Geology Glossary

aa flow a lava flow that develops a partially solidified surface as it moves forward, which breaks the surface into a rough, rubbly mass.

ablation the loss of ice and snow from a glacier, generally through melting or evaporation.

abrasion of a stream, the process by which a stream's rocky bed is eroded away by the constant friction and impact of tumbling rock fragments, gravel, and sediment.

abyssal fan a fan-shaped accumulation of sediment that forms at the mouth of submarine canyons.

abyssal plain a very flat expanse of horizontally deposited sediment that accumulates on the ocean floor at the base of a continental rise.

accreted terrane a terrane that appears to have formed in place along a continent's margin through accumulation and orogeny.

accretionary wedge an accumulation of marine sediment, derived from the subducting plate, that builds up at the edge of a subduction zone.

acid rain an environmentally harmful acidic rain that results from rain mixing with chemical pollutants in the atmosphere.

active continental margin marked by a landward continental shelf, a continental slope that forms a sidewall of an oceanic trench and is much steeper than that of a passive continental margin, and an irregular ocean bottom that may contain volcanic seamounts; an area of earthquake and volcano activity.

advancing glacier a glacier that exhibits outward or downslope movement.

aftershock one of the small earthquakes that may follow the main earthquake.

A horizon the uppermost soil horizon; characterized by the downward movement of water.

alkali soil a pedocal type of soil toxic to plant growth because of its high salt content.

alluvial fan a feature similar to a delta; a large, fanlike accumulation of sediment dropped where a stream emerges from rugged terrain, such as the edge between a mountain canyon and a flat plain.

alpine referring to high mountain regions.

alpine glaciation glaciation usually restricted to deep valleys in high mountainous terrain.

angle of repose the steepest angle at which loose material will remain in place.

angular unconformity the contact that separates a younger, gently dipping rock unit from older underlying rocks that are tilted or deformed layered rock.

anticline a fold that is arched upward to form a ridge.

aphanitic fine grained.

aquiclude an impermeable layer such as clay that retards the flow of groundwater.

aquifer a porous, permeable, saturated formation of rock or soil through which groundwater flows easily.

aquitard a formation such as shale, clay, or unfractured igneous rocks that retards water flow.

arc-continent convergence the result when intervening ocean is destroyed by subduction, welding an island arc to the continental edge.

arete a sharp ridge that commonly extends downward from a horn to separate two adjacent glacial valleys.

arroyo a narrow gorge with steep walls and a gravel bottom; produced over time by flash floods.

artesian well a well that taps water from a confined aquifer.

aseismic ridge a chain of seamounts and guyots; so called because it is not associated with earthquakes.

assimilation the process by which pieces of the country rock melt and mix within a body of magma.

asthenosphere an area composed of the flexible mantle beneath the lithosphere.

atoll a circular reef in deep water that shelters a lagoon; the result of reef development around the flank of a volcano that has since subsided but to which the corals are still anchored.

aulacogen see failed rift.

aureole see halo.

axial plane of a fold, the plane that separates rocks on one side of a fold from those that dip in the opposite direction on the other side.

backarc basin the area on the continental side of an island arc or magmatic arc.

backarc thrust belt the belt of rocks that has been thrust toward the continental interior from the magmatic arc area along low-angle faults.

backswamp a poorly drained and marshy area behind a natural levee.

backwash the water that flows back down the beach into the surf zone.

bajada the joining of alluvial fans at the front of a mountain range in a rolling surface of sediment and gravel.

balanced budget of a glacier, a situation in which there is neither advancement nor recession.

bar an elongate sedimentary accumulation of sand or gravel deposited by current action in a stream or other body of water.

barchan dune a solitary, crescent-shaped dune that forms in areas of sparse sand.

barchanoid dune a variety of dune intermediate between barchan and transverse dunes; barchanoid dunes form scalloped rows of sand perpendicular to the wind.

barrier island a large, elongate mass of sand that parallels a coast and forms an island.

barrier reef an elongate reef that develops offshore parallel to a coastline and is separated from the coastline by a deep lagoon.

basal sliding the movement of a glacier generated by its sliding on a thin film of water resulting from the pressure of the glacier's weight.

Basin and Range topography a series of steep mountain ranges separated by broad valley floors.

batholith a pluton larger than 100 kilometers at the earth's surface; usually granitic and made up of diapirs.

baymouth bar see spit.

beach the strip of sand or gravel (more rarely silt) that covers a shoreline from the low-water edge to a well-defined point of higher elevation.

beach drift the zig-zag pattern by which sediment is moved across a beach face by breaking waves.

beach face the side of a beach facing the ocean.

bedding the pattern (usually horizontal) of layering in which sediments are deposited.

bedding planes demarcations of different layers of sediments.

bed load of a stream, the heavier, coarser-grained earth material that travels on or near the bed of a stream.

bed load of wind, the heavier grains (usually sand) that hop and skip along the ground by saltation.

Benioff zone a zone that slopes downward from an oceanic trench and underneath the overlying crustal plate at 30 to 60 degrees; an area of earthquake origination.

bergschrund a crevasse, commonly filled with rock fall debris, that forms where a glacier separates from a cirque wall.

berm the landward edge of a beach.

B horizon the middle soil horizon into which the leached materials from the A horizon often precipitate.

bituminous coal a common form of coal that is soft and black.

black smoker a submarine hot spring that results from high heat flows and convection currents at divergent plate boundaries and that deposits solid masses of metallic minerals.

blowout a bowl-like depression caused by deflation.

body wave a seismic wave that radiates out from the focus of an earthquake and travels though solid rock.

bottomset bed the finest silt and clay particles that are carried out from a delta into deeper water or slide down a delta front into deeper water.

Bowen's reaction series a description of the progression of mineral formation as magmas cool and crystallize.

braided stream a stream in which the water has lost its main channel and flows in an interconnecting network of rivulets around numerous bars.

breaker a high wave in which the crest falls forward in front of the main body of the wave.

breakwater a wall built parallel to the shoreline to provide quiet water.

breccia rock composed of coarse-grained, angular fragments of broken rocks that have been cemented together and lithified.

brittle strain strain that occurs when a stress is great enough to break or fracture a rock.

budget of a glacier, the ratio between ice gained and ice lost.

butte a landform resulting from the erosion of a mesa.

caldera a depression larger than a crater, at least a kilometer in diameter, that forms at the top of a volcano when the summit is destroyed during an eruption or when the crater floor collapses into the magma chamber below.

caliche a hardpan formed by the precipitation of salt by evaporation.

calving the breaking off of large blocks of ice from a glacier.

capillary action the process by which surface tension causes water to rise up into unfilled pore spaces.

capillary fringe the lower part of the unsaturated zone that draws water upward from the saturated zone.

cast a fossil formed when the organic remains dissolved, leaving an opening (mold) shaped like the organism and later filled with calcite or silica.

cementation the step in lithification in which minerals fill some or all of the pore space and adhere to the sediment fragments, thus producing a sedimentary rock.

chemical sedimentary rock a sedimentary rock resulting from biological or chemical processes, generally underwater, that crystallizes minerals that accumulate on the sea floor.

chemical weathering the process by which rain, water, and atmospheric gases decompose minerals, destroy chemical and mineralogical bonds, and form new minerals.

chill zone the fine-grained edge of a rock intrusion.

C horizon the lowest soil horizon, which lies directly above the bedrock; composed partly of soil and partly of decomposing bedrock fragments.

cinder cone a feature composed of pyroclastic material (not lavas) ejected from a volcanic vent.

circum-Pacific belt an earthquake belt that follows the rim of the Pacific Ocean.

cirque a steep-sided, circular hollow carved in the top of a mountain from an alpine glacier.

clastic sedimentary rock a sedimentary rock formed from the consolidation of material such as gravel, sand, or clay (sediment) derived from the weathering and breakdown of preexisting rocks.

cleavage the ability of a mineral to break along preferred directions, usually along the faces of layered crystals.

coal a dark-colored sedimentary rock that contains a high percentage of organic plant material.

coast the strip of land near the ocean that includes the beach and the immediate inland area beside it.

coastal straightening the process of the headlands being cut back and the flanking beaches being widened.

collision boundary a convergent boundary that separates two continental plates that are pushed into contact.

columnar jointing see columnar structures.

columnar structures cooled and contracted flood basalt in vertical, parallel, generally six-sided columns.

compaction the step in lithification in which the grains of sediment are packed more tightly together.

complex mountain see folded mountain.

composite volcano a volcano that consists of alternating layers of lava and pyroclastic debris; built up over millions of years, such volcanos are characterized by long periods of dormancy.

compressive stress stress applied to a rock from opposite directions, compressing and flattening the rock mass.

cone of depression the area and shape of a drawdown around a well.

confined aquifer an aquifer overlain by a less permeable bed that keeps the water in the aquifer under pressure.

confining pressure see geostatic pressure.

connate water water trapped in the original sediments during deposition and lithification.

contact the plane of separation between any two different kinds of rocks.

contact metamorphic deposit a hydrothermal deposit that results from hot solutions that leave a cooling intrusion and deposit minerals in cracks in country rock.

contact metamorphism the process by which country rock surrounding a hot magma intrusion is metamorphosed by the high heat flow coming from the intrusion.

contamination plume the elongate area of contaminated groundwater that is downgradient from the point source of leakage.

continental divide that topographic ridgeline that separates the streams that flow in opposite directions and empty into different oceans.

continental drift the theory that the continents were once joined together and somehow then split and moved apart.

continental glaciation glaciation that affects a broader, flatter part of a continental land mass than does alpine glaciation.

continental rise a very low-angle ridge of sediment that forms between the lower part of the continental slope and the abyssal plain.

continental shelf a shallow, very gently sloping platform that extends seaward from the edge of a continent.

continental slope an area that extends from the seaward edge of a continental shelf into the deep ocean at an average angle of 4 to 5 degrees.

continent-continent convergence the result when two continents collide.

continuous branch the type of magmatic differentiation in which minerals form continuously during cooling.

contour current a current that flows parallel to the edge of a continental slope.

convection currents currents within a material that are driven mostly by changing temperature gradients.

convergent boundary a fault boundary marked by plates that come together.

core the zone of the earth that includes the inner and outer core.

country rock the surrounding rock that magma invades in the formation of intrusive rocks.

crater the circular depression at the top of a volcano.

craton a continental interior that has been structurally inactive for a prolonged time, usually hundreds of millions of years or longer.

creep a mass-wasting event slow enough that it cannot be detected as it is occurring.

crevasse a deep crack or fissure in a glacier.

cross-bedding a sedimentary structure in which the bedding planes of a particular unit are inclined compared to the bedding of the enclosing rocks.

crude oil a liquid containing hydrocarbons that forms in organic- or fossil-rich sediments and rocks.

crust the outermost zone of the earth, its exterior layer.

crustal rebound the process by which crustal rocks that were down-warped by a glacier's weight slowly return to normal elevation after the glacier's retreat.

daughter products products created as an element undergoes radioactive decay.

debris avalanche a rapidly churning mass of rock debris, soil, water, and air that races down very steep slopes.

debris flow a mass-wasting event in which movement and turbulence occur throughout the mass.

debris slide the rapid movement of a mass of debris as a single unit.

deep sea fan see abyssal fan.

deflation the removal of sediment from a land surface by wind.

delta a thick, roughly wedge-shaped accumulation of sediment deposited at the mouth of a stream.

dendritic drainage pattern a veinlike drainage pattern that develops in a rock type that erodes uniformly, such as granite.

depositional coast a gently sloped coast that has been built up by sediments deposited from longshore drift.

depth of focus of an earthquake, the distance between the epicenter and the focus.

desert an area that receives less than 25 centimeters (10 inches) of rain annually.

desert pavement a large surface of the desert floor that is covered by pebbles and stones that resemble rounded paving stones; caused by deflation or temperature changes.

desiccation crack a crack that develops when a muddy sediment is exposed to air and begins to dry out; these cracks combine to form a polygonal pattern.

detachment fault a low-angle fault above which is often a series of thrust faults and below which is undeformed bedrock.

dewatering the step in lithification in which increasing pressure squeezes out some of the water between sediment particles.

diapir a small magma blob resulting from localized melting of the crust; a component of batholiths.

differential stress stress usually caused by tectonic forces applied to a body of rock from different, but not opposite, directions, stretching the rock mass into an elongate shape.

differential weathering the result of the resistance of some rocks more than other rocks to weathering, creating uneven rates of erosion and sometimes spectacular formations.

differentiation the process by which a magma forms different minerals according to changes in temperature and pressure.

dike an intrusive rock that generally occupies a discordant, or cross-cutting, crack or fracture that crosses the trend of layering in the country rock.

dip angle the angle between the horizontal plane and a tilted bedding plane.

dip-slip fault a fault in which movement is parallel to the dip of the fault plane in an up or down direction between the two blocks.

disconformity an erosional contact usually parallel to the bedding planes of the upper and lower rock units.

discontinuous branch the type of magmatic differentiation in which minerals form at discrete temperatures and not continuously during cooling.

disseminated deposit a hydrothermal deposit in which the metal ore is evenly distributed in generally low concentrations throughout large masses of rock.

dissolved load earth material in a stream that has been dissolved into ions and carried in solution.

distributary a small, shifting channel that spreads out across a delta from the main river channel and disperses the sediment load.

divergent boundary a fault boundary marked by plates that move away from one another.

downcutting the erosion directly downward by a stream channel.

drainage basin the area drained by a stream and its tributaries.

drainage divide a ridge that separates one drainage basin from another.

drawdown a local lowering of the water table around a well.

drumlin a long, narrow, rounded ridge of till whose long axes parallel the direction a glacier traveled.

dry wash see arroyo.

ductile of a rock, flowing plastically in response to stress.

dust storm a windstorm that carries large amounts of sand or sediment through the air.

earthflow the movement of earth material down a hillside as a viscous fluid; earthflows typically occur on steep slopes with thick soil cover that becomes saturated by heavy rains.

earthquake the ground shaking caused by rocks that suddenly move or jolt in response to tectonic stress.

ebb currents tidal currents preceding low tide.

elastic rebound theory the theory that suggests that in some cases energy is stored in rock that is being bent (deformed) by tectonic forces until the energy in the rock exceeds the rock's chemical bonds and it breaks, releasing the energy and causing motion.

elastic strain strain after which the body of a rock returns to its previous shape when stress has been removed.

end moraine an extensive and typically crescent shaped pile of till built up at the front of a glacier.

ephemeral stream a stream that flows intermittently as a result of periods of sudden rainfall.

epicenter the point on the surface directly above the focus of an earthquake.

erosion the picking up of sediment and soil particles by an agent such as wind or water.

erratic a boulder that has been deposited by a glacier and is not derived from the local bedrock.

esker a long, winding ridge of outwash deposited in streams flowing through ice caves and tunnels at the base of a glacier.

estuary a drowned river mouth from an older coastline; appears as a long arm of ocean water extending inland from the coast.

evaporitic rock a rock formed from minerals that chemically precipitated from water.

exfoliation the process by which curved sheets of rock loosen and fall from a weathered rock surface.

exfoliation dome a large rounded landform (usually composed of intrusive rocks) that results from exfoliation.

exotic terrane a terrane that did not form naturally through accretion and has likely collided with the continental margin.

extrusive igneous rock igneous rock that crystallized from liquid magmas that reached the surface and were generally vented as volcanic lavas.

facies see metamorphic grades.

failed rift rifting that ceases before the crustal mass has been separated into parts.

fall a mass-wasting movement in which earth material free-falls from a steep face or cliff and generally collects at the base as talus.

fault an area in which rock has been displaced along a fracture, such as having one side that is moved up or down.

fault block mountain a mountain that is bordered on both sides by steeply dipping faults, such as a horst.

fault gouge the broken material within a fault.

fault plane a plane of fracture in a rock along which movement has occurred.

fault zone a series of parallel fault planes that are close together and form a wider zone of structural weakness.

felsic rock a rock that is rich in silica, potassium, sodium, and aluminum and that contains only small amounts of iron, magnesium, and calcium.

fetch the distance that wind travels over a surface of water.

fiord a steep-walled, fingerlike coastal inlet that was carved by glacial action and later flooded by the rising sea.

firn rounded granules formed by the compaction of snow by pressure from overlying snow and cemented by ice.

first-motion studies studies that indicate whether the first rock motion in an earthquake was a push (the rock moved toward the seismograph station) or a pull (the rock moved away from the station).

fissile splitting naturally along layers.

flash flood flood resulting from very heavy rainfall over short periods.

flood currents tidal currents preceding high tide.

floodplain the area created on both sides of a stream when periodic flooding deposits mud and silt over extensive, low-lying areas.

flow a mass-wasting movement in which the mass moves downslope like a viscous fluid.

flowing artesian well a well that taps an aquifer under confining pressure that is sufficient to force the water to rise naturally to the surface through the well.

focus the point of origin of an earthquake.

fold a bend in a layered rock.

fold and thrust belt a mountain-building event in which rocks are folded during tectonic stress and detached as thin layers along thrust zones, which vertically stack the layers; typically occurs on the continental side of a magmatic arc.

folded mountain a mountain created by intense compressional forces that fold, fault, and metamorphose the rocks, a process that resulted in many of the world's biggest mountain belts.

foliation the alignment of parallel layers or bands of mineral grains in a rock subjected to prolonged differential stress and/or shearing.

footwall the block that underlies an inclined dip-slip fault.

forearc basin the relatively undisturbed expanse of ocean floor between an accretionary wedge and an island arc.

foreland basin a shallow continental basin behind a magmatic arc, a result of subsidence.

foreset bed a sandy bed that composes the main body of a delta.

foreshock one of the small earthquakes that may precede the main earthquake.

fossil the trace of a plant or animal in a sedimentary rock.

fossil assemblage a group of different kinds of fossils that coexisted; more useful than single types of fossils in determining the age of a formation.

fossil fuel coal, oil, or gas derived from organic-rich rocks.

fracture a crack in a rock along which no motion has taken place.

fringing reef a flat expanse of reef that is attached directly to shore.

frost heaving the process by which rock and soil are lifted vertically by the formation of ice and repeated freezing and thawing.

frost wedging the widening and deepening of cracks by ice, breaking off pieces and slabs of rock.

fumerole a vent in or near a volcano from which steam and other gases escape from molten rock below.

gaining stream a stream into which groundwater flows from the saturated zone.

geologic cross section a vertical slice across a map area; depicts the spatial relationships of rock units and structures beneath the surface.

geophysics a field concerning the application of the laws of physics to the study of the earth.

geostatic pressure pressure that is equally applied to all sides of a deeply buried mass of rock.

geothermal energy the energy produced when exceptionally hot water underground is tapped by wells and used to generate electricity.

geothermal gradient the rate at which temperature increases with depth.

geyser an explosive hot spring that periodically erupts scalding water and steam; water temperatures in a geyser are generally near boiling.

geyserite a build-up of ledgelike layers, generally of calcite or silica, around a geyser.

glaciation the movement of an ice mass over a land surface.

glacier a large mass of ice that forms on land during cooler climatic periods.

Gondwanaland a paleocontinent that consisted of what is now Africa, India, South America, Australia, and Antarctica.

gossan a rusty, iron-bearing cap; a remnant of a weathered metallic ore deposit at the surface.

graben a feature formed when a block that is bounded by normal faults slips downward, usually because of a tensional force, creating a valleylike depression.

graded bed a bed in which the base consists of coarser material and subsequent beds grade upward through sand and silt to the finest clay sizes at the top.

graded stream a stream that has smoothed out its longitudinal profile to resemble a smooth, concave-upward curve.

gravity meter a device that measures the force of gravity between a mass inside the instrument and the earth.

groin one of a series of walls built perpendicular to the coast to widen beaches that are losing sand to longshore drift.

ground moraine a thin, widespread layer of till deposited across the surface as an ice sheet melts.

groundwater water derived from rain and melting snow that percolates downward from the surface and collects in the open pore spaces between soil particles or in cracks and fissures in bedrock.

guyot a submerged, flat-topped seamount that was once above sea level and was eroded flat by continual wave action.

half-life the time it takes for half of a known quantity of radioactive material to convert to daughter products.

halo the zone of metamorphism surrounding an intrusion in contact metamorphism.

hanging valley a valley that forms a cliff face with the main valley it enters because its lower part has been eroded away by glacial action.

hanging wall the block that overlies the inclined fault plane in a dip-slip fault.

hardness a quality of minerals determined by the Mohs hardness scale.

hardpan a layer of soil, usually the B horizon, that is so hard (usually cemented by calcite or quartz) that even a backhoe cannot break through it.

headland a rocky arm of a coastline that juts into the sea.

headward erosion erosion that results when a valley is extended upward above its original source by gullying, mass wasting, and sheet erosion.

headwaters the origination of a stream; usually in higher elevations of mountainous terrain.

heat flow of the earth in general, the amount of heat from the earth's interior that is lost at the surface.

heavy crude oil a dense, viscous petroleum that flows so slowly it is usually left behind in an oil field.

herringbone cross-bedding a distinctive pattern of alternating cross-bedding directions that is reflective of a rhythmic, high-energy environment, such as a tidal zone.

hinge line the center axis of a fold.

horn a sharply defined peak that has formed from erosional processes along the rim of a cirque.

horst a feature that results when a block that is bounded by normal faults experiences a compressive force that forces the block upward, forming mountainous terrain.

hot spring a spring with water 6 to 9 degrees centigrade (11 to 16 degrees Fahrenheit) warmer than the mean annual air temperature for the locality where it occurs.

hot springs deposit a disseminated metal deposit formed in response to hot spring activity at the surface of the earth.

hydraulic action the ability of flowing water to dislodge, pick up, and transport rock particles or sediment.

hydrogenous sediments sediments on the ocean floor that have chemically precipitated from seawater.

hydrologic cycle the continuous exchange of water between the atmosphere, oceans, continents, plants, and animals.

hydrothermal deposit of metallic ore, the result of the formation of rich deposits from hydrothermal solutions that circulate through fractured country rock.

hydrothermal rocks those rocks whose minerals crystallized from hot water or whose minerals have been altered by hot water passing through them.

hydrothermal vein minerals that are deposited from hydrothermal processes and fill a crack in the country rock.

iceberg a floating mass of ice that results from calving from a glacial face into the water of a lake or ocean.

ice cap a glacial ice mass that is centered on a highland area and migrates outward in all directions.

ice fall a blocky, piled-up ice surface that results when rapid ice movements rupture glacial crevasses.

ice sheet a glacier that covers a broad expanse of land and is not restricted to a channel.

igneous rock rock that at one time was molten and part of magmas or lavas and that then cooled.

incised meander a steep-walled canyon that results from the down-cutting of a meandering stream.

inclinometer a device on a compass used to measure dip angle.

inclusion a rock fragment enclosed within an intrusive rock unit.

index fossil a fossil of those species that lived only during a restricted period and that identifies the narrow time range during which the host rock could have formed.

inner core the spherical, solid, innermost part of the earth.

inselberg an isolated bedrock remnant of a former mountain front; may project through the pediment cover.

interior drainage pattern a drainage pattern in which streams empty into landlocked basins.

intermediate rock a general term for rock between mafic and felsic classifications.

intrusive igneous rock igneous rock that formed from magmas that moved upward into cracks and voids deep in the crust and that never reached the surface.

island arc a curved chain of islands that develops between an oceanic trench and a continental landmass.

isoclinal fold a fold that has undergone stress great enough to compress its limbs tightly together.

isostasy the equilibrium, or balance, between adjacent blocks of crust overlying the mantle.

isotope an atom of an element that contains a different number of neutrons in its nuclei than does another atom of that element.

jetty a wall that is built on both sides of a harbor and that extends into the ocean to protect the harbor from sedimentation and destructive waves.

joint an opening in a rock along which the rock exhibits no displacement; generally an equilibrium response to cooling or unloading.

joint set a series of roughly parallel joints that occur in one direction.

jovian planets those outer planets that have densities of less than 2 g/cm3: Saturn, Jupiter, Uranus, Neptune, and Pluto.

kame a steep-sided mound of stratified till that was deposited by meltwater in depressions or openings in the ice or as short-lived deltas or fans at the mouths of meltwater streams.

karst topography an irregular land surface dotted with numerous sinkholes and depressions related to underlying cave systems.

kettle a depression in glacial till that results when a buried block of glacial ice melts.

kettle lake a body of water occupying a kettle.

komatiite a typical ultramafic extrusive rock that is mostly olivine and pyroxene, with lesser feldspar.

laccolith an intrusive feature similar to a sill but formed from a more viscous magma that creates a lens-shaped mass between sedimentary layers, arching the overlying strata upward.

lahar a mudflow originating on a volcanic slope.

landslide a destructive, rapid mass-wasting event.

lateral erosion erosion that occurs when a stream meanders or braids back and forth across its valley floor or channel, undercutting and eroding its banks.

lateral moraine a moraine consisting of rock debris and sediment that have worked loose from the walls beside a valley glacier and have accumulated in ridges along the sides of the glacier.

laterite a typically bright red, highly leached, residual soil that forms in tropical regions.

lateritic weathering weathering that results in residual deposits that become enriched through the chemical breakdown and removal of more reactive elements of a rock.

Laurasia the paleocontinent that once included the present-day landmasses of North America and Eurasia.

lava magma that is extruded at the earth's surface, as from a volcano.

lava flood nonvolcanic lava that vents from deep cracks in the continental crust.

law of faunal succession a law that states that fossil species succeed one another in undisturbed rocks in a definite and recognizable order around the world.

law of original horizontality a law that states that most sedimentary rocks formed as nearly horizontal layers.

law of superposition a law that states that in an undisturbed sequence of sedimentary rocks or lava flows the overlying rock is younger than the underlying rock.

left-lateral strike-slip fault a strike-slip fault in which the block across the fault appears to have moved to the left.

lignite a soft, brown coal produced by increasing temperature and pressure on peat.

limb one side of a fold.

liquefaction of a landslide, an occurrence in which water-saturated soil moves downslope like a liquid.

lithification the hardening of sediment into a rock.

lithosphere an area composed of the crust and the uppermost part of the mantle.

loam soil that contains about equal amounts of sand, silt, and clay as well as abundant organic matter.

loess silt and clay deposited by wind and weakly cemented by calcite.

longitudinal dune a large, symmetrical ridge of sand that parallels the wind direction; it can be over 100 meters high and over 100 kilometers long.

longshore current a strong current resulting from water being pushed parallel to the shore by repeated wave action; primary transporter of sand in the shoreline environment.

longshore drift the movement of sediment parallel to the shore by wave action.

losing stream a stream whose channel lies above the water table and loses water into the unsaturated zone through which it is flowing.

luster the appearance or quality of light reflected from a mineral's surface.

mafic rock an igneous rock containing approximately 50 percent silica and relatively high percentages of iron, magnesium, and calcium.

magma molten rock that forms below the surface of the earth, usually at depths of 100 kilometers or greater.

magmatic arc a general term for belts of andesitic island arcs or inland andesitic mountain ranges (volcanic arcs) that develop along continental edges.

magmatic deposit of metallic ore, the result when the minerals settle to the bottom of an intrusive body and form thin, high-grade layers.

magmatic water water derived from magmas.

magnetic anomaly an area of magnetism that is either higher or lower than the average magnetic field for that region.

magnetic field of a planet, a magnetic force that surrounds the planet and probably originates from its metallic core.

magnetic pole a locality at which magnetic lines of force converge to create the strongest point in the magnetic field.

magnetometer a device for measuring the intensity of the magnetic field at the earth's surface.

mantle the middle zone of the earth, between the core and the crust.

mantle plume a "hot spot" in the crust where hot mantle material has ascended along deep penetrating cracks in the crust.

marine terrace a broad, gently sloping platform offshore from a beach face.

marker beds those distinctive layers in a sedimentary sequence that allow exposures in different areas to be definitely correlated, or linked.

mass wasting the process of erosion whereby rock, soil, and other earth materials move down a slope because of gravitational forces.

meander a hairpinlike feature of a stream caused by erosion on the outside of a curve and deposition on the inside.

mechanical weathering the process by which rocks are physically broken down into smaller pieces by external conditions, such as the freezing and thawing of water in cracks in the rock.

medial moraine a long ridge of till that results when lateral moraines join as two tributary glaciers merge to form a single glacier.

Mediterranean-Himalayan belt a zone that runs through the Mediterranean region eastward through Asia and to the East Indies marked by frequent earthquake and volcanic activity.

member a distinctive rock layer that is part of a larger rock formation.

mesa a remnant flat-topped tower or column resulting from the weathering and erosion of a plateau's slopes.

metamorphic grades the different groups of minerals that crystallize and are stable at the different pressure and temperature ranges during regional metamorphism.

metamorphic rock a rock created by solid-state transformation (no melting) of a rock mass into a rock of generally the same chemistry but with different textures and minerals.

metasomatism the process by which hot-water solutions carrying ions from an outside source move through a rock mass via fractures or pore space.

meteoric water water that is derived from the atmosphere as rain or snow and that moves down into the bedrock from the earth's surface.

microseism a very small seismic tremor.

midoceanic ridge a deep crustal fault on the ocean floor that separates crustal plates and generates new ocean crust.

mineral a combination of elements that forms an inorganic, naturally occurring crystalline solid of a definite chemical composition.

Mississippi Valley-type deposit a concentration of lead and zinc thought to be deposited in porous limestones and sandstones by low-temperature water that was driven out of deeper sediments by compaction.

modified Mercalli scale a ranking system for the intensity of an earthquake, ranking it from 1 to 12 depending on the amount of resulting damage.

Moho see Mohorovicic discontinuity.

Mohorovicic discontinuity the first major boundary of the earth's interior; separates the crust from the underlying mantle.

Mohs hardness scale a scale from 1 to 10 on which the relative hardness of minerals is measured; named after its originator, Friedrich Mohs, a German mineralogist.

moraine an accumulation of till either left behind when a glacier recedes or carried on top of alpine glaciers.

mountain belt long chains of mountain ranges; typically thousands of kilometers long.

mountain-building the process of building mountains through tectonic forces that deform, metamorphose, and uplift crustal rocks.

mountain range a group of mountain peaks or ridges that form a discrete topographic area.

mountain root a bulge of continental crust downward into the mantle beneath a mountain.

mouth the terminus of a stream.

mud crack see desiccation crack.

mud flow the movement of a liquidy mass of soil, rock debris, and water down a well-defined channel.

mudpot a kind of hot spring that produces boiling mud and releases sulfurous gases.

natural attenuation the natural breakdown of groundwater contaminants over time and distance from the point source.

natural gas a gaseous mixture of hydrocarbons that usually occurs with crude oil.

natural levee a ridge of sand and silt deposited near the edge of a stream's channel.

neap tides the lowest tides, which occur close to the first and third quarters of the moon.

nebular hypothesis the hypothesis that suggests that the planets and moons in the solar system formed from a huge hydrogen-helium cloud.

negative budget of a glacier, the losing of more volume than that gained from new snowfall.

negative gravity anomaly the gravity reading of a rock if it is lower than the normal regional gravity value.

negative magnetic anomaly a magnetic reading that is lower than the average regional magnetic field strength.

negative polarity of a rock, the polarity created when the earth's magnetic field was reversed, which reduced the earth's net field strength.

nonconformity the erosional contact that separates a younger sedimentary rock unit from a plutonic or metamorphic rock unit.

nonflowing artesian well a well in which water from the tapped aquifer must be pumped to the surface.

normal dip-slip fault a dip-slip fault in which the hanging wall block has moved downward relative to the footwall block.

normal force force that is parallel to the surface of the slope.

normal polarity see positive polarity.

nose of a fold, its tip.

obduction the process by which a crustal plate overrides another plate for a short distance.

oblique-slip fault a fault in which the fault blocks show both horizontal and vertical displacement.

ocean-continent convergence convergence that occurs when oceanic crust is subducted under continental crust.

oceanic trench a narrow deep trough that parallels the edge of a continent, island arc, or convergence of two oceanic plates and forms at the edge of a subduction zone.

ocean-ocean convergence convergence that occurs when two plates carrying ocean crust converge, with one slab subducted under the other at an ocean trench.

oil field the occurrence of multiple oil pools in one area.

oil sands sandstone deposits that have been cemented with tar or asphalt.

oil shales organic-rich shale formations from which oil can be extracted.

open fold a broad feature in which the limbs of a fold dip at a gentle angle away from the crest of the fold.

ophiolite a mafic rock sequence at the earth's surface that is believed to be pieces of ancient oceanic crust that were thrust onto the continent during subduction and mountain-building.

organic sedimentary rock a sedimentary rock composed primarily of accumulations of organic remains from plants or animals.

orogenesis see orogeny.

orogeny the folding, faulting, deformation, and metamorphism from the onset of intense tectonic stress that results in mountain-building.

outcrop a bedrock exposure at the surface of the earth.

outer core the outer shell of the core, between the mantle and the inner core, that is inferred to be molten (liquid).

outwash the sediments deposited by glacial meltwater.

outwash plain the broad front of outwash associated with an ice sheet.

overturned fold a fold whose limbs dip in the same direction, indicating that the upper part of the fold has overridden the lower part.

oxbow lake a body of water shaped roughly like a U and formed when a meander begins to close on itself and the stream breaks through and bypasses the meander.

pahoehoe flow a basalt flow with a ropy or undulating surface resulting from quick cooling and solidification of the lava.

paleocoast an old coastline that has been preserved in the geologic record.

paleocurrent a direction of sediment transport or ice movement that is revealed by sedimentary or glacial features.

paleomagnetic field an ancient magnetic field that can be detected from the orientation of magnetic crystals in rocks such as basalt.

paleontology the study of fossils.

Pangaea a single continental mass that rifted to form our present-day continents.

parabolic dune a deeply curved dune with the tips pointing into the wind; usually forms around a blowout in vegetated areas.

parallel retreat the retention by slopes of their original steepness as they erode.

parent rock the original rock from which a metamorphic rock was formed.

partial melting the process by which a portion of the magma that is forming from a melting mass of rock separates and rises as a distinct magma.

passive continental margin marked by a landward, continental shelf followed by a deeper continental slope, continental rise, and flat abyssal plain; characterized by a lack of earthquake activity.

P body wave a compressional (longitudinal) body wave that induces rock to vibrate parallel to the direction the wave is traveling.

peat an unlithified organic material that can be cut into blocks and burned for fuel.

pedalfer a thick soil high in aluminum and iron that develops in response to abundant rainfall, organic acids, and strong downward leaching.

pediment a low-angle erosion surface at the foot of a mountain range that is typically covered by up to 100 feet of sediment; occurs between the bajada and the range front.

pedocal a thin, poorly leached soil formed in arid climates by the upward movement of soil water by subsurface evaporation and capillary action.

pegmatite a dike that contains very coarse-grained crystals.

pelagic sediment a sea-floor sediment that is composed of finegrained clay particles and microskeletons of marine organisms that settle slowly to the ocean floor; its clay component (and sometimes volcanic ash) is generally carried from land by wind and deposited on the surface of the ocean.

peneplain an area reduced by erosion nearly to a plain.

perched water table an accumulation of groundwater that is held above the water table in the unsaturated zone by an impermeable bed such as clay.

permeability of a rock, the ease with which fluid is transmitted through its pore space.

petroleum a general term that includes both natural gas and crude oil.

physical geology the study of the earth's rocks, minerals, and soils and how they have formed through time.

piedmont glacier the forwardmost extension of a valley glacier; forms where the ice emerges at the front of the mountain range.

pillow structures blobs of submarine lava that break through the thin, hardened exterior of a lava flow and chill immediately in the cold water, forming small rounded shapes.

placer deposit a deposit of heavy metallic minerals, such as iron or titanium minerals, or native gold or diamonds, that have been concentrated by wave or water action in a river or beach environment.

plan geologic map a two-dimensional map showing the locations and shapes of the outcrops at an appropriate scale and indicating, through a variety of geologic symbols, features such as folds, faults, contacts between different rock units, and strike and dip.

plastic deformation the physical, permanent changes, such as folds or stretching, in a rock from tectonic forces that do not result in fracturing.

plastic flow the ability of a material, such as glacial ice, to flow plastically without breaking.

plastic strain strain that results in a permanent change in the shape of a rock.

plate a segment of the earth's crust that is bounded by deep faults and moves in response to internal forces.

plateau a flat-lying hill underlain by resistant rock.

plateau basalt see lava flood.

plate tectonics the theory that the earth's surface is divided into large, slow-moving crustal plates that are driven by internal forces, such as convection currents in the mantle.

playa lake a lake formed from water that drains from mountains into the central part of a valley.

plunge the angle between the horizontal and the hinge line in a plunging fold.

plunging fold a fold that has been tipped by tectonic forces and that has a hinge line, or axis, that is not horizontal.

pluton see plutonic rock.

plutonic rock an intrusive, discordant, generally coarse-grained rock that was formed deep in the earth's crust.

pluvial lake a lake formed during the wetter climates that existed during and after glacial retreat.

point source the point of contamination.

polar wandering the apparent movement of the earth's geographic and magnetic poles through geologic time.

pore space open space between sediment grains.

porosity of a rock or sedimentary deposit, the percentage of volume that consists of voids and open space.

porphyritic of an igneous rock, containing coarser crystals that are supported in a fine-grained groundmass.

porphyry copper deposit a disseminated deposit in which copper and molybdenum are found in porphyritic intrusive rocks.

positive budget of a glacier, the gaining of more volume from new snowfall than the losing from melting.

positive gravity anomaly the gravity reading of a rock if it is higher than the normal regional gravity value.

positive magnetic anomaly a magnetic reading that exceeds the average magnetic field strength.

positive polarity of a rock, when its magnetic field is the same as the earth's field today.

pothole a circular depression eroded into the bedrock of a stream by abrasive sediments.

primary body wave see P body wave.

protolith see parent rock.

provenance area the source from which sediment originated.

P-wave shadow zone that area on the earth's surface in which P waves from an earthquake cannot be detected.

pyroclastic cone see cinder cone.

pyroclastic debris fragments of rock ejected from a volcano.

pyroclastic flow a dense mixture of hot gas and pyroclastic debris.

radial drainage pattern a drainage pattern that resembles the spokes on a wheel; occurs when the streams originate on the flanks of conical mountains.

radioactive decay the spontaneous breakdown of isotopes that contain unstable nuclei.

rain shadow an area on the lee side of a mountain range that is arid because most of the rain is precipitated on the other side of the range.

receding glacier a glacier that, although it can move downslope, cannot overtake its rate of uphill recession.

recessional moraine a moraine that develops at the front of a receding glacier.

recharge the process by which new water is added to the saturated zone, replenishing water that is lost.

rectangular drainage pattern a drainage pattern created in bedrock that is regularly fractured or jointed in 90-degree angles.

recumbent fold a fold so overturned that its limbs are essentially horizontal and parallel.

reef an accumulation of organisms (typically corals and algae) that forms in warm, shallow ocean environments; resistant ridge that rims islands, lagoons, and other shorelines.

regional metamorphism metamorphism of rocks typically exposed to tectonic forces and associated high pressures and temperatures.

regolith the interface between bedrock and overlying sedimentary material; consists of solid fragments of weathered rock.

renewable resource a resource or commodity that can be replenished, such as trees and crops.

reserves that subgroup of a resource that has been discovered and can be extracted at a profit.

reservoir rock a rock with the required permeability and porosity to hold large accumulations of petroleum.

residual soil soil developed from the weathering of the underlying bedrock.

resource that amount of a geologic commodity that exists in both discovered and undiscovered deposits.

reverse dip-slip fault a dip-slip fault in which the hanging wall block has moved upward relative to the footwall block.

Richter scale a numerical scale that lists earthquake magnitude in logarithmic increments from about 2 to 8.6.

ridge-push a term that refers to the cooling and sinking of new crust as it moves away from a midoceanic ridge along a deeper lithospheric plane of weakness.

rift valley a large crack in the crest of a midoceanic ridge that typically forms a graben-type valley.

right-lateral strike-slip fault a strike-slip fault in which the block across the fault appears to have moved to the right.

rill a concentration of sheetwash into a small channel; rills merge to form larger streams.

Rim of Fire see circum-Pacific belt.

rip current a narrow channel of water that flows straight back out to sea after its waves have broken on the beach.

ripple marks gentle, repeated ridges, usually in sand or silt, that form perpendicular to the flow of wind or water.

rock a solid aggregate of bonded mineral crystals.

rock avalanche the rapid descent of a mass of variously sized rock fragments.

rock-basin lake a depression that is scoured out by an advancing glacier and later fills with water.

rock cycle the various interrelated ways rock types form from geological processes.

rock formation an occurrence of rock with a set of characteristics that distinguishes it from the rocks above or below it.

rock slide the rapid movement of loose rock along an inclined plane.

rounding the smoothing of rock fragments during transportation.

saltation of water, the process in which turbulent or eddying currents temporarily lift larger sediment grains into the overlying flow of water.

saltation of wind, the process in which air currents temporarily lift larger sediment grains into the air.

salt dome a vertical column of rock salt that extends upward through a sedimentary sequence, creating folds and faults that trap petroleum.

salt flat a flat surface area covered by salt that precipitated by evaporation.

sand dune a heap of loose sand deposited by wind action.

sand fall a mass of sand that dislodges and falls in a submarine canyon.

saturated (saturation) zone rock and soil in which all the porosity is filled with water.

S body wave a body wave, only about half as fast as a P wave, that causes rock to vibrate perpendicularly to the direction of wave travel.

scarp a steep hillside or cliff that typically results from faulting or mass wasting.

sea arch a stack whose center has been eroded through, producing a bridgelike shape, because the rock is softer or more fractured.

sea cave a cavity eroded into a sea cliff in the wave zone.

sea cliff a steep slope along a coastline that results from the slope's base being eroded by waves.

sea floor spreading the process by which new basaltic oceanic crust forms at a midoceanic ridge and is slowly pushed away on both sides toward the continents as more new crust is produced.

seamount a conical, usually basaltic volcanic mountain that forms on the ocean floor.

secondary body wave see S body wave.

sedimentary rock a rock that is composed of sediment grains that have been compacted and lithified.

sedimentary structures features that were part of sediments when they were deposited and which were preserved when the sediments became lithified.

seif see longitudinal dune.

seismic gap a stretch along an active fault zone that has not produced earthquakes for a significant time.

seismic reflection the return of some of the energy from seismic waves that have penetrated downward from the surface or nearsurface, hit a rock boundary, and bounded back to the surface.

seismic refraction a change in the direction of travel of a seismic wave as it passes through different mediums; occurs only if the mediums have different densities or strengths, which change the velocity of the seismic wave.

seismic sea wave see tidal wave.

seismic wave a wave of energy that is released by an earthquake.

seismogram the series of squiggly lines recorded by a seismograph.

seismograph a device used to record the motion of a seismometer during an earthquake.

seismometer a suspended pendulumlike device used to detect seismic waves.

shear force force that is parallel to the surface of the slope.

shearing the sliding motion that is parallel to and results from compressive forces applied to a rock mass.

shear plane the surface along which shearing occurs.

shear strength an object's resistance to movement that needs to be overcome in order to make it move.

shear stress stress that results when forces from opposite directions create a shear plane in an area in which the forces run parallel to one another.

sheet joint a crack that parallels the outer surface of a rock.

sheetwash a thin layer of unchanneled water that flows downhill during very heavy rains.

shield volcano a broad, cone-shaped hill or mountain made from solidified lava flows.

silica tetrahedron four oxygen atoms connected to a smaller, central silicon atom.

sill an intrusive body formed from magma that entered country rock parallel to the bedding and is thus concordant with the country rock.

sinkhole a basinlike depression at the surface caused when a portion of a cave system collapses.

sinter a build-up of ledgelike layers, generally of calcite or silica, around a hot spring.

slab-pull a term that refers to the result of the cold edge of a plate subducting at a steep angle through the mantle, its downward motion tending to pull the plate away from the ridge crest.

slide a mass-wasting movement that moves along a surface parallel to the slope of the surface.

slip a mass-wasting movement in which the mass moves as a single unit along a well-defined surface or plane.

slip face the steeper, downwind slope of a sand dune.

slot canyon a vertical-walled canyon where mass-wasting processes have been very limited.

slump a mass-wasting movement along a curved surface where the downward movement of the upper part of the mass leaves a steep scarp and the bottom part is pushed outward along a more horizontal plane.

snow line of a glacier, the irregular boundary between the zone of accumulation and the zone of wastage.

soil layers of weathered, unconsolidated particles of earth material that contain organic material and can support vegetation.

soil horizon one of the three layers of mature soil.

solifluction a variety of earthflow in which the flow of watersaturated earth is over an impermeable surface such as permafrost; usually occurs in bitterly cold regions.

solum the O, A, and B soil horizons.

solution weathering the process by which certain minerals are completely dissolved by acidic solutions.

sorting the process by which large, coarse, angular pieces of sediment are deposited near a source area, while progressively smaller and smoother sediments are carried farther.

spatter cone a smaller feature usually associated with an already extruded and cooling lava flow from a shield volcano.

spherical weathering weathering that occurs when the corners of an angular rock are broken down more quickly than the flat surfaces, forming rounded shapes.

spit a fingerlike ridge of sand that projects into a bay.

spreading axis see spreading center.

spreading center a divergent boundary (midoceanic ridge) along which new oceanic crust is formed and pushed outward.

spring tides tides that occur at the times of the new and full moons; spring tides exhibit the greatest difference in tidal elevations.

stability field the ranges of temperature and pressure in which a particular mineral is stable.

stack an erosional remnant of a sea cliff; a stack is rooted to the wave-cut platform and stands above the surface of the water.

star dune an isolated hill of sand formed by variable winds in the Sahara and Arabian deserts; the base of the dune resembles a multipointed star.

stock a pluton that occupies less than 100 square kilometers at the earth's surface.

strain a change in the volume and/or shape of a rock because of stress.

stratigraphic trap a naturally occurring change in a sedimentary sequence that traps migrating oil and gas; examples include a lens of sandstone in a larger bed of shale or a porous reef structure in a limestone unit.

stream base level the elevation of a stream's most horizontal flow and lowest velocity.

stream capacity the total load of sediment a stream is capable of carrying.

stream competence a measure of the largest-sized particle a stream can transport.

stream discharge the volume of water that flows past a certain point in a certain amount of time.

stream gradient the downhill slope of a channel; typically measured in feet per mile.

stream terrace a steplike bench that occurs above a stream bed and floodplain and that is cut into bedrock or is a remnant of older river sediments that have since been eroded.

stream valley a topographically low area, typically centered on a stream, that is produced by mass wasting and erosion.

stream velocity the speed at which a stream flows.

stress an applied force (usually tectonic) that tends to physically alter a rock mass.

strike the compass bearing of the line formed by the intersection of a tilted bedding plane with the horizontal plane.

strike-slip fault a fault in which the blocks on either side of the fault move horizontally in relation to each other, parallel to the strike of the fault.

structural basin a variation of a syncline in which all the beds dip inward toward the center of the basin.

structural dome a variety of anticline, a feature of which is that the central area has been warped and uplifted and all the surrounding rock units dip away from the center.

structural geology the study of the processes that result in the formation of geologic structures.

structural trap a structure such as a fault between reservoir rocks and impermeable rocks, a thrust fault, or a fold such as an anticline that traps migrating petroleum.

subbituminous coal weakly metamorphosed, black, soft, sooty coal.

subduction the process by which oceanic crust is pushed against, and finally underneath, continental or oceanic crust.

subduction boundary a convergent boundary marked by the oceanic crust of one plate that is being pushed downward beneath the continental or oceanic crust of another plate.

subduction complex see accretionary wedge.

subduction zone the gently dipping zone along which subduction occurs.

submarine canyon a V-shaped erosion feature that cuts a continental shelf and slope.

subsoil the layer of soil that underlies the topsoil.

supergene deposit a high-grade metal deposit enriched through the processes of weathering.

surf the zone where waves break against a shoreline.

surface wave the slowest of the seismic waves; surface waves travel outward on the earth's surface from the epicenter much as ripples do from a stone thrown into the water.

suspect terrane a terrane that does not fit the regional pattern or has conflicting age dates.

suspended load of a stream, the fine-grained sediment that remains in the water in a stream during transportation.

suspended load of wind, the fine-grained clay and silt that is carried long distances.

suture zone the line of collision at a convergent boundary, typically continent-to-continent.

swash the still-turbulent sheet of water that sweeps up the slope of a beach.

S-wave shadow zone the area on the earth's surface in which S waves from an earthquake cannot be detected.

syncline a fold that arches downward to form a trough.

talus the accumulation of rock debris at the base of a steep slope.

tarn see rock-basin lake.

tar sands see oil sands.

tectonostratigraphic terrane see terrane.

tensional stress stress that occurs when a rock is subjected to forces that tend to elongate it or pull it apart.

tephra pyroclastic debris that is ejected from a volcano.

terminal moraine a ridge of till that marks the farthest advance of a glacier before it started to recede.

terminus of a glacier, the front.

terrane a region of geologic continuity distinct from neighboring regions.

terrestrial planets those that have densities of 3g/cm3 or more: Mercury, Venus, Earth, and Mars.

terrigenous sediment a sea-floor sediment derived from land and usually deposited on the continental shelf, continental rise, and abyssal plain.

texture a term describing the sizes and orientations of a rock's mineral or rock fragment components.

theory of glacial ages a theory proposed by Swiss naturalist Louis Agassiz that parts of the earth's surface in the geologic past were covered with larger glaciers than we see today.

thermal metamorphism see contact metamorphism.

thrust fault a reverse fault in which the hanging block (upper plate) has overridden the footwall block (lower plate) at a very shallow angle for an extensive distance.

tidal current the horizontal flow of water that accompanies the changing tide and flows in two opposite directions.

tidal delta sediments deposited by the back-and-forth tidal action between barrier islands.

tidal flat a flat, muddy zone of coastline affected by tidal currents.

tidal wave a gigantic wall of water, sometimes as high as 90 meters, caused by a submarine earthquake.

tide the rhythmic rise and fall of sea level along a coastline.

till the unsorted and unlayered rock debris and sediment that is carried or later deposited by a glacier.

tombolo a bar of sediment that connects an island to the mainland, forming a small peninsula.

topset bed a nearly horizontal layer of sediment deposited by distributaries as they flow toward a delta front.

topsoil the upper part of a section of loam; topsoil has the highest organic content of types of soil and is considered to be the most fertile.

transform boundary a fault boundary marked by plates that slide past one another.

transported soil soil deposited by agents such as ice and water and not derived from the underlying bedrock.

trap a stratigraphic or structural feature of high porosity that traps migrating petroleum.

travel-time curve a plot of the arrival times of seismic waves relative to distance.

trellis drainage pattern a drainage pattern consisting of a main stream with short tributaries on either side; forms in areas of tilted sedimentary rocks that create parallel ridges and valleys.

trench-suction a term that refers to the subduction of a plate at a steep angle, which pulls the overlying plate and the trench toward the midoceanic ridge.

triple junction (point) the junction of three major faults, thought to be in response to an underlying mantle plume, that signals the onset of rifting.

truncated spurs topographic spurs along a valley that have been truncated by glacial erosion in the valley.

tsunami see tidal wave.

tuff a volcanic rock consisting of small particles such as ash and dust.

tuff breccia a volcanic rock that contains angular, coarse rock fragments in a matrix of finer-grained ash and dust.

turbidites sediments that are deposited by turbidity currents and that typically show graded bedding.

turbidity current a large volume of dense, sediment-laden water that results when sand and mud on a continental slope are dislodged by landslides or earthquakes and become suspended in the water.

turbidity flow see turbidity current.

ultramafic rock rock consisting almost entirely of ferromagnesian minerals and having no feldspars or quartz.

unconfined aquifer an aquifer that does not have a confining bed that separates the zone of saturation from the unsaturated units above it.

unconformity an erosional contact between two rocks in which the upper unit is usually much younger than the lower unit.

ungraded stream a stream that is still actively downcutting its course and smoothing out irregularities in its gradient through erosion.

uniformitarianism the principle that the geologic processes we see today were active in the geologic past.

unloading the removal of the overlying weight and pressure through erosion when a rock mass is uplifted to the surface, resulting in the mass's slow expansion.

unsaturated zone rock and soil in which pore spaces contain both air and water and therefore are not saturated.

uplifted coast a former coast that has been lifted above the present coastline by tectonic activity.

uplifted marine terrace a former marine terrace that has been lifted above the present coastline by tectonic activity.

upwarped mountain a mountain that is the result of broad arching of the crust or great vertical displacement along a high-angle fault.

valley glacier a mass of ice restricted to high mountain valleys.

valley train the outwash plain of an alpine glacier.

varve one light-colored bed and one dark-colored bed of sediment that form at the bottom of a glacial lake and that represent a single year's deposition.

ventifact a rock that has flattened surfaces formed by windblown sand.

viscosity resistance to flow; a lava with low viscosity spreads quickly, and one with high viscosity flows sluggishly.

volcanic arc a range of andesitic volcanic mountains that forms on the continental edge above a subduction zone.

volcanic dome a rounded volcanic feature, formed from thick, viscous magma, that creates a plug in the vent of a volcano.

volcanic mountain the result of the accumulation of a large amount of volcanic lavas and pyroclastic material around a volcanic vent.

volcanic neck a rock that formed in the vent or throat of a volcano at the end of its eruptive life and remains standing after the flanks of the volcano have eroded away.

volcanism the venting of liquid magma at the surface of the earth.

volcano a hill or mountain that forms around a volcanic vent and that consists of cooled lava, rock fragments, and dust from the eruptions.

water table the contact between the saturated and unsaturated zones.

wave crest the top of a wave.

wave cut platform a flat-lying bench of eroded rock left behind by a sea cliff's retreat.

wave height the vertical distance between the top of the wave and the low point of the wave.

wavelength the horizontal distance between two crests or two troughs of adjacent waves.

wave refraction a process by which breaking waves become more parallel with the shore.

waves of oscillation waves in the open sea; so named because of the orbital motion of water particles in them.

waves of translation waves that begin to break as they meet the shore.

wave trough the low point of a wave.

weathering the breaking apart of rock at the surface through chemical and physical processes.

xenolith a fragment of country rock torn away during the emplacement of magma; generally most abundant near the contact with the country rock.

zone of accumulation of a glacier, the higher portion that is perennially covered with snow.

zone of accumulation of soil, see B horizon.

zone of fracture the more rigid portion of a glacier near its surface.

zone of wastage of a glacier, the lower portion, where the ice is lost.