM BEGDEF" 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. "
Subduction zone ENDTERM BEGDEF"An elongated region along which a plate descends relative to another plate, for example, the descent of the Nazca plate beneath the South American plate along the Peru-Chile Trench. "
Subduction ENDTERM BEGDEFThe process in which one lithospheric plate collides with and is forced down under another plate and drawn back into the Earth's mantle.
Sublimation ENDTERM BEGDEF" A phase change from the solid to the gaseous state, without passing through the liquid state. "
Sublimation ENDTERM BEGDEFThe direct change from the solid to the vapor phase (without passing through the liquid phase). Commonly occurs in ice and snow fields on sunny days above the snowline.
Submarine canyon ENDTERM BEGDEF An underwater canyon in the continental shelf.
Submicroscopic ENDTERM BEGDEF"Not resolvable in a microscope.
Subsidence ENDTERM BEGDEF A gentle epeirogenic movement where a broad area of the crust sinks without appreciable deformation.
Subsolidus convection ENDTERM BEGDEF"Convection via solid-state deformational processes.
Superposed stream ENDTERM BEGDEF A stream that flows through resistant formations because its course was established at a higher level on uniform rocks before down-cutting began.
Supersaturation ENDTERM BEGDEF The unstable state of a solution that contains more solute than its solubility allows.
Surf zone ENDTERM BEGDEF An offshore belt along which the waves collapse into breakers as they approach the shore.
Surf ENDTERM BEGDEF The breaking or tumbling forward of water waves as they approach the shore.
Surface of the earth ENDTERM BEGDEF"The value given is the depth below the surface of the mean spheroid. The mean spheroid is a uniform approximation to the true shape of the Earth. No adjustment is made to the depth due to any differences between the true Earth and the mean spheroid. For example, the minimum depth that will be given is 0 kilometers even though a quake directly under Mount Everest (elevation 8848 meters) could legitimately have a depth of -6 kilometers and still be 2 kilometers underground. On the other hand, a depth of 10 kilometers would actually be more than 1 kilometer above the ocean floor of Challenger Deep (elevation -11,033 meters) in the Marianas Trench of the Pacific Ocean. "
Surface waves ENDTERM BEGDEFWaves that move over the surface of the Earth. Rayleigh waves and Love waves are surface waves.
Suspended load ENDTERM BEGDEF The fine sediment kept suspended in a stream because the settling velocity is lower than the upward velocity of eddies.
Suspended load ENDTERM BEGDEF"Part of the total stream load that is carried in suspension free from contact with the stream bed; it consists mainly of clay, silt, and fine sand.
Suture zone ENDTERM BEGDEF"Lineation in continental lithosphere marking the location of an earlier plate collision that led to the amalgamation of formnerly separate continental masses.
Swash ENDTERM BEGDEF The landward rush of water from a breaking wave up the slope of the beach.
Swell ENDTERM BEGDEF 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 ENDTERM BEGDEF The interaction of two mutually supporting species who do not compete with or prey upon each other.
Syncline ENDTERM BEGDEF A large fold whose limbs are higher than its center; a fold with the youngest strata in the center.
System (stratigraphy) ENDTERM BEGDEF" A stratigraphic unit larger than a series, consisting of all the rocks deposited in one period of an era. "
T-phase ENDTERM BEGDEF"Tertiary wave, a late-arriving short-period phase often recorded at stations of island or coastal regions, containing energy that travels through a layer of water in the ocean known as the SOFAR channel. Can be caused by earthquakes or submarine volcanic eruptions.
Tableland ENDTERM BEGDEF A large elevated region with a relatively low relief surface.
Talus ENDTERM BEGDEF" A deposit of large angular fragments of physically weathered bedrock, usually at the base of a cliff or steep slope. "
Tar sand ENDTERM BEGDEF A sandstone containing the densest aspbaltic components of petroleum - the end-product of evaporation of volatile components or of some thickening process.
Tectonic ENDTERM BEGDEF"Pertaining to the deformation of the earth's crust.
Tectonics ENDTERM BEGDEF" The study of the movements and deformation of the crust on a large scale, including epeirogeny, metamorphism, folding, faulting, and plate teetonics. "
Teleseism ENDTERM BEGDEFAn earthquake that is distant (usually more than 20 degrees) from the recording station.
Telluric currents ENDTERM BEGDEF"Natural electric currents that flow in the ground.
TEM ENDTERM BEGDEF"Transient Electromagnetic Method in which the primary energizing field is a repetitive pulse
Tephra ENDTERM BEGDEFMaterials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air.
Term ENDTERM BEGDEFDefinition
Terminal moraine ENDTERM BEGDEF A sinuous ridge of unsorted glacial till deposited by a glacier at the line of its farthest advance.
Terrestrial planet ENDTERM BEGDEF" A planet similar in size and composition to the Earth; especially Mars, Earth, Venus, and Mercury. "
Terrestrial sediment ENDTERM BEGDEF" A deposit of sediment that accumulated above sea level in lakes, alluvial fans, floodplains, moraines, etc., regardless of its present elevation. "
Texture (rock) ENDTERM BEGDEF" The rock characteristics of grain or crystal size, size variability, rounding or angularity, and preferred orientation. "
Thalweg ENDTERM BEGDEF A sinuous imaginary line following the deepest part of a stream.
Thermal conductivity ENDTERM BEGDEF A measure of a rock's capacity for heat conduction.
Thermal expansion ENDTERM BEGDEF The property of increasing in volume as a result of an increase in internal temperature.
Thermonuclear reaction ENDTERM BEGDEF A reaction in which atomic nuclei fuse into new elements with a large release of heat; especially a reaction that is self-sustaining. Occasionally used to include fission reactions as well.
Thermophile ENDTERM BEGDEF"Solute diffuses into warmer bottom waters.
Thermoremanent magnetization ENDTERM BEGDEF A permanent magnetization acquired by igneous rocks in the presence of the Earth's magnetic field as they cool through the Curie point.
Tholelite ENDTERM BEGDEF"Relatively silica-rich and alkali-poor basaltic rock containing intermediate to calcic, plagioclase, clinopyroxene, and low-Ca pyroxene.
Thrust fault ENDTERM BEGDEF" A dip-slip fault in which the upper block above the fault plane moves up and over the lower block, so that older strata are placed over younger. "
Thrust fault ENDTERM BEGDEF"General term for a fault dipping between horizontal and 30ø and with the hanging wall moving up with respect to the foot wall.
Tidal current ENDTERM BEGDEF" A horizontal displacement of ocean water under the gravitational influence of Sun and Moon, causing the water to pile up against the coast at high tide and move ourward at low tide. "
Tidal flat ENDTERM BEGDEF" A broad, flat region of muddy or sandy sediment, covered and uncovered in each tidal cycle. "
Till (glacial) ENDTERM BEGDEFGeneral term for material deposited by a glacier. See also moraine.
Till ENDTERM BEGDEF" An unconsolidated sediment containing all sizes of fragments from clay to boulders deposited by glacial action, usually unbedded. "
Time scale ENDTERM BEGDEF" The division of geologic history into eras, periods, and epochs accomplished through stratig-raphy and paleontology. "
Topographic map ENDTERM BEGDEF" See Contour map; also a schematic drawing of prominent landforms indicated by conventionalized symbols, such as hachures or contours. "
Topography ENDTERM BEGDEF" The shape of the Earth's surface, above and below sea level; the set of landforms in a region; the distribution of elevations. "
Topset bed ENDTERM BEGDEF A horizontal sedimentary bed formed at the top of a delta and overlying the foreset beds.
Trace element ENDTERM BEGDEF An element that appears in minerals in a concentration of less than l percent (often less than 0.001 percent).
Transcurrent fault ENDTERM BEGDEF"One class of vertical faults with a horizontal displacement vector that does not link two or more plate tectonic scale boundaries.
Transform fault ENDTERM BEGDEF A strike-slip fault connecting the ends of an offset in a mid-ocean ridge. Some pairs of plates slide past each other along transform faults.
Transform fault ENDTERM BEGDEF"A class of vertical faults with a horizontal displacement vector that does not link two or more plate tectonic scale boundaries.
Transgression ENDTERM BEGDEF A rise in sea level relative to the land which causes areas to be submerged and marine deposition to begin in that region.
Transition element ENDTERM BEGDEF" Elements of atomic number 21 to 29, 38 to 46, and 71 to 78, whose second outermost electron shell is only partially filled. "
Translocation crossover ENDTERM BEGDEF"Soil depth at which mass accumulation of an element by downward translocation changes from positive to zero and then to negative, thereby separating an upper soil system dominated by invasion of foreign detrital minerals from a lower, less contaminated, soil system referred to as saprolite.
Translocation ENDTERM BEGDEF"Physical or chemical migration of suspended particles or aqueous solutes by moving water in a hydrochemical system or by animals and plants.
Transpiration ENDTERM BEGDEF" The removal of water from the ground into plants, ultimately to be evaporated into the atmosphere by them. "
Transverse dune ENDTERM BEGDEF" A dune that has its axis transverse to the prevailing winds or to a current. The upwind or upcurrent side has a gentle slope, and the downwind side lies at the angle of repose. "
Trap (oil) ENDTERM BEGDEF A sedimentary or tectonic structure that impedes the upward movement of oil and gas and allows it to collect beneath the barrier.
Travel-time curve ENDTERM BEGDEF A curve on a graph of travel time versus distance for the arrival of seismic waves from distant events. Each type of seismic wave has its own curve.
Travel time ENDTERM BEGDEFThe time required for a wave train to travel from its source to a point of observation.
Travertine ENDTERM BEGDEF" A terrestrial deposit of limestone formed in caves and around hot springs where cooling, carbonate-saturated groundwater is exposed to the air. "
Trellis drainage ENDTERM BEGDEF A system of streams in which tributaries tend to lie in parallel valleys formed in steeply dipping beds in folded belts.
Trench ENDTERM BEGDEF A long and narrow deep trough in the sea floor; interpreted as marking the line along which a plate bends down into a subduction zone.
Triple junction ENDTERM BEGDEF" A point that is common to three plates and which must also be the meeting place of three boundary features, such as divergence zones, convergence zones, or transform faults. "
Tsunami ENDTERM BEGDEF A large destructive wave caused by sea-floor movements in an earthquake.
Tsunami ENDTERM BEGDEF"Literally harbor wave in Japanese. Tsunamis are great sea waves produced by a submarine earthquake, volcanic eruption or large landslide. Tsunamis can cause great damage due to flooding of low coastal areas. "
Tuff ENDTERM BEGDEF" A consolidated rock composed of pyroclastic fragments and fine ash. If particles are melted slightly together from their own heat, it is a welded tuff. "
Tuff ENDTERM BEGDEFRock formed of pyroclastic material.
Turam ENDTERM BEGDEF"Electromagnetic survey method in which the energizing source is a long, grounded, insulated cable or a large, rectangular, horizontal loop excited by one or more frequencies in the range of 100-1000 Hz.
Turbidire ENDTERM BEGDEF" The sedimentary deposit of a turbidity current, typically showing graded bedding and sedimentary structures on the undersides of the sandstones. "
Turbidity current ENDTERM BEGDEF A mass of mixed water and sediment that flows downhill along the bottom of an ocean or lake because it is denser than the surrounding water. It may reach high speeds and erode rapidly (see also Density current).
Turbulent flow ENDTERM BEGDEF A high-velocity flow in which streamlines are neither parallel nor straight but curled into small tight eddies (compare Laminar flow).
U-shaped valley ENDTERM BEGDEF" A deep valley with steep upper walls that grade into a flat floor, usually eroded by a glacier. "
Ultramafic rock ENDTERM BEGDEF" An igneous rock consisting dominantly of mafic minerals, containing less than 10 percent feldspar. Includes dunite, peridotite, amphibolite, and pyroxenite. "
Unconformity ENDTERM BEGDEF" A surface that separates two strata. It represents an interval of time in which deposition stopped, erosion removed some sediments and rock, and then deposition resumed (see also Angular unconformity ). "
Unconsolidated material ENDTERM BEGDEF Nonlithified sediment that has no mineral cement or matrix binding its grains.
Unconsolidated ENDTERM BEGDEF"Loosely arranged, not cemented together,  so particles separate easily. "
Unit cell ENDTERM BEGDEF The smallest contiguous group of atomic structural units in a mineral that can be repeated in three directions to form a crystal.
Unsaturated zone ENDTERM BEGDEF"Zone between the land surface and the water table in which fluid pressures are less than atmospheric.
Uplift ENDTERM BEGDEF A broad and gentle epeirogenic increase in the elevation of a region without a eustatic change of sea level.
Upwelling current ENDTERM BEGDEF" The upward movement of cold bottom water in the sea, which occurs when wind or currents displace the lighter surface water. "
V-shaped valley ENDTERM BEGDEF" A valley whose walls have a more-or-less uniform slope from top to bottom, usually formed by stream erosion. "
Vadose zone ENDTERM BEGDEF The region in the ground between the surface and the water table in which pores are not filled with water. Also called the unsaturated zone.
Valence electron ENDTERM BEGDEF An electron of the outermost shell of an atom; one of those most active in bonding.
Valley glacier ENDTERM BEGDEF" A glacier that is smaller than a continental glacier or an icecap, and which flows mainly along well-defined valleys, many with tributaries. "
Van der Waals bond ENDTERM BEGDEF" A bond much weaker than the ionic or covalent, which bonds atoms by small electrostatic attraction. "
Varve ENDTERM BEGDEF" A thin layer of sediment grading upward from coarse to fine and light to dark, found in a lake bed and representing one year's deposition of glacial outwash. "
Varve ENDTERM BEGDEF"A layer of sediment deposited in lakes during one year. Each layer consists of two parts, which are deposited at different seasons and which differ in color and texture; thus the layers can be counted and measured. In a complete series the number of layers gives the date on which the ground was vacated by retreating ice.
Vector ENDTERM BEGDEF" A mathematical element that has a direction and magnitude, but no fixed position. Examples are force and gravity. "
Vein ENDTERM BEGDEF A deposit of foreign minerals within a rock fracture or joint.
Vent ENDTERM BEGDEFThe opening at the earth's surface through which volcanic materials reach the surface.
Ventifact ENDTERM BEGDEF" A rock that exhibits the effects of sand-blasting or snowblasting on its surfaces, which become fiat with sharp edges in between. "
Vertical exaggeration ENDTERM BEGDEF" The ratio of the horizontal scale (for example, 100,000"
Vesicle ENDTERM BEGDEF A cavity in an igneous rock that was formerly occupied by a bubble of escaping gas.
Vesicular basalt ENDTERM BEGDEF"Holes and other openings in basaltic flow which are the result of trapped gas bubbles. Vesicles are often filled at a later date with a wide variety of materials, including quartz, agate, zeolites, and many other minerals. "
Viscosity ENDTERM BEGDEF A measure of resistance to flow in a liquid.
Viscosity ENDTERM BEGDEFA measure of resistance to flow in a liquid (molasses in January has high viscosity while molasses in August has lower viscosity).
Volatiles ENDTERM BEGDEF"Elements that are soluble in magmas at elevated pressure and temperature that exsolve as gas from magmas during ascent and eruption at the earth's surface. They include sulfur, water, carbon dioxide, chlorine and fluorine.
Volcanic aerosol ENDTERM BEGDEF"Very small (microns to fraction of micron in diameter) particles or droplets, composed mainly of sulfuric acid and water, produced by the gas-to-particle conversion of sulfur dioxide gas to sulfuric acid in the volcanic eruption plume.
Volcanic ash fall ENDTERM BEGDEF A deposit of volcanic ash resting where it was dropped by eruptions and winds.
Volcanic ash flow ENDTERM BEGDEF A mixture of volcanic ash and gases that moves downhill as a density current in the atmosphere.
Volcanic ash ENDTERM BEGDEF" A volcanic sediment of rock fragments, usually glass, less than 4 millimeters in diameter that is formed when escaping gases force out a fine spray of magma. "
Volcanic block ENDTERM BEGDEF A pyroclastic rock fragment ranging from about fist- to car-sized.
Volcanic bomb ENDTERM BEGDEF A pyroclastic rock fragment that shows the effects of cooling in flight in its streamlined or bread-crust surface.
Volcanic breccia ENDTERM BEGDEF A pyroclastic rock in which all fragments are more than 2 millimeters in diameter.
Volcanic cone ENDTERM BEGDEF The deposit of lava and pyroclastic materials that has settled close to the volcano's central vent.
Volcanic dome ENDTERM BEGDEF A rounded accumulation around a volcanic vent of congealed lava too viscous to flow away quickly; hence usually rhyolite lava. Volcanic dust
Volcanic ejecta blanket ENDTERM BEGDEF" A collective term for all the pyroclastic rocks deposited around a volcano, especially by a volcanic explosion. "
Volcanic emanations ENDTERM BEGDEF" Gases, especially steam, emitted from a vent or released from lava. "
Volcanic front ENDTERM BEGDEF"Elongate zone of maximum volcanic activity that defines the main volcanic (or magmatic) arc.
Volcanic neck ENDTERM BEGDEFSolidified lava that fills the conduit of a volcano. Volcanic necks (also called plugs) are usually more resistant to erosion than the material making up the surrounding cone and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away.
Volcanic pipe ENDTERM BEGDEF" The vertical chamber along which magma and gas ascend to the surface; also, a formation of igneous rock that cooled in a pipe and remains after the erosion of the volcano. "
Volcano ENDTERM BEGDEF" Any opening through the crust that has allowed magma to reach the surface, including the deposits immediately surrounding this vent. "
Wadi ENDTERM BEGDEF A steep-sided valley containing an intermittent stream in an arid region.
Warping ENDTERM BEGDEF" In tectonics, refers to the gentle, regional bending of the crust, which occurs in epeirogenic movements. "
Water cycle ENDTERM BEGDEF"The transfer of water between numerous temporary storage reservoirs. These include the ocean, rivers and streams, glacial ice, groundwater and the atmosphere. "
Water mass ENDTERM BEGDEF" A mass of water that fills part of an ocean or lake and is distinguished by its uniform physical and chemical properties, such as temperature and salinity. "
Water table ENDTERM BEGDEF A gently-curved surface below the ground at which the vadose zone ends and the phreatic zone begins; the level to which a well would fill with water.
Water table ENDTERM BEGDEF"Surface along which fluid pressure is equal to atmospheric pressure. ENDTERM BEGDEF"
Wave-cut terrace ENDTERM BEGDEF A level surface formed by wave erosion of coastal bedrock to the bottom of the turbulent breaker zone. May appear above sea level if uplifted.
Wave steepness ENDTERM BEGDEF The maximum height or amplitude of a wave divided by its wavelength.
Wavelength ENDTERM BEGDEF" The distance between two successive peaks, or between troughs, of a cyclic propagating disturbance. "
Weathering (surface) ENDTERM BEGDEFWeathering includes the processes which mechanically and chemically wear and fragment rock.
Weathering ENDTERM BEGDEF" The set of all processes that decay and break up bedrock, by a combination of physically fracturing or chemical decomposition. "
Weights of evidence modeling ENDTERM BEGDEF"Estimation of posterior probability that a hypothesis holds true from its prior probability and a combination of weights associated with presence or absence of features supporting the hypothesis. ENDTERM BEGDEF"
Wireline logging ENDTERM BEGDEF"Measurement of depth variations (logs) of physical properties using a tool or sonde deployed by means of a conducting cable (wireline). ENDTERM BEGDEF"
X-ray diffraction ENDTERM BEGDEF" In mineralogy, the process of identifying mineral structures by exposing crystals to X-rays and studying the resulting diffraction pattern. "
Xenolith ENDTERM BEGDEF A piece of country rock found engulfed in an intrusion.
Xenolith ENDTERM BEGDEF"Rock sample with mineralogy foreign to the igneous host in which it occurs. Thought to be a fragment of wall rock from volcanic conduits in the deep lithosphere rapidly transported to the surface during eruptions. ENDTERM BEGDEF"
Youth (geomorphology) ENDTERM BEGDEF A stage in the geomorphic cycle in which a landscape has just been uplifted and is beginning to be dissected by canyons cut by young streams.
Zeolite ENDTERM BEGDEF" A class of silicates containing H=O in cavities within the crystal structure. Formed by alteration at low temperature and pressure of other silicates, often volcanic glass. "
Zone of ablation (or wastage) ENDTERM BEGDEFThe area below the snowline where snow melt exceeds snowfall and material is lost from a glacier.
Zone of accumulation ENDTERM BEGDEFThe area above the snowline where snowfall exceeds snow melt and material is added to a glacier.
Zoned crystal ENDTERM BEGDEF" A single crystal of one mineral that has a different chemical composition in its inner and outer parts. Formed from minerals belonging to a solid-solution series, and caused by the changing concentration of elements in a cooling magma that results from crystals settling out. "