Glossary in Essentials textbook

Aa A type of lava flow that has a jagged, blocky surface.
Ablation A general tenn for the loss of ice and snow from a glacier.
Abrasion The grinding and scraping of a rock surface by the friction and impact of rock particles carried by water, wind, or ice.
Abyssal plain Very level area of the deep-ocean floor, usually lying at the foot of the continental rise.
Accretionary wedge A large wedge-shaped mass of sediment that accumulates in subduction zones. Here sediment is scraped from the subducting oceanic plate and accreted to the overriding crustal block.
Active continental margin Usually narrow and consisting of highly deformed sediments. They occur where oceanic lithosphere is being subducted beneath the margin of a continent.
Active layer The zone above the permafrost that thaws in summer and refreezes in winter.
Aftershock A smaller earthquake that follows the main earthquake.
Alluvial fan A fan-shaped deposit of sediment formed when a stream's slope is abruptly reduced.
Alluvium (or fluvial deposit) Unconsolidated sediment deposited by a stream.
Alpine glacier (or Valley glacier) A glacier confined to a mountain valley, which in most instances had previously been a stream valley.
Andesitic Igneous rocks having a mineral makeup between that of granite and basalt, after the common volcanic rock andesite. (also Intermediate Compositional category for igneous rocks found near the middle of Bowen's reaction series, mainly amphibole and the intermediate plagioclase feldspars.)
Angle of repose The steepest angle at which loose material remains stationary without sliding downslope.
Angular unconformity An unconformity in which the older strata dip at an angle different from that of the younger beds.
Anthracite A hard, metamorphic form of coal that burns clean and hot.
Anticline A fold in sedimentary strata that resembles an arch.
Aphanitic texture A texture of igneous rocks in which the crystals are too small for individual minerals to be distinguished with the unaided eye.
Aquifer Rock or sediment through which groundwater moves easily.
Aquitard An impermeable bed that hinders or prevents groundwater movement.
Archean eon  The second (reading down the geologic column) eon of Precambrian time. The eon  between the (younger) Proterozoic eon and the (older) Hadean eon.
Aręte A narrow, knifelike ridge separating two adjacent glaciated valleys.
Arkose A feldspar-rich sandstone.
Artesian well A well in which the water rises above the level where it was initially encountered.
Asthenosphere A subdivision of the mantle situated below the lithosphere. This zone of weak material exists below a depth of about 100 kilometers and in some regions extends as deep as 700 kilometers. The rock within this zone is easily deformed.
Atmosphere The gaseous portion of a planet; the planet's envelope of air. One of the traditional subdivisions of Earth's physical environment.
Atoll A continuous or broken ring of coral reef surrounding a central lagoon.
Atom The smallest particle that exists as an element.
Atomic mass unit A mass unit equal to exactly one-twelfth the mass of a carbon-12 atom.
Atomic mass number The number of protons in the nucleus of an atom.
Atomic weight (Atomic mass) The average of the atomic masses of isotopes of a given element.
Aureole A zone or halo of contact metamorphism found in the host rock surrounding an igneous intrusion.
Avultion A process in which a river locally overflows (and so breachs) its natural levee to flow in another direction.
Back swamp A poorly drained area on a floodplain resulting when natural levees are present.
Barchan dune A solitary sand dune shaped like a crescent with its tips pointed downwind.
Barchanoid dunes Dunes forming scalloped rows of sand oriented at right angles to the wind. This form is intermediate between isolated barchans and extensive waves of transverse dunes.
Barrier island A low, elongated ridge of sand that parallels the coast.
Basal slip A mechanism of glacial movement in which the ice mass slides over the surface below.
Basalt An aphanitic igneous rock of mafic composition.
Basaltic Term used to describe igneous rocks that contain abundant dark (ferromagnesian) minerals and about 50 percent silica.
Base level The level below which a stream cannot erode.
Basin A circular downfolded structure.
Batholith A large mass of igneous rock that formed when magma was emplaced at depth, crystallized, and was subsequently exposed by erosion.
Baymouth bar A sandbar that completely crosses a bay, sealing it off from the main body of water.
Beach drift The transport of sediment in a zigzag pattern along a beach. It is caused by the uprush of water from obliquely breaking waves.
Beach nourishment Large quantities of sand are added to the beach system to offset losses caused by wave erosion. By building beaches seaward, beach quality and storm protection are both improved.
Bed (See Strata Parallel layers of sedimentary rock.)
Bedding plane A nearly flat surface separating two beds of sedimentary rock. Each bedding plane marks the end of one deposit and the beginning of another having different characteristics.
Bed load Sediment rolled along the bottom of a stream by moving water, or particles robed along the ground surface by wind.
Belt of son moisture A zone in which water is held as a film on the surface of soil particles and may be used by plants or withdrawn by evaporation. The uppetmost subdivision of the zone of aeration.
Biochemical Describing a type of chemical sediment that forms when material dissolved in water is precipitated by water-dwelling organisms. Shells are common examples.
Biogenous sediment Seafloor sediments consisting of material of marine-organic origin.
Bituminous coal The most common form of coal, often called soft, black coal.
Blowout (deflation hollow) A depression excavated by wind in easily eroded materials.
Body wave A seismic wave that travels through Earth's interior.
Bottomset bed A layer of fine sediment deposited beyond the advancing edge of a delta and then buried by continuous delta growth.
Bowen's reaction series A concept proposed by N.L. Bowen that illustrates the relationship between magma and the minerals crystallizing from it during the formation of igneous rock.
Braided stream A stream consisting of numerous intertwining channels.
Breakwater A Structure protecting a nearshore area from breaking waves.
Breccia A sedimentary rock composed of angular fragments that were lithified.
Caldera A large depression typically caused by collapse of the sumrnit area of a volcano following a violent eruption.
Caliche A hard layer, rich in calcium carbonate, that forms beneath the B horizon in soils of arid regions.
Calving Wastage of a glacier that occurs when large pieces of ice break off into water.
Capacity The total amount of sediment a stream is able to transport.
Capillary fringe A relatively narrow zone at the base of the zone of aeration. Here water rises from the water table in tiny threadlike openings between grains of soil or sediment.
Cap rock A necessary part of an oil trap. The cap rock is impermeable and hence keeps upwardly mobile oil and gas from escaping at the surface.
Catastrophism The concept that Earth was shaped by catastrophic events of a short-term nature.
Cavem A naturally formed underground chamber or series of chambers most commonly produced by solution activity in limestone.
Cementation One way in which sedimentary rocks are lithified. As material precipitates from water that percolates through the sediment, open spaces are filled and particles are joined into a solid mass.
Cenozoic era A time span on the geologic time scale that followed the Mesozoic era.
Chemical sedimentary rock Sedimentary rock consisting of material that was precipitated from water by either inorganic or organic means.
Chemical weathering The processes by which the internal structure of a mineral is altered by the removal and/or addition of elements.
Cinder cone A rather small volcano built primarily of pyroclastics ejected from a single vent.
Cirque An amphitheater-shaped basin at the head of a glaciated valley produced by frost wedging and plucking.
Clastic A sedimentary rock texture consisting of broken fragments of preexisting rock.
Cleavage The tendency of a mineral to break along planes of weak bonding to yield flat surfaces.
Col A pass between mountain valleys where the headwalls of two cirques intersect.
Color A phenomenon of light by which otherwise identical objects may be differeptiated.
Column A feature found in caves that is formed when a stalactite and stalagmite join.
Columnar joints A pattern of cracks that forms during cooling of molten rock to generate columns.
Compaction A type of lithification in which the weight of overlying material compresses more deeply buried sediment. It is most important in fine-grained sedimentary rocks such as shale.
Competence. A measure of the largest particle a stream can transport; a factor dependent on velocity.
Composite cone A volcano composed of both lava flows and proclastic material.
Compound A substance formed by the chemical combination of two or more elements in definite proportions and usually having properties different from those of its constituent elements.
Concordant A term used to describe intrusive igneous masses that form parallel to the bedding of the surrounding rock.
Cone of depression A cone-shaped depression immediately surrounding a well.
Conformable layers Rock layers that were deposited without interruption.
Conglomerate A sedimentary rock consisting of rounded, gravel-sized particles.
Contact metamorphism Changes in rock caused by the heat of a nearby magma body.
Continental drift A hypothesis, credited largely to Alfred Wegener, that suggested all present continents once existed as a single supercontinent. Further, beginning about 200 million years ago, the supercontinent began breaking into smaller continents, which then "drifted" to their present positions.
Continental margin See Active continental margin and Passive continental margin.
Continental rise The gently sloping surface at the base of the continental slope.
Continental shelf The gently sloping submerged portion of the continental margin extending from the shoreline to the continental slope.
Continental slope The steep gradient that leads to the deep-ocean floor and that marks the seaward edge of the continental shelf.
Continental volcanic arc Mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent.
Convergent boundary A boundary in which two plates move together, causing one of the slabs of lithosphere to be consumed into the mantle as it descends beneath the overriding plate.
Core Located beneath the mantle, it is Earth's innermost layer. The core is divided into an outer core and an inner core.
Correlation Establishing the equivalence of rocks of similar age in different areas.
Covalent bond A chemical bond produced by the sharing of electrons.
Crater The depression at the summit of a volcano, or that which is produced by a meteorite impact.
Creep The slow downhill movement of soil and regolith.
Crevasse A deep crack in the brittle surface of a glacier.
Cross-bedding Structure in which relatively thin layers are inclined at an angle to the main bedding. Formed by currents of wind or water.
Cross-cutting A principle of relative dating. A rock or fault is younger than any rock (or fault) through which it cuts.
Crust The very thin, outermost layer of Earth.
Crystal An orderly arrangement of atoms.
Crystal form The external appearance of a mineral as determined by its internal arrangement of atoms.
Crystallization The formation and growth of a crystalline solid from a liquid or gas.
Curie point The temperature above which a material loses its magnetization.
Cut bank The area of active erosion on the outside of a meander.
Cutoff A short channel segment created when a river erodes through the narrow neck of land between meanders.
Daughter product An isotope resulting from radioactive decay.
Debris flow A relatively rapid type of mass wasting that involves a flow of soil and regolith containing a large amount of water. Also called mudflows.
Deep-focus earthquake An earthquake focus at a depth of more than 300 kilometers.
Deep-ocean basin The portion of seafloor that lies between the continental margin and the oceanic ridge system. This region comprises almost 30 percent of Earth's surface.
Deep-ocean trench A narrow, elongated depression of the seafloor.
Deflation The lifting and removal of loose material by wind.
Delta An accumulation of sediment formed where a stream enters a lake or ocean.
Dendritic pattern A stream system that resembles the pattern of a branching tree.
Density The weight per unit volume of a particular material.
Desert One of the two types of dry climate; the drier of the dry climates.
Desert pavement A layer of coarse pebbles and gravel created when wind removes the fmer material.
Detrital sedimentary rocks Rocks that form from the accumulation of materials that originate and are transported as solid particles derived from both mechanical and chemical weathering.
Dike A tabular-shaped intrusive igneous feature that cuts through the surrounding rock.
Dip The angle at which a rock layer is inclined from the horizontal. The direction of dip is at a right angle to the strike.
Dip-slip fault A fault in which the movement is parallel to the dip of the fault.
Discharge The quantity of water in a stream that passes a given point in a given period of time.
Disconformity A type of unconformity in which the beds above and below are parallel.
Discontinuity A sudden change with depth in one or more of the physical properties of the material making up Earth's interior. The boundary between two dissimilar materials in Earth's interior as deterrnined by the behavior of seismic waves.
Discordant A term used to describe plutons that cut across existing rock structures, such as bedding planes.
Disseminated deposit Any economic mineral deposit in which the desired mineral occurs as scattered particles in the rock but in sufficient quantity to make the deposit an ore.
Dissolved load The portion of a stream's load carried in solution.
Distributary A section of a stream that leaves the main flow.
Diurnal tide A tide characterized by a single high and low water height each tidal day.
Divergent boundary A boundary in which two plates move apart, resulting in upwelling of material from the mantle to create new seafloor.
Divide An imaginary line that separates the drainage of two streams; often found along a ridge.
Dome A roughly circular, upfolded structure.
Drainage basin The land area that contributes water to a stream.
Drawdown The difference in height between the bottom of a cone of depression and the original height of the water table.
Drift The general term for any glacial deposit.
Drumlin A streamlined asymmetrical hill composed of glacial till. The steep side of the hill faces the direction from which the ice advanced.
Dry climate A climate in which the yearly precipitation is less than the potential loss of water by evaporation.
Dune A hill or ridge of wind-deposited sand.
Earthflow The downslope movement of water-saturated, clay-rich sediment. Most characteristic of humid regions.
Earthquake Vibration of Earth produced by the rapid release of energy.
Echo sounder An instrument used to determine the depth of water by measuring the time interval between emission of a sound signal and the return of its echo from the bottom.
Effluent stream A stream channel that intersects the water table. Consequently, groundwater feeds into the stream.
Elastic deformation Nonpermanent deformation in which rock returns to its original shape when the stress is released.
Elastic rebound The sudden release of stored strain in rocks that results in movement along a fault.
Electron A negatively charged subatomic particle that has a negligible mass and is found outside the atom's nucleus.
Element A substance that cannot be decomposed into simpler substances by ordinary chemical or physical means.
Eluviation The washing out of fine soil components from the A horizon by downward-percolating water.
Emergent coast A coast where land formerly below the sea level has been exposed either by crustal uplift or a drop in sea level or both.
End moraine A ridge of till marking a fonner position of the front of the glacier.
Energy-Ievel shell The region occupied by electrons with a specific energy level.
Eon The largest time unit on the geologic time scale, next in order of magnitude above era.
Epicenter The location on Earth's surface that lies directly above the focus of an earthquake.
Epoch A unit of the geological time scale that is a subdivision of a period.
Era A major division on the geologic time scale; eras are dividedJ{1to shorter units called periods.
Erosion The incorporation and transportation of material by a mobile agent, such as water, wind, or ice.
Esker Sinuous ridge composed largely of sand gravel deposited by a stream flowing in ~ tunnel beneath a glacier near its teriminus.
Estuary A partially enclosed coastal water body that is connected to the ocean. Salinity here is measurably reduced by the freshwater flow of rivers.
Evaporite A sedimentary rock fonned of material deposited from solution by evaporation of water.
Evapotranspiration The combined effect of evaporation and transpiration.
Exfoliation dome Large, domeshaped structure, usually composed of granite, formed by sheeting.
Exotic stream A permanent stream that traverses a desert and has its source in well-watered areas outside the desert.
External process Process such as weathering, mass wasting, or erosion that is powered by the Sun and transforms solid rock into sediment.
Extrusive Igneous activity that occurs at Earth's surface.
Fall A type of movement common to mass-wasting processes that refers to the free falling of detached individual pieces of any size.
Fault A break in a rock mass along which movement has occurred.
Fault-block mountain A mountain formed by the displacement of rock along a fault.
Fault creep Slow, gradual displacement along a fault that occurs relatively smoothly and with little noticeable seismic activity.
Felsic A term derived from feldspar and silica (quartz). It is a term used to describe granitic igneous rocks.
Fetch The distance that the wind has traveled across the open water.
Fiord A steep-sided inlet of the sea formed when a glacial trough was partially submerged.
Fissure eruption An eruption in which lava is extruded from narrow fractures or cracks in the crust.
Flood basalts Flows of basaltic lava that issue from numerous cracks or fissures and commonly cover extensive areas to thicknesses of hundreds of meters.
Floodplain The flat, low-lying portion of a stream valley subject to periodic inundation.
Flow A type of movement common to rnass-wasting processes in which water-saturated material moves downslope as a viscous fluid.
Fluorescence The absorption of ultraviolet light, which is re-ernitted as visible light.
Fluvial deposit (or alluvium ) Unconsolidated sediment deposited by a stream.
Focus (earthquake) The zone within Earth where rock displacement produces an earthquake.
Fold A bent layer or series of layers that were originally horizontal and subsequently deformed.
Foliated A texture of metamorphic rocks that gives the rock a layered appearance.
Foliation A term for a linear arrangement of textural features often exhibited by metamorphic rocks.
Foreset bed An inclined bed deposited along the front of a delta.
Foreshocks Small earthquakes that often precede a major earthquake.
Fossil The remains or traces of organisms preserved from the geologic past.
Fossil fuel General term for any hydrocarbon that may be used as a fuel, including coal, oil, natural gas, bitumen from tar sands, and shale oil.
Fossil succession Fossil organisms succeed one another in a definite and determinable order, and any time period can be recognized by its fossil content.
Fractional crystallization The process that separates magma into components having varied compositions and melting points.
Fracture (mineral) One of the basic physical properties of minerals. It relates to the breakage of minerals when there are no planes of weakness in the crystalline structure. Examples include conchoidal, irregular, and splintery.
Fracture (rock) Any break or rupture in rock along which no appreciable movement has taken place.
Frost wedging The mechanical breakup of rock caused by the expansion of freezing water in cracks and crevices.
Fumarole A vent in a volcanic area from which fumes or gases escape.
Geology The science that examines Earth, its form and composition, and the changes it has undergone and is undergoing.
Geothermal energy Natural steam used for power generation.
Geothermal gradient The gradual increase in temperature with depth in the crust. The average is 30°C per kilometer in the upper crust.
Geyser A fountain of hot water ejected periodically from the ground.
Glacial budget The balance, or lack of balance, between accumulation at the upper end of a glacier, and loss at the other end.
Glacial erratic An ice-transported boulder that was not derived from the bedrock near its present site.
Glacial striations Scratches and grooves on bedrock caused by glacial abrasion.
Glacial trough A mountain valley that has been widened, deepened, and straightened by a glacier.
Glacier A thick mass of ice originat ing on land from the compaction and recrystallization of snow. The ice shows evidence of past or present flow.
Glass (volcanic) Natural glass produced when molten lava cools too rapidly to permit crystallization. Volcanic glass is a solid composed of unordered atoms.
Glassy texture A term used to describe the texture of certain igneous rocks, such as obsidian, that contain no crystals.
Gneissic texture The texture displayed by the metamorphic rock gneiss in which dark and light silicate minerals have separated, giving the rock a banded appearance.
Gondwanaland The southern portion of Pangaea consisting of South America, Africa, Australia, India, and Antarctica.
Graben A valley formed by the downward displacement of a faultbounded block.
Graded bed A sediment layer characterized by a decrease in sediment size from bottom to top.
Graded stream A stream that has the correct channel characteristics to maintain the exact velocity required to transport the material supplied to it.
Gradient The slope of a stream; generally measured in feet per mile.
Granitic Igneous rocks composed mainly of light-colored silicates (quartz and feldspar) are said to have this composition.
Greenhouse effect Carbon dioxide and water vapor in a planet's atmosphere absorb and reradiate infrared wavelengths, effectively trapping solar energy and raising the temperature.
Groin A short wall built at a right angle to the seashore to trap moving sand.
Groundmass The matrix of smaller crystals within an igneous rock that has porphyritic texture.
Ground moraine An undulating layer of till deposited as the ice front retreats.
Groundwater Water in the zone of saturation.
Guyot A submerged flat-topped seamount.
Half-life The time required for onehalf of the atoms of a radioactive substance to decay.
Hanging valley A tributary valley that enters a glacial trough at a considerable height above the floor of the trough.
Hardness A mineral's resistance to scratching and abrasion.
Head The vertical distance between the recharge and discharge points of a water table. Also, the source area or beginning of a valley.
Headward erosion The extension up slope of the head of a valley due to erosion.
Historical geology A major division of geology that deals with the origin of Earth and its development through time. Usually involves the study of fossils and their sequence in rock beds.
Hogback A narrow, sharp-crested ridge formed by the upturned edge of a steeply dipping bed of resistant rock.
Horn A pyramid-like peak formed by glacial action in three or more cirques surrounding a mountain summit.
Horst An elongated, uplifted block of crust bounded by faults.
Hot spot A proposed concentration of heat in the mantle capable of introducing magma that, in turn, extrudes onto Earth's surface. The intraplate volcanism that produced the Hawaiian Islands is one example.
Hot spring A spring in which the water is 6° to 9°C (10° to 15° F) warmer than the mean annual air temperature of its locality.
Humus Organic matter in soil produced by the decomposition of plants and animals.
Hydrogenous sediment Seafloor sediments consisting of minerals that crystallize from seawater. The principal example is manganese nodules.
Hydrologic cycle The unending circulation of Earth's water supply. The cycle is powered by energy from the Sun and is characterized by continuous exchanges of water among the oceans, the atmosphere, and the continents.
Hydrolysis A chemical weathering process in which minerals are altered by chemically reacting with water and acids.
Hydrosphere The water portion of our planet; one of the traditional subdivisions of Earth's physical environment.
Hydrothermal solution The hot, watery solution that escapes from a mass of magma during the latter stages of crystallization. Such solutions may alter the surrounding country rock and are frequently the source of significant ore deposits.
Hypothesis A tentative explanation that is then tested to determine if it is valid.
Ice cap A mass of glacial ice covering a high upland or plateau and spreading out radially.
Ice-contact deposit An accumulation of stratified drift deposited in contact with a supporting mass of ice.
Ice sheet A very large, thick mass of glacial ice flowing outward in all directions from one or more accumulation centers.
Igneous rock A rock formed by the crystallization of molten magma.
Immature soil A soil lacking horizons.
Incised meander Meandering channel that flows in a steep, narrow valley. These meanders form either when an area is uplifted or when base level drops.
Inclusion A piece of one rock unit contained within another. Inclusions are used in relative dating. The rock mass adjacent to the one containing the inclusion must have been there first in order to provide the fragment.
Index fossil A fossil that is associated with a particular span of geologic time.
Index mineral A mineral that is a good indicator of the metamorphic environment in which it formed. Used to distinguish different zones of regional metamorphism.
Inertia Objects at rest tend to remain at rest and objects in motion tend to stay in motion unless either is acted upon by an outside force.
Infiltration The movement of surface water into rock or soil through crack and pore spaces.
Infiltration capacity The maximum rate at which soil can absorb water.
Influent stream A stream channel that is above the water table level. Water seeps downward from the channel to the zone of saturation to produce an upward bulge in the water table.
Inner core The solid, innermost layer of Earth, about 1216 kilometers (754 miles) in radius.
Inselberg An isolated mountain remnant characteristic of the late stage of erosion in a mountainous region.
Intensity (eariliquake) An indication of the destructive effects of an earthquake at a particular place. Intensity is affected by such factors as distance to the epicenter and the nature of the surface materials.
Interior drainage A discontinuous pattern of intermittent streams that do not flow to the ocean.
Intermediate Compositional category for igneous rocks found near the middle of Bowen's reaction series, mainly amphibole and the intermediate plagioclase feldspars.
Intermediate focus An earthquake focus at a depth of between 60 and 300 kilometers.
Intrusive rock Igneous rock that formed below Earth's surface.
Ion An atom or molecule that possesses an electrical charge.
Ionic bond A chemical bond between two oppositely charged ions formed by the transfer of valence electrons from one atom to another.
Isostasy The concept that Earth's crust is "floating" in gravitational balance upon the material of the mantle.
Isotopes Varieties of the same element that have different mass numbers; their nuclei contain the same number of protons but different numbers of neutrons.
Joint A fracture in rock along which there has been no movement.
Kame A steep-sided hill composed of sand and gravel originating when sediment collected in openings in stagnant glacial ice.
Kame terrace A narrow, terrace-like mass of stratified drift deposited between a glacier and an adjacent valley wall.
Karst A topography consisting of numerous depressions called sinkholes.
Kettle holes Depressions created when blocks of ice become lodged in glacial deposits and subsequently melt.
Laccolith A massive, concordant igneous body intruded between preexisting strata.
Lahar Mudflows on the slopes of volcanoes that result when unstable layers of ash and debris become saturated and flow downslope, usually following stream channels.
Laminar flow The movement of water particles in straight-line paths that are parallel to the channel. The water particles move downstream without mixing.
Lateral moraine A ridge of till along the sides of a valley glacier composed primarily of debris that fell to the glacier from the valley walls.
Laterite A red, highly leached soil type found in the tropics and rich in oxides of iron and aluminum.
Laurasia The northern portion of Pangaea consisting of North America and Eurasia.
Lava Magma that reaches Earth's surface.
Lava dome A bulbous mass associated with an old-age volcano, produced when thick lava is slowly squeezed from the vent. Lava domes may act as plugs to deflect subsequent gaseous eruptions.
Law of superposition In any undeformed sequence of sedimentary rocks or surface-deposited igneous materials, each layer is older than the one above it and younger than the one below.
Leaching The depletion of soluble materials from the upper soil by downward-percolating water.
Lithification The process, generally cementation and/or compaction, of converting sediments to solid rock.
Lithosphere The rigid outer layer of Earth, including the crust and upper mantle.
Loess Deposits of windblown silt, lacking visible layers, generally buff colored, and capable ofmaintaining a nearly vertical cliff.
Longitudinal dunes Long ridges of sand oriented parallel to the prevailing wind; these dunes form where sand supplies are limited.
Longitudinal profile A cross section of a stream channel along its descending course from the head to the mouth.
Longshore current A nearshore current that flows parallel to the shore.
Luster The appearance or quality of light reflected from the surface of a mineral.
Mafic Because basaltic rocks contain a high percentage of ferromagnesian minerals, they are also called mafic (from magnesium and ferrum, the Latin name for iron).
Magma A body of molten rock found at depth, including any dissolved gases and crystals.
Magnetometer A sensitive instrument used to measure the intensity of Earth's magnetic field at various points.
Magnitude (earthquake ) The total amount of energy released during an earthquake.
Manganese nodules A type of hydrogenous sediment scattered on the ocean floor, consisting mainly of manganese and iron, and usually containing small amounts of copper, nickel, and cobalt.
Mantle The 2885-kilometer (1789mile) thick layer of Earth located below the crust.
Mantle plume The source of some intraplate basaltic magma. Rising plumes may originate at the core-mantle boundary. Partial melting of this hot rock as it enters the lower pressure environment near the surface generates a volcanic area known as a bot spot.
Mass number The sum of the number of neutrons and protons in the nucleus of an atom.
Mass wasting The downslope movement of rock, regolith, and soil under the direct influence of gravity.
Meander A loop like bend in the course of a stream.
Meander scar A floodplain feature created when an oxbow lake becomes filled with sediment.
Mechanical weadlering The physical disintegration of rock, resulting in smaller fragments.
Medial moraine A ridge of till formed when lateral moraines from two coalescing valley glaciers join.
Melt The liquid portion of magma excluding the solid crystals.
Mercalli intensity scale A 12-point scale originally developed to evaluate earthquake intensity based upon the amount of damage to various types of structures.
Mesozoic era A time span on the geologic time scale between the Paleozoic and Cenozoic eras.
Metallic bond A chemical bond present in all metals that may be characterized as an extreme type of electron sharing in which the electrons move freely from atom to atom.
Metamorphic rock Rock formed by the alteration of preexisting rock deep within Earth (but still in the solid state) by heat, pressure, and/or chemically active fluids.
Metamorphism The changes in mineral composition and texture of a rock subjected to high temperature and pressure within earth.
Mid-ocean ridge A continuous 70,000-kilometer-long elevated zone on the floor of all major ocean basins, varying in width from 3000 to 4000 kilometers. The rift valley at the crest of the ridge represents a divergent plate boundary.
Migmatite A rock exhibiting both igneous and metamorphic rock characteristics. Such rocks may form when light-colored silicate minerals melt and then crystallize, while the dark silicate minerals remain solid.
Mineral A naturally occurring, inorganic crystalline material with a unique chemical structure.
Mineral resource All discovered and undiscovered deposits of a useful mineral that can be extracted now or at some time in the future.
Mohorovicic discontinuity (Moho) The boundary separating the crust and the mantle, discernible by an increase in seismic velocity.
Mohs scale A series of 10 minerals used as a standard in determining hardness.
Monocline A one-limbed flexure in strata. The strata are usually flat lying or very gently dipping on both sides of the monocline.
Mouth The point downstream where a river empties into another stream or water body.
Mud crack A feature in some sedimentary rocks that forms when wet mud dries out, shrinks, and cracks.
Mudflow A relatively rapid type of mass wasting that involves a flow of soil and regolith containing a large amount of water. Also called Debris flow.
Natural levees The elevated landforms composed of alluvium that parallel som~ streams and act to confine their waters, except during floodstage.
Neap tide The lowest tidal range, occurring near the times of the fIrst and third quarters of the Moon.
Nebular hypothesis A model for the origin of the solar system that assumes a rotating nebula of dust and gases that contracted to produce the Sun and planets.
Neutron A subatomic particle found in the nucleus of an atom. The neutron is electrically neutral with a mass approximately equal to that of a proton.
Nonclastic A term for the texture of sedimentary rocks in which the minerals form a pattern of interlocking crystals.
Nonconformity An unconformity in which older metamorphic or intrusive igneous rocks are overlain by younger sedimentary strata.
Nonfoliated Metamorphic rocks that do not exhibit foliation.
Nonmetallic mineral resource Mineral resource that is not a fuel or processed for the metals it contains.
Nonrenewable resource Resource that forms or accumulates over such long time spans that it must be considered as fixed in total quantity.
Normal fault A fault in which the rock above the fault plane has moved down relative to the rock below.
Normal polarity A magnetic field the same as that which presently exists.
Nucleus The small, heavy core of an atom that contains all of its positive charge and most of its mass.
Nuee ardente Incandescent volcanic debris that is buoyed up by hot gases and moves downslope in an avalanche fashion.
Numerical date Date that specifies the actual number of years that have passed since an event occurred.
Oblique-slip fault A fault having both vertical and horizontal movement.
Octet rule Atoms combine in order that each may have the electron arrangement of a noble gas; that is, the outer energy level contains eight electrons.
Oil trap A geologic structure that allows for significant amounts of oil and gas to accumulate.
Ore Usually a useful metallic mineral that can be mined at a profit. The term is also applied to certain nonmetallic minerals such as fluorite and sulfur.
Original horizontality Layers of sediment are generally deposited in a horizontal or nearly horizontal position.
Orogenesis The processes that collectively result in the formation of mountains.
Outer core A layer beneath the mantle about 2270 kilometers (1407 miles) thick that has the properties of a liquid.
Outgassing The release of gases dissolved in molten rock.
Outwash Sediments deposited by glacial meltwater.
Outwash plain A relatively flat, gently sloping plain consisting of materials deposited by meltwater streams in front of the margin of an ice sheet.
Oxbow lake A curved lake produced when a stream cuts off a meander.
Oxidation The removal of one or more electrons from an atom or ion. So named because elements commonly combine with oxygen.
Pahoehoe A lava flow with a smooth-to-ropy surface.
Paleomagnetism The natural remnant magnetism in rock bodies. The permanent magnetization acquired by rock that can be used to determine the location of the magnetic poles and the latitude of the rock at the time it became magnetized.
Paleontology The systematic study of fossils and the history of life on earth.
Paleozoic era A time span on the geologic calendar between the Precambrian and Mesozoic eras.
Pangaea The proposed supercontinent that 200 million years ago began to break apart and form the present landmasses.
Parabolic dune A sand dune similar in shape to a barchan dune except that its tips point into the wind. These dunes often form along coasts that have strong onshore winds, abundant sand, and vegetation that partly covers the sand.
Parasitic cone A volcanic cone that forms on the flank of a larger volcano.
Parent material The material upon which a soil develops.
Partial melting The process by which most igneous rocks melt. Because individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results.
Passive continental margin Margins that consist of a continental shelf, continental slope, and continental rise. They are not associated with plate boundaries and therefore experience little volcanism and few earthquakes.
Pater noster lakes A chain of small lakes in a glacial trough that occupy basins created by glacial erosion.
Pedalfer Soil of humid regions characterized by the accumulation of iron oxides and aluminum-rich clays in the B horizon.
Pedocal Soil associated with drier regions and characterized by an accumulation of calcium carbonate in the upper horizons.
Pegmatite A very coarse-grained igneous rock (typically granite) commonly found as a dike associated with a large mass of plutonic rock that has smaller crystals. Crystallization in a water-rich environment is believed to be responsible for the very large crystals.
Peneplain In the idealized cycle of landscape evolution in a humid region, an undulating plain near base level associated with old age.
Perched water table A localized zone of saturation above the main water table created by an impermeable layer (aquitard).
Peridotite An igneous rock of ultramafic composition thought to be abundant in the upper mantle.
Period A basic unit of the geologic calendar that is a subdivision of an era. Periods may be divided into smaller units called epochs.
Permafrost Any permanently frozen subsoil. Usually found in the subarctic and arctic regions.
Permeability A measure of a material's ability to transmit water.
Phaneritic texture An igneous rock texture in which the crystals are roughly equal in size and large enough so that individual minerals can be identified with the unaided eye.
Phanerozoic eon That part of geologic time represented by rocks containing abundant fossil evidence. The eon extending from the end pf the Proterozoic eon to the present.
Phenocryst Conspicuously large crystals in a porphyry that are imbedded in a matrix of finer-grained crystals (the groundmass).
Physical geology A major division of geology that examines the materials of Earth and seeks to understand the processes and forces acting upon Earth's surface from below. (This definition is worse than useless - HR)
Piedmont glacier A glacier that forms when one or more valley glaciers emerge from the confining walls of mountain valleys and spread out to create a broad sheet in the lowlands at the base of the mountains.
Pillow lava Basaltic lava that solidifies in an underwater environment and develops a structure that resembles a pile of pillows.
Pipe A vertical conduit through which magmatic materials have passed.
Placer Deposit formed when heavy mine~als are mechanically concentrated by currents, most commonly streams and waves. Placers are sources of gold, tin, platinum, diamonds, and other valuable minerals.
Plastic deformation Permanent deformation that results in a change in size and shape through folding or flowing.
Plastic flow A type of glacial movement that occurs within the glacier, below a depth of approximately 50 meters, in which the ice is not fractured.
Plate One of numerous rigid sections of the lithosphere that moves as a unit over the material of the asthenosphere.
Plate tectonics The theory that proposes Earth's outer shell consists of individual plates, which interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself.
Playa The flat central area qf an undrained desert basin.
Playa lake A temporary lake in a playa.
Pleistocene epoch An epoch of the Quaternary period beginning about 1.6 million years ago and ending about 10,000 years ago. Best known as a time of extensive continental ice sheets.
Plucking The process by which pieces of bedrock are lifted out of place by a glacier.
Pluton A structure that results from the emplacement and crystallization of magma beneath Earth's surface.
Pluvial lake A lake formed during a period of increased rainfall. For example, this occurred in many nonglaciated areas during periods of ice advance elsewhere.
Point bar A crescent-shaped accumulation of sand and gravel deposited on the inside of a meander.
Polar wandering hypothesis As the result of paleomagnetic studies in the 1950s, researchers proposed that either the magnetic poles migrated greatly through time or the continents had gradually shifted their positions.
Polymorphs Two or more minerals having the same chemical composition but different crystalline structures. Exemplified by the diamond and graphite forms of carbon.
Porosity The volume of open spaces in rock or soil.
Porphyritic texture An igneous rock texture characterized by two distinctively different crystal sizes. The larger crystals are called pbenocrysts, and the matrix of smaller crystals is termed the groundmass.
Porphyry An igneous rock with a porphyritic texture.
Pothole A depression formed in a stream channel by the abrasive action of the water's sediment load. ,
Precambrian All geologic time prior to the Paleozoic era.
Principle of fossil succession Fossil organisms succeed one another in a definite and determinable order, and any time period can be recognized by its fossil content.
Principle of original horizontality Layers of sediment are generally deposited in a horizontal or nearly horizontal position.
Proterozoic eon The eon following the Archean and preceding the Phanerozoic eon. It extends between 2500 million and 570 million years ago.
Proton A positively charged subatomic particle found in the nucleus of an atom.
P wave The fastest earthquake wave; travels by compression and expansion of the medium.
Pyroclastic flow A highly heated mixture, largely of ash and pumice fragments, traveling down the flanks of a volcano or along the surface of the ground.
Pyroclastic material The volcanic rock ejected during an eruption. Pyroclastics include ash, bombs, and blocks.
Pyroclastic texture An igneous rock texture resulting from the consolidation of individual rock fragments that are ejected during a violent eruption.
Radial drainage A system of streams running in all directions away from a central elevated structure, such as a volcano.
Radioactivity The spontaneous decay of certain unstable atomic nuclei.
Radiocarbon (carbon-14) The radioactive isotope of carbon produced continuously in the atmosphere and used in dating events as far back as 75,000 years.
Radiometric dating The procedure of calculating the absolute ages of rocks and minerals containing certain radioactive isotopes.
Rainshadow desert A dry area on the lee side of a mountain range. Many middle-latitude deserts are of this type.
Rapids A part of a stream channel in which the water suddenly begins flowing more swiftly and turbulently because of an abrupt steepening of the gradient.
Recessional moraine An end moraine formed as the ice front stagnated during glacial retreat.
Rectangular pattern A drainage pattern that develops on jointed or fractured bedrock and is characterized by numerous right-angle bends.
Refraction A change in direction of waves as they enter shallow water. The portion of the wave in shallow water is slowed, which causes the wave to bend and align with the underwater contours.
Regional metamorphism Metamorphism associated with largescale mountain building.
Regolith The layer of rock and mineral fragments that nearly everywhere covers Earth's land surface.
Relative dating Rocks are placed in their proper sequence or order. Only the chronological order of events is determined.
Renewable resource A resource that is virtually inexhaustible or that can be replenished over relatively short time spans.
Reserve Already identified deposits from which minerals can be extracted profitably.
Reservoir rock The porous, permeable portion of an oil trap that yields oil and gas.
Residual son Soil developed directly from the weathering of the bedrock below.
Reverse fault A fault in which the material above the fault plane moves up in relation to the material below.
Reverse polarity A magnetic field opposite to that which presently exists.
Richter scale A scale of earthquake magnitude based on the motion of a seismograph.
Rift A region of Earth's crust along which divergence (separation) is taking place.
Ripple marks Small waves of sand that develop on the surface of a sediment layer by the action of moving water or air.
Roche moutonnee An asymmetrical knob of bedrock formed when glacial abrasion smooths the gentle slope facing the advancing ice sheet and plucking steepens the opposite side as the ice overrides the knob.
Rock A consolidated mixture of minerals.
Rock avalanche The very rapid downslope movement of rock and debris. These rapid movements may be aided by a layer of air trapped beneath the debris, and they have been known to reach speeds in excess of 200 kilometers per hour.
Rock cleavage The tendency of rock to split along parallel, closely spaced surfaces. These surfaces are often highly inclined to the bedding planes in the rock.
Rock cycle A model that illustrates the origin of the three basic rock types and the interrelatedness of Earth's materials and processes.
Rock flour Ground-up rock produced by the grinding effect of a glacier.
Rockslide The rapid slide of a mass of rock downslope along planes of weakness.
Runoff Water that flows over the land rather than infiltrating into the ground.
Saltation Transportation of sediment through a series of leaps or bounces.
Salt flat A white crust on the ground produced when water evaporates and leaves its dissolved materials behind.
Schistosity A type of foliation characteristic of coarser-grained metamorphic rocks. Such rocks have a parallel arrangement of platy rninerals such as the micas.
Scoria Hardened lava that has retained the vesicles produced byescaping gases.
Sea arch An arch formed by wave erosion when caves on opposite sides of a headland unite.
Seafloor spreading The hypothesis first proposed in the 1960s by Harry Hess that suggested that new oceanic crust is produced at the crests of mid-ocean ridges, which are the sites of divergence.
Seamount An isolated volcanic peak that rises at least 1000 meters (3300 feet) above the deep-ocean floor.
Sea stack An isolated mass of rock standing just offshore, produced by wave erosion of a headland.
Seawall A barrier constructed to prevent waves from reaching the area behind the wall. Its purpose is to defend property from the force of breaking waves.
Secondary enrichment The concentration of minor amounts of metals that are scattered through unweathered rocks into economically valuable concentrations by weathering processes.
Secondary (S) wave A seismic wave that involves oscillation perpendicular to the direction of propagation.
Sediment Unconsolidated particles created by the weathering and erosion of rock, by chemical precipitations from solution in water, or from the secretions of organisms, and transported by water, wind, or glaciers.
Sedimentary rock Rock formed from the weathered products of preexisting rocks that have been transported, deposited, and lithified.
Seiche The rhythmic sloshing of water in lakes, reservoirs, and other smaller enclosed basins. Some seiches are initiated by earthquake activity.
Seismic sea wave A rapidly moving ocean wave generated by earthquake activity and capable of inflicting heavy damage in coastal regions.
Seismogram The record made by a seismograph.
Seismograph An instrument that records earthquake waves.
Seismology The study of earthquakes and seismic waves.
Settling velocity The speed at which a particle falls through a still fluid. The size, shape, and specific gravity of particles influence setding velocity.
Shadow zone The zone between 105 and 140 degrees distance from an earthquake epicenter that direct waves do not penetrate because of refraction by Earth's core.
Shallow-focus earthquake An earthquake focus at a depth of less than 60 kilometers.
Shear Stress that causes two adjacent parts of a body to slide past one another.
Sheet flow Runoff moving in unconfined thin sheets.
Sheeting A mechanical weathering process characterized by the splitting off of slablike sheets of rock.
Shelf break The point at which a rapid steepening of the gradierit occurs, marking the outer edge of the continental shelf and the beginning of the continental slope.
Shield A large, relatively flat expanse of ancient metamorphic rock within the stable continental interior.
Shield volcano A broad, gently sloping volcano built from fluid basaltic lavas.
Silicate Anyone of numerous minerals that have the silicon-oxygen tetrahedron as their basic structure.
Silicon-oxygen tetrahedron A structure composed of four oxygen atoms surrounding a silicon atom that constitutes the basic building block of silicate minerals.
Sill A tabular igneous body that was intruded parallel to the layering of preexisting rock.
Sinkhole A depression produced in a region where soluble rock has been removed by groundwater.
Slaty cleavage The type of foliation characteristic of slates in which there is a parallel arrangement of fme-grained metamorphic minerals.
Slide A movement common to mass-wasting processes in which the material moving downslope remains fairly coherent and moves along a well-defined surface.
Slip face The steep, leeward surface of a sand dune that maintains a slope of about 34 degrees.
Slump The downward slipping of a mass of rock or unconsolidated material moving as a unit along a curved surface.
Snowfield An area where snow persists year-round.
Snowline Lower limit of perennial snow.
Soil A combination of mineral and organic matter, water, and air; that portion of the regolith that supports plant growth.
Soil horizon A layer of soil that has identifiable characteristics produced by chemical weathering and other soil-forming processes.
Soil profile A vertical section through a soil showing its succession of horizons and the underlying parent material.
Solitluction Slow, downslope flow of water-saturated materials common to permafrost areas.
Solum The 0, A, and B horizons in a soil profile. Living roots and other plant and animal life are largely confined to this zone.
Solution The change of matter from the solid or gaseous state into the liquid state by its combination with a liquid.
Sorting The degree of similarity in particle size in sediment or sedimentary rock.
Specific gravity The ratio of a substance's weight to the weight of an equal volume of water.
Speleothem A collective term for the drips tone features found in caverns.
Spheroidal weathering Any weathering process that tends to produce a spherical shape from an initially blocky shape.
Spit An elongated ridge of sand that projects from the land into the mouth of an adjacent bay.
Spring A flow of groundwater that emerges naturally at the ground surface.
Spring tide The highest tidal range. Occurs near the times of the new and full moons.
Stalactite The icicle-like structure that hangs from the ceiling of a cavern.
Stalagmite The columnlike form that grows upward from the floor of a cavern.
Star dune Isolated hill of sand that exhibits a complex form and develops where wind conditions are variable.
Steppe One of the two types of dry climate. A marginal and more humid variant of the desert that separates the desert from bordering humid climates.
Stock A pluton similar to but smaller than a batholith.
Strata Parallel layers of sedimentary rock.
Stratovolcano A volcano composed of both lava flows and proclastic material. (Also called a Composite cone.)
Streak The color of a mineral in powdered form.
Stream A genernl tenn to denote the flow of water within any natural channel. Thus, a small creek and a large river are both streams.
Stress The force per unit area acting on any surface within a solid. Also known as directed pressure.
Striations The multitude of fine parallel lines found on some cleavage faces of plagioclase feldspars but not present on orthoclase feldspar.
Striations {glacial) Scratches or grooves in a bedrock surface caused by the grinding action of a glacier and its load of sediment.
Strike The compass direction of the line of intersection created by a dipping bed or fault and a horizontal surface. Strike is always perpendicular to the direction of dip.
Strike-slip fault A fault along which the movement is horizontal.
Stromatolite Structures that are deposited by algae and that consist of layered mounds or columns of calcium carbonate.
Subduction The process of thrusting oceanic lithosphere into the mantle along a convergent zone.
Subduction zone A long, narrow zone where one lithospheric plate descends beneath another.
Submarine canyon A seaward extension of a valley that was cut on the continental shelf during a time when sea level was lower, or a canyon carved into the outer continental shelf, slope, and rise by turbidity currents.
Submergent coast A coast whose form is largely the result of the partial drowning of a former land surface either due to a rise of sea level or subsidence of the crust, or both.
Subsoil A term applied to the B horizon of a soil profile.
Superposition, law of In any undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below.
Surf A collective term for breakers; also the wave activity in the area between the shoreline and the outer limit of breakers.
Surface son The upper portion of a soil profile consisting of the O and A horizons.
Surface waves Seismic waves that travel along the outer layer of Earth.
Surge A period of rapid glacial advance. Surges are typically sporadic and short-lived.
Suspended load The fine sediment carried within the body of flowing water or air.
S wave An earthquake wave, slower than a P wave, that travels only in solids.
Swells Wind-generated waves that have moved into an area of weaker winds or calm.
Syncline A linear downfold in sedimentary strata; the opposite of anticline.
Talus An accumulation of rock debris at the base of a cliff.
Tarn A small lake in a cirque.
Tectonics The study of the large scale processes that collectively deform Earth's crust.
Temporary (local) base level The level of a lake, resistant rock layer, or any other base level that stands above sea level.
Terminal moraine The end moraine marking the farthest advance of a glacier.
Terrace A flat, benchlike structure produced by a stream and that was left elevated as the stream cut downward.
Terrane A crustal block bounded by faults whose geologic history is distinct from the histories of adjoining crustal blocks.
Terrigenous sediment Seafloor sediment derived from terrestrial weathering and erosion.
Texture The size, shape, and distribution of the particles that collectively constitute a rock.
Theory A well-tested and widely accepted view that explains certain observable facts.
Thrust fault A low-angle reverse fault.
Tide Periodic change in the elevation of the ocean's surface.
Till Unsorted sediment deposited directly by a glacier.
Tillite A rock formed when glacial till is lithified.
Tombolo A ridge of sand that connects an island to the mainland or to another island.
Topset bed An essentially horizontal sedimentary layer deposited on top of a delta during floodstage.
Transform fault boundary A boundary in which two plates slide past one another without creating or destroying lithosphere.
Transpiration The release of water vapor to the atmosphere by plants.
Transported son Soils that form on unconsolidated deposits.
Transverse dunes A series of long ridges oriented at right angles to the prevailing wind; these dunes form where vegetation is sparse and sand is very plentiful.
Travertine A form of limestone (CaCO₃) that is deposited by hot springs or as a cave deposit.
Trellis drainage A system of streams in which nearly parallel tributaries occupy valleys cut in folded strata.
Trench An elongated depression in the seafloor produced by bending of oceanic crust during subduction.
Truncated spurs Triangular-shaped cliffs produced when spurs of land that extend into a valley are removed by the great erosional force of a valley glacier.
Tsunami The Japanese word for a seismic sea wave.
Turbidite Turbidity current deposit characterized by graded bedding.
Turbidity current A downslope movement of dense, sediment-laden water created when sand and mud on the continental shelf and slope are dislodged and thrown into suspension.
Turbulent flow The movement of water ifi an erratic fashion often characterized by swirling, whirlpool-like eddies. Most streamflow is of this type.
Ultimate base level Sea level; the lowest level to which stream erosion could lower the land.
Ultramafic Compositional category for igneous rocks made up almost entirely of ferromagnesian minerals (mostly olivine and pyroxene).
Unconformity A surface that represents a break in the rock record; caused by erosion or nondeposition.
Uniformitarianism The concept that the processes that have shaped Earth in the geologic past are essentially the same as those operating today.
Valence electron The electrons involved in the bonding process; the electrons occupying the highest principal energy level of an atom.
Valley glacier A glacier confined to a mountain valley, which in most instances had previously been a stream valley.
Valley train A relatively narrow body of stratified drift deposited on a valley floor by meltwater streams that issue from the terminus of a valley glacier.
Vein deposit A mineral fIlling a fracture or fault in a host rock. Such deposits have a sheetlike, or tabular, form.
Ventifact A cobble or pebble polished and shaped by the sandblasting effect of wind.
Vesicles Spherical or elongated openings on the outer portion of a lava flow that were created by escapinggases.
Vesicular A term applied to igneous rocks that contain small cavities, termed vesicles, which are formed when gases escape from lava.
Viscosity A measure of a fluid's resistance to flow.
Volcanic Pertaining to the activities, structures, or rock types of a volcano.
Volcanic island arc A chain of volcanic islands generally located a few hundred kilometers from a trench where active subduction of one oceanic plate beneath another is occurring.
Volcanic bomb A streamlined pyroclastic fragment ejected from a volcano while the fragment is still molten.
Volcanic neck An isolated, steep-sided, erosional remnant consisting of lava that once occupied the vent of a volcano.
Volcano A mountain formed from lava and/or pyroclastics.
Wash A desert stream course that is typically dry except for brief periods immediately following rainfall.
Water gap A pass through a ridge or mountain in which a stream flows.
Water table The upper level of the saturated zone of groundwater.
Wave-cut cliff A seaward-facing cliff along a steep shore line formed by wave erosion at its base and by mass wasting.
Wave-cut platform A bench or shelf along a shore at sea level, cut by wave erosion.
Wave height The vertical distance between the trough and crest of a wave.
Wave length The horizontal distance separating successive crests or troughs.
Wave of oscillation A water wave in which the wave form advances as the water particles move in circular orbits.
Wave of translation The turbulent advance of water created by breaking waves.
Wave period The time interval between the passage of successive crests at a stationary point.
Weathering The disintegration and decomposition of rock at or near the surface of the Earth.
Welded tuff A pyroclastic deposit composed of particles fused together by the combination of heat still contained in the deposit after it has come to rest and by the weight of overlying material.
Well An opening bored into the zone of saturation.
Wind gap An abandoned water gap. These gorges typically result from stream piracy.
Xenolith An inclusion of unmelted country rock in an igneous pluton.
Xerophyte A plant highly tolerant of drought.
Yazoo tributary A tributary that flows parallel to the main stream because a natural levee is present.
Zone of accumulation The part of a glacier characterized by snow accumulation and ice formation. The outer limit of this zone is the snowline.
Zone of aeration Area above the water table where openings in soil, sediment, and rock are not saturated but are filled mainly with air.
Zone of fracture The upper portion of a glacier consisting of brittle ice.
Zone of saturation Zone where all open spaces in sediment and rock are completely filled with water.