Geology Glossary

(Most definitions taken from Physical Geology 11e)

(Last update - January 25, 2009)





§  0/00 – The symbol for parts per thousand.

§  Aa – A lava flow that solidifies with a spiny, rubbly surface.

§  Ablation – The loss of the glacial ice or snow by melting, evaporation, or breaking off into icebergs. (Also called wastage.)

§  Abrasion – The grinding away of rock by friction and impact during transportation.

§  Absolute age – Age given in years or some other unit of time. (Also known as numerical age.)

§  Abyssal fan – Great fan-shaped deposit of sediment on the deep-sea floor at the base of many submarine canyons.

§  Abyssal plain – Very flat, sediment-covered region of the deep-sea floor, usually at the base of the continental rise.

§  Accreted terrane – Terrane that did not form at its present site on a continent.

§  Accretionary wedge (subduction complex) – A wedge of thrust-faulted and folded sediment scraped off a subducting plate by the overlying plate.

§  Active continental margin – A margin consisting of a continental shelf, a continental slope, and an oceanic trench.

§  Actualism – The principle that the same processes and natural laws that operated in the past are those we can actually observe or infer from observations as operating at present. Under present usage, uniformitarianism has the same meaning as actualism for most geologists.

§  Advancing glacier – Glacier with a positive budget, so that accumulation results in the lower edges being pushed outward and downward.

§  Aftershock – Small earthquake that follows a main shock.

§  A horizon – The top layer of soil, characterized by the downward movement of water. (Also called zone of leaching.)

§  Alkali soil – Soil containing such a great quantity of sodium salts precipitated by evaporating ground water that it is generally unfit for plant growth.

§  Alluvial fan – Large, fan-shaped pile of sediment that usually forms where a stream's velocity decreases as it emerges from a narrow canyon onto a flat plain at the foot of a mountain range.

§  Alpine glaciation – Glaciation of a mountainous area.

§  Amphibole group – Mineral group in which all members are double-chain silicates.

§  Amphibolite – Amphibole (hornblende), plagioclase schist.

§  Andesite – Fine-grained igneous rock of intermediate composition. Up to half of the rock is plagioclase feldspar with the rest being ferromagnesian minerals.

§  Angle of dip – A vertical angle measured downward from the horizontal plane to an inclined plane.

§  Angular – Sharp-edged; lacking rounded edges or corners.

§  Angular unconformity – An unconformity in which younger strata overlie an erosion surface on tilted or folded layered rock.

§  Anorthosite – A crystalline rock composed almost entirely of calcium-rich plagioclase feldspar.

§  Antecedent stream – A stream that maintains its original course despite later deformation of the land.

§  Anthracite – Coal that has undergone low-grade metamorphism. Burns dust-free and smokeless.

§  Anticline – An arched fold in which the rock layers usually dip away from the axis of the fold.

§  Aquifer – A body of saturated rock or sediment through which water can move readily.

§  Arch (sea arch) – Bridge of rock left above an opening eroded in a headland by waves.

§  Archean Eon – The oldest eon of Earth's history.

§  Arκte – A sharp ridge that separates adjacent glacially carved valleys.

§  Arid region – An area with less than 25 centimeters of rain per year.

§  Arkose – A sandstone in which more than 25% of the grains are feldspar.

§  Artesian aquifer – See confined aquifer.

§  Artesian well – A well in which water rises above the aquifer.

§  Artificial recharge – Groundwater recharge increased by engineering techniques.

§  Aseismic ridge – Submarine ridge with which no earthquakes are associated.

§  Ash (volcanic) – Fine pyroclasts (less than 4 millimeters).

§  Assimilation – The process in which very hot magma melts country rock and assimilates the newly molten material.

§  Asteroid – A small, generally rocky, solid body orbiting the Sun and ranging in diameter from a few meters to hundreds of kilometers.

§  Asthenosphere – A region of Earth's outer shell beneath the lithosphere. The asthenosphere is of indeterminate thickness and behaves plastically.

§  Astronomical unit (AU) – A distance unit based on the average distance of the Earth from the Sun.

§  Atmosphere – Gases that envelop Earth.

§  Atoll – A circular reef surrounding a deeper lagoon.

§  Atom – Smallest possible particle of an element that retains the properties of that element.

§  Atomic mass number – The total number of neutrons and protons in an atom.

§  Atomic number – The total number of protons in an atom.

§  Atomic weight – The sum of the weight of the subatomic particles in an average atom of an element, given in atomic mass units.

§  Augite – Mineral of the pyroxene group found in mafic igneous rocks.

§  Aulacogen – See failed rift.

§  Aureole – Zone of contact metamorphism adjacent to a pluton.

§  Axial plane – A plane containing all of the hinge lines of a fold.

§  Axis – See hinge line.




§  Backarc spreading – A type of seafloor spreading that moves an island arc away from a continent, or tears an island arc in two, or splits the edge of a continent, in each case forming new sea floor.

§  Backshore – Upper part of the beach, landward of the high-water line.

§  Bajada – A broad, gently sloping, depositional surface formed at the base of a mountain range in a dry region by the coalescing of individual alluvial fans.

§  Bar – A ridge of sediment, usually sand or gravel, that has been deposited in the middle or along the banks of a stream by a decrease in stream velocity.

§  Barchan – A crescent-shaped dune with the horns of the crescent pointing downwind.

§  Barrier island – Ridge of sand paralleling the shoreline and extending above sea level.

§  Barrier reef – A reef separated from the shoreline by the deeper water of a lagoon.

§  Basal sliding – Movement in which the entire glacier slides along as a single body on its base over the underlying rock.

§  Basalt – A fine-grained, mafic, igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar.

§  Base level – A theoretical downward limit for stream erosion of Earth's surface.

§  Batholith – A large discordant pluton with an outcropping area greater than 100 square kilometers.

§  Bauxite – The principal ore of aluminum; Al203 ∙ nH2O.

§  Baymouth bar – A ridge of sediment that cuts a bay off from the ocean.

§  Beach – Strip of sediment, usually sand but sometimes pebbles, boulders, or mud, which extends from the low-water line inland to a cliff or zone of permanent vegetation.

§  Beach face – The section of the beach exposed to wave action.

§  Bedding – An arrangement of layers or beds of rock.

§  Bedding plane – A nearly flat surface separating two beds of sedimentary rock.

§  Bed load – Heavy or large sediment particles in a stream that travel near or on the stream-bed.

§  Bedrock – Solid rock that underlies soil.

§  Benioff zone – Distinct earthquake zone that begins at an oceanic trench and slopes landward and downward into Earth at an angle of about 30° to 60°.

§  Bergschrund – The crevasse that develops where a glacier is pulling away from a cirque wall.

§  Berm – Platform of wave-deposited sediment that is flat or slopes slightly landward.

§  B horizon – A soil layer characterized by the accumulation of material leached downward from the A horizon above; also called zone of accumulation.

§  Biochemical – Precipitated by the action of organisms.

§  Bioclastic limestone – A limestone consisting of fragments of shells, corals, and algae.

§  Biosphere – All of the living or once-living material on Earth.

§  Biotite – Iron/magnesium-bearing mica.

§  Block – Large angular pyroclast.

§  Blowout – A depression on the land surface caused by wind erosion.

§  Body wave – Seismic wave that travels through Earth's interior.

§  Bomb – Large spindle- or lens-shaped pyroclast.

§  Bonding – Attachment of an atom to one or more adjacent atoms.

§  Bottomset bed – A delta deposit formed from the finest silt and clay, which are carried far out to sea by river flow or by sediments sliding downhill on the sea floor.

§  Boulder – A sediment particle with a diameter greater than 256 millimeters.

§  Bowen's reaction series – The sequence in which minerals crystallize from a cooling basaltic magma.

§  Braided stream – A stream that flows in a network of many interconnected rivulets around numerous bars.

§  Breaker – A wave that has become so steep that the crest of the wave topples forward, moving faster than the main body of the wave.

§  Breakwater – An offshore structure built to absorb the force of large breaking waves and provide quiet water near shore.

§  Brittle strain – Cracking or rupturing of a body under stress.

§  Butte – A narrow pinnacle of resistant rock with a flat top and very steep sides.




§  Calcareous – Containing calcium carbonate.

§  Calcite – Mineral with the formula CaCO3.

§  Caldera – A volcanic depression much larger than the original crater.

§  Capacity (of stream) – The total load that a stream can carry.

§  Capillary action – The drawing of water upward into small openings as a result of surface tension.

§  Capillary fringe – A thin zone near the water table in which capillary action causes water to rise above the zone of saturation.

§  Carbonaceous chondrite – Stony meteorite containing chondrules and composed mostly of serpentine and large quantities of organic materials.

§  Carbonic acid – H2CO3, a weak acid common in rain and surface waters.

§  Cave (cavern) – Naturally formed underground chamber.

§  Cement – The solid material that precipitates in the pore space of sediments, binding the grains together to form solid rock.

§  Cementation – The chemical precipitation of material in the spaces between sediment grains, binding the grains together into a hard rock.

§  Cenozoic Era – The most recent of the eras; followed the Mesozoic Era.

§  Chain silicate structure – Silicate structure in which two of each tetrahedron's oxygen ions are shared with adjacent tetrahedrons, resulting in a chain of tetrahedrons.

§  Chalk – A very fine-grained bioclastic limestone.

§  Channel (Mars) – Feature on the surface of the planet Mars that very closely resembles certain types of stream channels on Earth.

§  Chaotic terrain (Mars) – Patch of jumbled and broken angular slabs and blocks on the surface of Mars.

§  Chemical sedimentary rock – A rock composed of material precipitated directly from solution.

§  Chemical weathering – The decomposition of rock resulting from exposure to water and atmospheric gases.

§  Chert – A hard, compact, fine-grained sedimentary rock formed almost entirely of silica.

§  Chill zone – In an intrusion, the finer-grained rock adjacent to a contact with country rock.

§  Chondrule – Round silicate grain within some stony meteorites.

§  C horizon – A soil layer composed of incompletely weathered parent material.

§  Cinder (volcanic) – Pyroclast approximately the size of a sand grain. Sometimes defined as between 4 and 32 millimeters in diameter.

§  Cinder cone – A volcano constructed of loose rock fragments ejected from a central vent.

§  Circum-Pacific belt – Major belt around the edge of the Pacific Ocean on which most composite volcanoes are located and where many earthquakes occur.

§  Cirque A steep-sided, amphitheaterlike hollow carved into a mountain at the head of a glacial valley.

§  Clastic texture – An arrangement of rock fragments bound into a rigid network by cement.

§  Clay – Sediment composed of particles with diameter less than 1/256 millimeter.

§  Clay mineral – A hydrous aluminum-silicate that occurs as a platy grain of microscopic size with a sheet-silicate structure.

§  Clay mineral group – Collective term for clay minerals.

§  Cleavage – The ability of a mineral to break along preferred planes.

§  Coal – A sedimentary rock formed from the consolidation of plant material. It is rich in carbon, usually black, and burns readily.

§  Coal-bed methane – Gas trapped in coal.

§  Coarse-grained rock – Rock in which most of the grains are larger than 1 millimeter (igneous) or 2 millimeters (sedimentary).

§  Coast – The land near the sea, including the beach and a strip of land inland from the beach.

§  Coastal straightening – The gradual straightening of an irregular shoreline by wave erosion of headlands and wave deposition in bays.

§  Cobble – A sediment particle with a diameter of 64 to 256 millimeters.

§  Column – A dripstone feature formed when a stalactite growing downward and a stalagmite growing upward meet and join.

§  Columnar structure Volcanic rock in parallel, usually vertical columns, mostly six-sided; also called columnar jointing.

§  Comet – Small object in space, no more than a few kilometers in diameter, composed of frozen methane, frozen ammonia, and water-ice, with small solid particles and dust imbedded in the ices.

§  Compaction – A loss in overall volume and pore space of a rock as the particles are packed closer together by the weight of overlying material.

§  Competence – The largest particle that a stream can carry.

§  Composite volcano (stratovolcano) – A volcano constructed of alternating layers of pyroclastics and rock solidified from lava flows.

§  Compressive stress – A stress due to a force pushing together on a body.

§  Conchoidal fracture – Curved fracture surfaces.

§  Concordant – Parallel to layering or earlier developed planar structures.

§  Concretion – Hard, rounded mass that develops when a considerable amount of cementing material precipitates locally in a rock, often around an organic nucleus.

§  Cone of depression A depression of the water table formed around a well when water is pumped out; it is shaped like an inverted cone.

§  Confined aquifer (artesian aquifer) – An aquifer completely filled with pressurized water and separated from the land surface by a relatively impermeable confining bed, such as shale.

§  Confining pressure – Pressure applied equally on all surfaces of a body; also called lithostatic pressure.

§  Conglomerate – A coarse-grained sedimentary rock (grains coarser than 2 millimeters) formed by the cementation of rounded gravel.

§  Consolidation – Any process that forms firm, coherent rock from sediment or from liquid.

§  Contact – Boundary surface between two different rock types or ages of rocks.

§  Contact (thermal) metamorphism – Metamorphism under conditions in which high temperature is the dominant factor.

§  Continental crust – The thick, granitic crust under continents.

§  Continental drift – A concept suggesting that continents move over Earth's surface.

§  Continental glaciation – The covering of a large region of a continent by a sheet of glacial ice.

§  Continental rise – A wedge of sediment that extends from the lower part of the continental slope to the deep-sea floor.

§  Continental shelf – A submarine platform at the edge of a continent, inclined very gently seaward generally at an angle of less than 1°.

§  Continental slope – A relatively steep slope extending from a depth of 100 to 200 meters at the edge of the continental shelf down to oceanic depths.

§  Contour current – A bottom current that flows parallel to the slopes of the continental margin (along the contour rather than down the slope).

§  Contour line – A line on a topographic map connecting points of equal elevation.

§  Convection (convection current) – A very slow circulation of a substance driven by differences in temperature and density within that substance.

§  Convergent plate boundary – A boundary between two plates that are moving toward each other.

§  Coquina – A limestone consisting of coarse shells.

§  Core – The central zone of Earth.

§  Correlation – In geology, correlation usually means determining time equivalency of rock units. Rock units may be correlated within a region, a continent, and even between continents.

§  Country rock – Any rock that was older than and intruded by an igneous body.

§  Covalent bonding – Bonding due to the sharing of electrons by adjacent atoms.

§  Crater (of a volcano) – A basinlike depression over a vent at the summit of a volcanic cone.

§  Craton – Portion of a continent that has been structurally stable for a prolonged period of time.

§  Creep – Very slow, continuous downslope movement of soil or debris.

§  Crest (of wave) – The high point of a wave.

§  Crevasse – Open fissure in a glacier.

§  Cross-bedding – An arrangement of relatively thin layers of rock inclined at an angle to the more nearly horizontal bedding planes of the larger rock unit.

§  Crosscutting relationship – A principle or law stating that a disrupted pattern is older than the cause of disruption.

§  Cross section See geologic cross section.

§  Crude oil – A liquid mixture of naturally occurring hydrocarbons.

§  Crust – The outer layer of rock, forming a thin skin over Earth's surface.

§  Crustal rebound – The rise of Earth's crust after the removal of glacial ice.

§  Crystal – A homogeneous solid with an orderly internal atomic arrangement.

§  Crystal form – Arrangement of various faces on a crystal in a definite geometric relationship to one another.

§  Crystalline – Describing a substance in which the atoms are arranged in a regular, repeating, orderly pattern.

§  Crystalline texture – An arrangement of interlocking crystals.

§  Crystallization – Crystal development and growth.

§  Crystal settling – The process whereby the minerals that crystallize at a high temperature in a cooling magma move downward in the magma chamber because they are denser than the magma.

§  Cuesta – A ridge with a steep slope on one side and a gentle slope on the other side.

§  Curie point – The temperature below which a material becomes magnetized.




§  Data – What scientists regard as facts.

§  Daughter product – The isotope produced by radioactive decay.

§  Debris – Unconsolidated material (soil) in which coarse-grained fragments predominate.

§  Debris avalanche – Very rapid and turbulent mass wasting of debris, air, and water.

§  Debris flow – Mass wasting involving the flow of soil (unconsolidated material) in which coarse material (gravel, boulders) is predominant.

§  Decompression melting – Partial melting of hot mantle rock when it moves upward and the pressure is reduced to the extent that the melting point drops to the temperature of the body.

§  Deflation – The removal of clay, silt, and sand particles from the land surface by wind.

§  Delamination See lithospheric delamination.

§  Delta – A body of sediment deposited at the mouth of a river when the river velocity decreases as it flows into a standing body of water.

§  Dendritic pattern – Drainage pattern of a river and its tributaries that resembles the branches of a tree or veins in a leaf.

§  Density – Weight per given volume of a substance.

§  Deposition – The settling or coming to rest of transported material.

§  Depth of focus – Distance between the focus and the epicenter of an earthquake.

§  Desert – A region with low precipitation (usually defined as less than 25 centimeters per year).

§  Desertification – The expansion of barren deserts into once-populated regions.

§  Desert pavement – A thin layer of closely packed gravel that protects the underlying sediment from deflation; also called pebble armor

§  Detachment fault – Major fault in a mountain belt above which rocks have been intensely folded and faulted.

§  Detrital sedimentary rock – A sedimentary rock composed of fragments of preexisting rock.

§  Diapir – Bodies of rock (e.g., rock salt) or magma that ascend within Earth's interior because they are less dense than the surrounding rock.

§  Differential stress – When pressures on a body are not of equal strength in all directions.

§  Differential weathering – Varying rates of weathering resulting from some rocks in an area being more resistant to weathering than others.

§  Differentiation – Separation of different ingredients from an originally homogeneous mixture.

§  Dike – A tabular, discordant intrusive structure.

§  Diorite – Coarse-grained igneous rock of intermediate composition. Up to half of the rock is plagioclase feldspar and the rest is ferromagnesian minerals.

§  Dip – See angle of dip, direction of dip.

§  Dip-slip fault – A fault in which movement is parallel to the dip of the fault surface.

§  Directed pressure – See differential stress.

§  Direction of dip – The compass direction in which the angle of dip is measured.

§  Discharge – In a stream, the volume of water that flows past a given point in a unit of time.

§  Disconformity – A surface that represents missing rock strata but beds above and below that surface are parallel to one another.

§  Discordant – Not parallel to any layering or parallel planes.

§  Dissolved load – The portion of the total sediment load in a stream that is carried in solution.

§  Distributary – Small shifting river channel that carries water away from the main river channel and distributes it over a delta's surface.

§  Divergent plate boundary – Boundary separating two plates moving away from each other.

§  Divide – Line dividing one drainage basin from another.

§  Dolomite – A sedimentary rock composed mostly of the mineral dolomite.

§  Dolomitic marble – Marble in which dolomite, rather than calcite, is the prevalent mineral.

§  Dome – See structural dome.

§  Double refraction – The splitting of light into two components when it passes through certain crystalline substances.

§  Downcutting – A valley-deepening process caused by erosion of a streambed.

§  Drainage basin – Total area drained by a stream and its tributaries.

§  Drainage pattern – The arrangement in map view of a river and its tributaries.

§  Drawdown – The lowering of the water table near a pumped well.

§  Dripstone – Deposits of calcite (and, rarely, other minerals) built up by dripping water in caves.

§  Drumlin – A long, streamlined hill made of till.

§  Ductile – Capable of being molded and bent under stress.

§  Ductile strain – Strain in which a body is molded or bent under stress and does not return to its original shape after the stress is removed.

§  Dust (volcanic) – Finest-sized pyroclasts.




§  E horizon – Soil horizon that is the zone of leaching, characterized by the downward movement of water and removal of fine-grained soil components.

§  Earth – In mass wasting, soil in which fine-grained particles are predominant.

§  Earth systems – Study of Earth by analyzing how its components, or subsystems, interrelate.

§  Earthflow – Slow-to-rapid mass wasting in which fine-grained soil moves downslope as a very viscous fluid.

§  Earthquake – A trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath the surface.

§  Earthy luster – A luster giving a substance the appearance of unglazed pottery.

§  Echo sounder – An instrument used to measure and record the depth to the sea floor.

§  Elastic limit – The maximum amount of stress that can be applied to a body before it deforms in a permanent way by bending or breaking.

§  Elastic rebound theory – The sudden release of progressively stored strain in rocks results in movement along a fault.

§  Elastic strain – Strain in which a deformed body recovers its original shape after the stress is released.

§  Electron – A single, negative electric charge that contributes virtually no mass to an atom.

§  Element – A substance that cannot be broken down to other substances by ordinary chemical methods. Each atom of an element possesses the same number of protons.

§  Emergent coast – A coast in which land formerly under water has recently been placed above sea level, either by uplift of the land or by a drop in sea level.

§  End moraine – A ridge of till piled up along the front edge of a glacier.

§  Environment of deposition – The location in which deposition occurs, usually marked by characteristic physical, chemical, or biological conditions.

§  Eon – The largest unit of geological time.

§  Epicenter – The point on Earth's surface directly above the focus of an earthquake.

§  Epoch – Each period of the standard geologic time scale is divided into epochs (e.g., Pleistocene Epoch of the Quaternary Period).

§  Equilibrium – Material is in equilibrium if it is adjusted to the physical and chemical conditions of its environment so that it does not change or alter with time.

§  Equilibrium line – An irregular line marking the highest level to which the winter snow cover on a glacier is lost during a melt season; also called snow line.

§  Era – Major subdivision of the standard geologic time scale (e.g., Mesozoic Era).

§  Erosion – The physical removal of rock by an agent such as running water, glacial ice, or wind.

§  Erratic An ice-transported boulder that does not derive from bedrock near its present site.

§  Esker – A long, sinuous ridge of sediment deposited by glacial meltwater.

§  Estuary – Drowned river mouth.

§  Etch-pitted terrain (Mars) – A terrain on the surface of Mars characterized by small pits.

§  Evaporite – Rock that forms from crystals precipitating during evaporation of water.

§  Exfoliation – The stripping of concentric rock slabs from the outer surface of a rock mass.

§  Exfoliation dome – A large, rounded landform developed in a massive rock, such as granite, by the process of exfoliation.

§  Exotic terrane – Terrane that did not form at its present site on a continent and traveled a great distance to get to its present site.

§  Expansive clay – Clay that increases in volume when water is added to it.

§  Extension – Strain involving an increase in length. Extension can cause crustal thinning and faulting.

§  Extrusive rock – Any igneous rock that forms at Earth's surface, whether it solidifies directly from a lava flow or is pyroclastic.




§  Faceted – A rock fragment with one or more flat surfaces caused by erosive action.

§  Failed rift (aulacogen) – An inactive, sediment-filled rift that forms above a mantle plume. The rift becomes inactive as two other rifts widen to form an ocean.

§  Fall – The situation in mass wasting that occurs when material free-falls or bounces down a cliff.

§  Fault – A fracture in bedrock along which movement has taken place.

§  Fault-block mountain range A range created by uplift along normal or vertical faults.

§  Faunal succession – A principle or law stating that fossil species succeed one another in a definite and recognizable order; in general, fossils in progressively older rock show increasingly greater differences from species living at present.

§  Feldspar – Group of most common minerals of Earth's crust. All feldspars contain silicon, aluminum, and oxygen and may contain potassium, calcium, and sodium.

§  Felsic rock – Silica-rich igneous rock with a relatively high content of potassium and sodium.

§  Ferromagnesian mineral – Iron/magnesium­bearing mineral, such as augite, hornblende, olivine, or biotite.

§  Fine-grained rock – A rock in which most of the mineral grains are less than 1 millimeter across (igneous) or less than 1/16 millimeter (sedimentary).

§  Fiord – A coastal inlet that is a glacially carved valley, the base of which is submerged.

§  Firn – A compacted mass of granular snow, transitional between snow and glacier ice.

§  Firn limit – See equilibrium line.

§  Fissility – The ability of a rock to split into thin layers.

§  Flank eruption – An eruption in which lava erupts out of a vent on the side of a volcano.

§  Flash flood – Flood of very high discharge and short duration; sudden and local in extent.

§  Flood plain – A broad strip of land built up by sedimentation on either side of a stream channel.

§  Flow – A type of movement that implies that a descending mass is moving downslope as a viscous fluid.

§  Flowstone – Calcite precipitated by flowing water on cave walls and floors.

§  Focus – The point within Earth from which seismic waves originate in an earthquake.

§  Fold – Bend in layered bedrock.

§  Fold and thrust belt – A portion of a major mountain belt characterized by large thrust faults, stacked one upon another. Layered rock between the faults was folded when faulting was taking place.

§  Fold axis – See hinge line.

§  Foliation – Parallel alignment of textural and structural features of a rock.

§  Footwall – The underlying surface of an inclined fault plane.

§  Foreland basin – A sediment-filled basin on a continent, landward of a magmatic arc, and caused indirectly by ocean-continent convergence.

§  Foreset bed – A sediment layer in the main part of a delta, deposited at an angle to the horizontal.

§  Foreshock – Small earthquake that precedes a main shock.

§  Foreshore – The zone that is regularly covered and uncovered by the rise and fall of tides.

§  Formation – A body of rock of considerable thickness that has a recognizable unity or similarity making it distinguishable from adjacent rock units. Usually composed of one bed or several beds of sedimentary rock, although the term is also applied to units of metamorphic and igneous rock. A convenient unit for mapping, describing, or interpreting the geology of a region.

§  Fossil – Traces of plants or animals preserved in rock.

§  Fossil assemblage – Various different species of fossils in a rock.

§  Fracture – The way a substance breaks where not controlled by cleavage.

§  Fracture zone – Major line of weakness in Earth's crust that crosses the mid-oceanic ridge at approximately right angles.

§  Fracturing – Cracking or rupturing of a body under stress.

§  Framework silicate structure – Crystal structure in which all four oxygen ions of a silica tetrahedron are shared by adjacent ions.

§  Fretted terrain (Mars) – Flat lowland with some scattered high plateaus on the surface of Mars.

§  Fringing reef – A reef attached directly to shore. See barrier reef.

§  Frost action – Mechanical weathering of rock by freezing water.

§  Frost heaving – The lifting of rock or soil by the expansion of freezing water.

§  Frost wedging – A type of frost action in which the expansion of freezing water pries a rock apart.




§  Gabbro – A mafic, coarse-grained igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar.

§  Gaining stream – A stream that receives water from the zone of saturation.

§  Geode – Partly hollow, globelike body found in limestone or other cavernous rock.

§  Geologic cross section – A representation of a portion of Earth in a vertical plane.

§  Geologic map – A map representing the geology of a given area.

§  Geologic resources Valuable materials of geologic origin that can be extracted from Earth.

§  Geology The scientific study of the planet Earth.

§  Geophysics – The application of physical laws and principles to a study of Earth.

§  Geosphere – Solid Earth system. The rock and other inorganic material that make up the bulk of the planet.

§  Geothermal energy – Energy produced by harnessing naturally occurring steam and hot water.

§  Geothermal gradient – Rate of temperature increase associated with increasing depth beneath the surface of Earth (normally about 25°C per kilometer).

§  Geyser – A type of hot spring that periodically erupts hot water and steam.

§  Geyserite – A deposit of silica that forms around many geysers and hot springs.

§  Glacier – A large, long-lasting mass of ice, formed on land by the compaction and recrystallization of snow, which moves because of its own weight.

§  Glacier ice – The mosaic of interlocking ice crystals that form a glacier.

§  Glassy (or vitreous) luster – A luster that gives a substance a glazed, porcelainlike appearance.

§  Gneiss – A metamorphic rock composed of light and dark layers or lenses.

§  Gneissic – The texture of a metamorphic rock in which minerals are separated into light and dark-layers or lenses.

§  Goethite – The commonest mineral in the limonite group; Fe203 • nH2O.

§  Gondwanaland – The southern part of Pangaea (see definition) that formed South America, Africa, India, Australia, and Antarctica.

§  Graben – A downdropped block bounded by normal fault.

§  Graded bed – A single bed with coarse grains at the bottom of the bed and progressively finer grains toward the top of the bed.

§  Graded stream – A stream that exhibits a delicate balance between its transporting capacity and the sediment load available to it.

§  Granite – A felsic, coarse-grained, intrusive igneous rock containing quartz and composed mostly of potassium- and sodium-rich feldspars.

§  Gravel – Rounded particles coarser than 2 millimeters in diameter.

§  Gravitational collapse and spreading – When part of a mountain belt becomes too high, it moves vertically downward forcing rock at depth to spread out laterally.

§  Gravity – The force of attraction that two bodies exert on each other that is proportional to the product of their masses and inversely proportional to the square of the distance from the centers of the two bodies.

§  Gravity anomaly – A deviation from the average gravitational attraction between Earth and an object. See negative gravity anomaly, positive gravity anomaly.

§  Gravity meter – An instrument that measures the gravitational attraction between Earth and a mass within the instrument.

§  Graywacke – A sandstone with more than 15% fine-grained matrix between the sand grains.

§  Greenhouse effect – The trapping of heat by a planet's atmosphere, making the planet warmer than would otherwise be expected. Generally, the greenhouse effect operates if visible sunlight passes freely through a planet's atmosphere, but the infrared radiation produced by the warm surface cannot escape readily into space.

§  Groin – Short wall built perpendicular to shore to trap moving sand and widen a beach.

§  Ground moraine – A blanket of till deposited by a glacier or released as glacier ice melted.

§  Ground water – The water that lies beneath the ground surface, filling the cracks, crevices, and pore space of rocks.

§  Guyot – Flat-topped seamount.




§  Hadean Eon – The oldest eon.

§  Half-life – The time it takes for a given amount of a radioactive isotope to be reduced by one-half.

§  Hanging valley – A smaller valley that terminates abruptly high above a main valley.

§  Hanging wall – The overlying surface of an inclined fault plane.

§  Hardness – The relative ease or difficulty with which a smooth surface of a mineral can be scratched; commonly measured by Mohs scale.

§  Headland – Point of land along a coast.

§  Headward erosion – The lengthening of a valley in an uphill direction above its original source by gullying, mass wasting, and sheet erosion.

§  Heat engine – A device that converts heat energy into mechanical energy.

§  Heat flow – Gradual loss of heat (per unit of surface area) from Earth's interior out into space.

§  Heavy crude – Dense, viscous petroleum that flows slowly or not at all.

§  Hematite – A type of iron oxide that has a brick-red color when powdered; Fe203.

§  Highland (Moon) – A rugged region of the lunar surface representing an early period in lunar history when intense meteorite bombardment formed craters.

§  Hinge line – Line about which a fold appears to be hinged. Line of maximum curvature of a folded surface.

§  Hinge plane – See axial plane.

§  Hogback – A sharp-topped ridge formed by the erosion of steeply dipping beds.

§  Holocene Epoch – The youngest epoch which began around 10,000 years ago and is continuing presently.

§  Horn – A sharp peak formed where cirques cut back into a mountain on several sides.

§  Hornblende – Common amphibole frequently found in igneous and metamorphic rocks.

§  Hornfels – A fine-grained, unfoliated metamorphic rock.

§  Horst – An up-raised block bounded by normal faults.

§  Hot spot – An area of volcanic eruptions and high heat flow above a rising mantle plume.

§  Hot spring – Spring with a water temperature warmer than human body temperature.

§  Hydraulic action – The ability of water to pick up and move rock and sediment.

§  Hydrologic cycle – The movement of water and water vapor from the sea to the atmosphere, to the land, and back to the sea and atmosphere again.

§  Hydrology – The study of water's properties, circulation, and distribution.

§  Hydrosphere – The water on or near Earth's surface.

§  Hydrothermal rock – Rock deposited by precipitation of ions from solution in hot water.

§  Hydrothermal vein – Quartz or other minerals that have been deposited in a crack by hot fluids.

§  Hypocenter – Synonym for the focus of an earthquake.

§  Hypothesis – A tentative theory.




§  Iceberg – Block of glacier-derived ice floating in water.

§  Ice cap – A glacier covering a relatively small area of land but not restricted to a valley.

§  Icefall – A chaotic jumble of crevasses that split glacier ice into pinnacles and blocks.

§  Ice sheet – A glacier covering a large area (more than 50,000 square kilometers) of land.

§  Igneous rock – A rock formed or apparently formed from solidification of magma.

§  Incised meander – A meander that retains its sinuous curves as it cuts vertically downward below the level at which it originally formed.

§  Inclusion – A fragment of rock that is distinct from the body of igneous rock in which it is enclosed.

§  Inclusion, principle of – Fragments included in a host rock are older than the host rock.

§  Index fossil – A fossil from a very short-lived species known to have existed during a specific period of geologic time.

§  Inner planet – A planet orbiting in the inner part of the Solar System. Sometimes taken to mean Mercury, Venus, Earth, and Mars.

§  Intensity – A measure of an earthquake's size by its effect on people and buildings.

§  Intermediate rock – Rock with a chemical content between felsic and mafic compositions.

§  Intrusion (intrusive structure) – A body of intrusive rock classified on the basis of size, shape, and relationship to surrounding rocks.

§  Intrusive rock – Rock that appears to have crystallized from magma emplaced in surrounding rock.

§  Ion – An electrically charged atom or group of atoms.

§  Ionic bonding – Bonding due to the attraction between positively charged ions and negatively charged ions.

§  Iron meteorite – A meteorite composed principally of iron-nickel alloy.

§  Island arc – A curved line of islands.

§  Isoclinal fold – A fold in which the limbs are parallel to one another.

§  Isolated silicate structure – Silicate minerals that are structured so that none of the oxygen atoms are shared by silica tetrahedrons.

§  Isostasy – The balance or equilibrium between adjacent blocks of crust resting on a plastic mantle.

§  Isostatic adjustment – Concept of vertical movement of sections of Earth's crust to achieve balance or equilibrium.

§  Isotherm – A line along which the temperature of rock (or other material) is the same.

§  Isotopes – Atoms (of the same element) that have different numbers of neutrons but the same number of protons.

§  Isotopic dating – Determining the age of a rock or mineral through its radioactive elements and decay products (previously and somewhat inaccurately called radiometric or radioactive dating).




§  Jetty – Rock wall protruding above sea level, designed to protect the entrance of a harbor from sediment deposition and storm waves; usually built in pairs.

§  Joint – A fracture or crack in bedrock along which essentially no displacement has occurred.

§  Joint set – Joints oriented in one direction approximately parallel to one another.




§  Kame – Low mound or irregular ridge formed of outwash deposits on a stagnating glacier.

§  Kame and kettle topography – Irregular, bumpy landscape of hills and depressions associated with many moraines.

§  Karst topography – An area with many sinkholes and a cave system beneath the land surface and usually lacking a surface stream.

§  Kettle – A depression caused by the melting of a stagnant block of ice that was surrounded by sediment.

§  Kimberlite – An ultramafic rock that contains olivine along with mica, garnet, or both. Diamonds are found in some kimberlite bodies.




§  Laccolith – A concordant intrusive structure, similar to a sill, with the central portion thicker and domed upward. Laccoliths are not common and are not discussed in this textbook.

§  Laminar flow – Slow, smooth flow, with each drop of water traveling a smooth path parallel to its neighboring drops.

§  Laminated terrain (Mars) – Area where series of alternating light and dark layers can be seen on the surface of Mars.

§  Lamination – A thin layer in sedimentary rock (less than 1 centimeter thick).

§  Landform – A characteristically shaped feature of Earth's surface, such as a hill or a valley.

§  Lapilli (plural) – Pyroclasts in the 2-64 millimeter size range (singular, lapillus).

§  Landslide – The general term for a slowly to very rapidly descending mass of rock or debris.

§  Lateral continuity – Principle that states that an original sedimentary layer extends laterally until it tapers or thins at its edges.

§  Lateral erosion – Erosion and undercutting of stream banks caused by a stream swinging from side to side across its valley floor.

§  Lateral moraine – A low, ridgelike pile of till along the side of a glacier.

§  Laterite – Highly leached soil that forms in regions of tropical climate with high temperatures and very abundant rainfall.

§  Lava – Magma on Earth's surface.

§  Lava flow – Flow of lava from a crater or fissure.

§  Lava tube – Tunnel-like cave within a lava flow. It forms during the late stages of solidification of a mafic lava flow.

§  Left-lateral fault – A strike-slip fault in which the block seen across the fault appears displaced to the left.

§  Limb – Portion of a fold shared by an anticline and a syncline.

§  Limestone – A sedimentary rock composed mostly of calcite.

§  Limonite – A type of iron oxide that is yellowish-brown when powdered; Fe203 • nH2O.

§  Liquefaction – A type of ground failure in which water-saturated sediment turns from a solid to a liquid as a result of shaking, often caused by an earthquake.

§  Lithification – The consolidation of sediment into sedimentary rock.

§  Lithosphere – The rigid outer shell of Earth, 70 to 125 or more kilometers thick.

§  Lithospheric delamination – The detachment of part of the mantle portion of the lithosphere beneath a mountain belt.

§  Lithostatic pressure – Confining pressure due to the weight of overlying rock.

§  Loam – Soil containing approximately equal amounts of sand, silt, and clay.

§  Loess – A fine-grained deposit of wind-blown dust.

§  Longitudinal dune (seif) – Large, symmetrical ridge of sand parallel to the wind direction.

§  Longitudinal profile – A line showing a stream's slope, drawn along the length of the stream as if it were viewed from the side.

§  Longshore current – A moving mass of water that develops parallel to a shoreline.

§  Longshore drift – Movement of sediment parallel to shore when waves strike a shoreline at an angle.

§  Losing stream – Stream that loses water to the zone of saturation.

§  Love waves – A type of surface seismic wave that causes the ground to move side to side in a horizontal plane perpendicular to the direction the wave is traveling.

§  Low-velocity zone – Mantle zone at a depth of about 100 kilometers where seismic waves travel more slowly than in shallower layers of rock.

§  Luster – The quality and intensity of light reflected from the surface of a mineral.




§  Mafic rock – Silica-deficient igneous rock with a relatively high content of magnesium, iron, and calcium.

§  Magma – Molten rock, usually mostly silica. The liquid may contain dissolved gases as well as some solid minerals.

§  Magmatic arc – A line of batholiths or volcanoes. Generally the line, as seen from above, is curved.

§  Magmatic underplating – See underplating.

§  Magnetic anomaly – A deviation from the average strength of Earth's magnetic field. See negative magnetic anomaly, positive magnetic anomaly.

§  Magnetic field – Region of magnetic force that surrounds Earth.

§  Magnetic pole – An area where the strength of the magnetic field is greatest and where the magnetic lines of force appear to leave or enter Earth.

§  Magnetic reversal – A change in Earth's magnetic field between normal polarity and reversed polarity. In normal polarity, the north magnetic pole, where magnetic lines of force enter Earth, lies near the geographic North Pole. In reversed polarity, the south magnetic pole, where lines of force leave Earth, lies near the geographic North Pole (the magnetic poles have exchanged positions).

§  Magnetite – An iron oxide that is attracted to a magnet.

§  Magnetometer – An instrument that measures the strength of Earth's magnetic field.

§  Magnitude – A measure of the energy released during an earthquake.

§  Major mountain belt – A long chain (thousands of kilometers) of mountain ranges.

§  Mantle – A thick shell of rock that separates Earth's crust above from the core below.

§  Mantle diapir – A body of mantle rock, hotter than its surroundings, that ascends because it is less dense than the surrounding rock.

§  Mantle plume – Narrow column of hot mantle rock that rises and spreads radially outward.

§  Marble – A coarse-grained rock composed of interlocking calcite (or dolomite) crystals.

§  Maria (Moon) – Lava plains on Moon's surface (singular, mare).

§  Marine terrace – A broad, gently sloping platform that may be exposed at low tide.

§  Mass wasting (or mass movement) – Movement, caused by gravity, in which bedrock, rock debris, or soil moves downslope in bulk.

§  Matrix – Fine-grained material found in the pore space between larger sediment grains.

§  Meander – A pronounced sinuous curve along a stream's course.

§  Meander cutoff – A new, shorter channel across the narrow neck of a meander.

§  Meander scar – An abandoned meander filled with sediment and vegetation.

§  Mechanical weathering – The physical disintegration of rock into smaller pieces.

§  Medial moraine – A single long ridge of till on a glacier, formed by adjacent lateral moraines joining and being carried downglacier.

§  Mediterranean-Himalayan belt (Mediterranean belt) – A major concentration of earthquakes and composite volcanoes that runs through the Mediterranean Sea, crosses the Mideast and the Himalaya, and passes through the East Indies.

§  Melt – Liquid rock resulting from melting in a laboratory.

§  Mercalli scale – See modified Mercalli scale.

§  Mesa – A broad, flat-topped hill bounded by cliffs and capped with a resistant rock layer.

§  Mesozoic Era – The era that followed the Paleozoic Era and preceded the Cenozoic Era.

§  Metallic bonding – Bonding, as in metals, whereby atoms are closely packed together and electrons move freely among atoms.

§  Metallic luster – Luster giving a substance the appearance of being made of metal.

§  Metamorphic facies – Metamorphic rocks that contain the same set of pressure or temperature sensitive minerals are regarded as belonging to the same facies, implying that they formed under broadly similar pressure and temperature conditions.

§  Metamorphic rock – A rock produced by metamorphism.

§  Metamorphism – The transformation of preexisting rock into texturally or mineralogically distinct new rock as a result of high temperature, high pressure, or both but without the rock melting in the process.

§  Metasomatism – Metamorphism coupled with the introduction of ions from an external source.

§  Meteor – Fragment that passes through Earth's atmosphere, heated to incandescence by friction; sometimes incorrectly called "shooting" or "falling" stars.

§  Meteorite – Meteor that strikes Earth's surface.

§  Meteoroid – Small solid particles of stone and/or metal orbiting the sun.

§  Mica group – Group of minerals with a sheet-silicate structure.

§  Microcline (potassium) feldspar – A feldspar with the formula KAlSi3O8.

§  Mid-oceanic ridge – A giant mountain range that lies under the ocean and extends around the world.

§  Migmatite – Mixed igneous and metamorphic rock.

§  Milky Way galaxy – The galaxy to which the Sun belongs. Seen from Earth, the galaxy is a pale, milky band in the night sky.

§  Mineral – A crystalline substance that is naturally occurring and is chemically and physically distinctive.

§  Mineraloid – A substance that is not crystalline but otherwise would be considered a mineral.

§  Model – In science, a model is an image—graphic, mathematical, or verbal—that is consistent with the known data.

§  Modified Mercalli scale – Scale expressing intensities of earthquakes (judged on amount of damage done) in Roman numerals ranging from Ito XII.

§  Mohoroviĉić discontinuity – The boundary separating the crust from the mantle beneath it (also called Moho).

§  Mohs' hardness scale – Scale on which ten minerals are designated as standards of hardness.

§  Molecule – The smallest possible unit of a substance that has the properties of that substance.

§  Moment magnitude – An earthquake magnitude calculated from the strength of the rock, surface area of the fault rupture, and the amount of rock displacement along the fault.

§  Monocline – A local steeping in a gentle regional dip; a steplike fold in rock.

§  Moraine – A body of till either being carried on a glacier or left behind after a glacier has receded.

§  Mountain range – A group of closely spaced mountains or parallel ridges.

§  Mud – Term loosely used for silt and clay, usually wet.

§  Mud crack – Polygonal crack formed in very fine-grained sediment as it dries.

§  Mudflow – A flowing mixture of debris and water, usually moving down a channel.

§  Mudstone – A fine-grained sedimentary rock that lacks shale's laminations and fissility.

§  Muscovite – Transparent or white mica that lacks iron and magnesium.




§  Natural gas – A gaseous mixture of naturally occurring hydrocarbons.

§  Natural levee – Low ridges of flood-deposited sediment formed on either side of a stream channel, which thin away from the channel.

§  Nebula – A large volume of interstellar gas and dust.

§  Nebular Hypotheses – The hypothesis that the Solar System formed from a rotating cloud of gas and dust, the solar nebula.

§  Negative gravity anomaly – Less than normal gravitational attraction.

§  Negative magnetic anomaly – Less than average strength of Earth's magnetic field.

§  Neutron – A subatomic particle that contributes mass to an atom and is electrically neutral.

§  Nonconformity – An unconformity in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.

§  Nonmetallic luster – Luster that gives a substance the appearance of being made of something other than metal (e.g., glassy).

§  Nonrenewable resource – A resource that forms at extremely slow rates compared to its rate of consumption.

§  Normal fault – A fault in which the hanging-wall block moved down relative to the footwall block.

§  Nucleus – Protons and neutrons form the nucleus of an atom. Although the nucleus occupies an extremely tiny fraction of the volume of the entire atom, practically all the mass of the atom is concentrated in the nucleus.

§  Numerical age – Age given in years or some other unit of time.




§  Oblique-slip fault – A fault with both strike-slip and dip-slip components.

§  Obsidian – Volcanic glass.

§  Oceanic crust – The thin, basaltic crust under oceans.

§  Oceanic trench – A narrow, deep trough parallel to the edge of a continent or an island arc.

§  O Horizon - Dark-colored soil layer that is rich in organic material and forms just below surface vegetation.

§  Oil – See crude oil.

§  Oil field – An area underlain by one or more oil pools.

§  Oil pool – Underground accumulation of oil. oil sand Asphalt-cemented sand deposit.

§  Oil shale – Shale with a high content of organic matter from which oil may be extracted by distillation.

§  Oil trap – A set of conditions that hold petroleum in a reservoir rock and prevent its escape by


§  Olivine – A ferromagnesian mineral with the formula (Fe, Mg)2SiO4.

§  Oolite (ooid) – A small sphere of calcite precipitated from seawater.

§  Oolitic limestone – A limestone formed from oolites.

§  Opal – A mineraloid composed of silica and water.

§  Open fold – A fold with gently dipping limbs.

§  Open-pit mine – Mine in which ore is exposed at the surface in a large excavation.

§  Ophiolite – A distinctive rock sequence found in many mountain ranges on continents.

§  Ore – Naturally occurring material that can be profitably mined.

§  Ore mineral – A mineral of commercial value.

§  Organic sedimentary rock – Rock composed mostly of the remains of plants and animals.

§  Original horizontality – The deposition of most water-laid sediment in horizontal or near-horizontal layers that are essentially parallel to Earth's surface.

§  Orogeny – An episode of intense deformation of the rocks in a region, generally accompanied by metamorphism and plutonic activity.

§  Orthoclase (potassium) feldspar – A feldspar with the formula KAlSi3O8.

§  Outcrop – A surface exposure of bare rock, not covered by soil or vegetation.

§  Outer planet – A planet whose orbit lies in the outer part of the Solar System. Jupiter, Saturn, Uranus, Neptune, and Pluto are outer planets.

§  Outwash – Material deposited by debris-laden meltwater from a glacier.

§  Overburden – The upper part of a sedimentary deposit. Its weight causes compaction of the lower part.

§  Overturned fold – A fold in which both limbs dip in the same direction.

§  Oxbow lake – A crescent-shaped lake occupying the abandoned channel of a stream meander that is isolated from the present channel by a meander cutoff and sedimentation.




§  Pahoehoe – A lava flow characterized by a ropy or billowy surface.

§  Paired terraces – Stream terraces (see definition) that occur at the same elevation on each side of a river.

§  Paleomagnetism – A study of ancient magnetic fields.

§  Paleozoic Era – The era that followed the Precambrian and began with the appearance of complex life, as indicated by fossils.

§  Pangaea – A supercontinent that broke apart 200 million years ago to form the present continents.

§  Parabolic dune – A deeply curved dune in a region of abundant sand. The horns point upwind and are often anchored by vegetation.

§  Parent rock – Original rock before being metamorphosed.

§  Partial melting – Melting of the components of a rock with the lowest melting temperatures.

§  Passive continental margin – A margin that includes a continental shelf, continental slope, and continental rise that generally extends down to an abyssal plain at a depth of about 5 kilometers.

§  Paternoster lakes – A series of rock-basin lakes carved by glacial erosion.

§  Peat – A brown, lightweight, unconsolidated or semi-consolidated deposit of plant remains.

§  Pebble – A sediment particle with a diameter of 2 to 64 millimeters.

§  Pediment – A gently sloping erosional surface cut into the solid rock of a mountain range in a dry region; usually covered with a thin veneer of gravel.

§  Pegmatite – Extremely coarse-grained igneous rock.

§  Pelagic sediment – Sediment made up of fine-grained clay and the skeletons of microscopic organisms that settle slowly down through the ocean water.

§  Perched water table – A water table separated from the main water table beneath it by a zone that is not saturated.

§  Peridotite – Ultramafic rock composed of pyroxene and olivine.

§  Period – Each era of the standard geologic time scale is subdivided into periods (e.g., the Cretaceous Period).

§  Permafrost – Ground that remains permanently frozen for many years.

§  Permeability – The capacity of a rock to transmit a fluid such as water or petroleum.

§  Petrified wood – A material that forms as the organic matter of buried wood is either filled in or replaced by inorganic silica carried in by ground water.

§  Petroleum – Crude oil and natural gas. (Some geologists use petroleum as a synonym for oil.)

§  Phanerozoic – Eon of geologic time. Includes all time following the Precambrian.

§  Phenocryst – Any of the large crystals in porphyritic igneous rock.

§  Phyllite – A metamorphic rock in which clay minerals have recrystallized into microscopic micas, giving the rock a silky sheen.

§  Physical continuity – Being able to physically follow a rock unit between two places.

§  Physical geology – A large division of geology concerned with Earth materials, changes of the surface and interior of Earth, and the forces that cause those changes.

§  Pillow structure – Rocks, generally basalt, formed in pillow-shaped masses fitting closely together; caused by underwater lava flows.

§  Placer mine – Surface mines in which valuable mineral grains are extracted from stream bar or beach deposits.

§  Plagioclase feldspar – A feldspar containing sodium, calcium, or both, in addition to aluminum, silicon, and oxygen.

§  Planet – A body in orbit around a star.

§  Planetesimal – Small, planet-like body.

§  Plastic – Capable of being molded and bent under stress.

§  Plastic flow – Movement within a glacier in which the ice is not fractured.

§  Plate – A large, mobile slab of rock making up part of Earth's surface.

§  Plateau – Broad, flat-topped area elevated above the surrounding land and bounded, at least in part, by cliffs.

§  Plateau basalts – Layers of basalt flows that have built up to great thicknesses.

§  Plate tectonics – A theory that Earth's surface is divided into a few large, thick plates that are slowly moving and changing in size. Intense geologic activity occurs at the plate boundaries.

§  Playa – A very flat surface underlain by hard, mud-cracked clay.

§  Playa lake – A shallow temporary lake (following a rainstorm) on a flat valley floor in a dry region.

§  Pleistocene Epoch – An epoch of the Quaternary Period characterized by several glacial ages.

§  Plunging fold – A fold in which the hinge line (or axis) is not horizontal.

§  Pluton – An igneous body that crystallized deep underground.

§  Plutonic rock – Igneous rock formed at great depth.

§  Pluvial lake – A lake formed during an earlier time of abundant rainfall.

§  Point bar – A stream bar (see definition) deposited on the inside of a curve in the stream, where the water velocity is low.

§  Polarity – See magnetic reversal.

§  Polar wandering – An apparent movement of the Earth s poles.

§  Polymorphs – Substances having the same chemical composition but different crystal structures (e.g., diamond and graphite).

§  Pore space – The total amount of space taken up by openings between sediment grains.

§  Porosity – The percentage of a rock's volume that is taken up by openings.

§  Porphyritic rock – An igneous rock in which large crystals are enclosed in a matrix (or ground mass) of much finer-grained minerals or obsidian.

§  Positive gravity anomaly – Greater than normal gravitational attraction.

§  Positive magnetic anomaly – Greater than average strength of the Earth s magnetic field.

§  Potassium feldspar – A feldspar with the formula KAISi3O8.

§  Pothole – Depression eroded into the hard rock of a streambed by the abrasive action of the stream's sediment load.

§  Precambrian – The vast amount of time that preceded the Paleozoic Era.

§  Precambrian shield – A complex of old Precambrian metamorphic and plutonic rocks exposed over a large area.

§  Pressure release – A significant type of mechanical weathering that causes rocks to crack when overburden is removed.

§  Prograde metamorphism – Metamorphism in which progressively greater pressure and temperature act on a rock type with increasing depth in Earth's crust.

§  Proterozoic – Eon of Precambrian time.

§  Proton – A subatomic particle that contributes mass and a single positive electrical charge to an atom.

§  Pumice A frothy volcanic glass.

§  P wave – A compressional wave (seismic wave) in which rock vibrates parallel to the direction of wave propagation.

§  P-wave shadow zone – The region on Earth's surface, 103° to 142° away from an earthquake epicenter, in which P waves from the earthquake are absent.

§  Pyroclast – Fragment of rock formed by volcanic explosion.

§  Pyroclastic debris – Rock fragments produced by volcanic explosion.

§  Pyroclastic flow – Turbulent mixture of pyroclastics and gases flowing down the flank of a volcano.

§  Pyroxene group – Mineral group, all members of which are single-chain silicates.




§  Quartz – Mineral with the formula SiO2.

§  Quartzite – A rock composed of sand-sized grains of quartz that have been welded together during metamorphism.

§  Quartz sandstone – A sandstone in which more than 90% of the grains are quartz.

§  Quaternary Period – The youngest geologic period; includes the present time.




§  Radial pattern – A drainage pattern in which streams diverge outward like spokes of a wheel.

§  Radioactive decay – The spontaneous nuclear disintegration of certain isotopes.

§  Radioactivity – The spontaneous nuclear disintegration of atoms of certain isotopes.

§  Radon – A radioactive gas produced by the radioactive decay of uranium.

§  Rain shadow – A region on the downwind side of mountains that has little or no rain because of the loss of moisture on the upwind side of the mountains.

§  Rampart crater (Mars) – Meteorite crater that is surrounded by material that appears to have flowed from the point of impact.

§  Rayed crater (Moon) – Crater with bright streaks radiating from it on the Moon's surface.

§  Rayleigh waves – A type of surface seismic wave that behaves like a rolling ocean wave and causes the ground to move in an elliptical path.

§  Receding glacier – A glacier with a negative budget, which causes the glacier to grow smaller as its edges melt back.

§  Recent (Holocene) Epoch – The present epoch of the Quaternary Period.

§  Recessional moraine – An end moraine built during the retreat of a glacier.

§  Recharge – The addition of new water to an aquifer or to the zone of saturation.

§  Reclamation – Restoration of the land to usable condition after mining has ceased.

§  Recrystallization – The development of new crystals in a rock, often of the same composition as the original grains.

§  Rectangular pattern – A drainage pattern in which tributaries of a river change direction and join one another at right angles.

§  Recumbent fold – A fold overturned to such an extent that the limbs are essentially horizontal.

§  Reef – A resistant ridge of calcium carbonate formed on the sea floor by corals and coralline algae.

§  Regional (dynamothermal) metamorphism – Metamorphism that takes place at considerable depth underground.

§  Regolith – Loose, unconsolidated rock material resting on bedrock.

§  Relative time – The sequence in which events took place (not measured in time units).

§  Relief – The vertical distance between points on Earth's surface.

§  Reserves – The discovered deposits of a geologic material that are economically and legally feasible to recover under present circumstances.

§  Reservoir rock – A rock that is sufficiently porous and permeable to store and transmit petroleum.

§  Residual clay – Fine-grained particles left behind as insoluble residue when a limestone containing clay dissolves.

§  Residual soil – Soil that develops directly from weathering of the rock below.

§  Resources – The total amount of a geologic material in all its deposits, discovered and undiscovered. See reserves.

§  Reverse fault – A fault in which the hanging-wall block moved up relative to the footwall block.

§  Rhyolite – A fine-grained, felsic, igneous rock made up mostly of feldspar and quartz.

§  Richter scale – A numerical scale of earthquake magnitudes.

§  Ridge push – The concept that oceanic plates diverge as a result of sliding down the sloping lithosphere-asthenosphere boundary.

§  Rift valley – A tensional valley bounded by normal faults. Rift valleys are found at diverging plate boundaries on continents and along the crest of the mid-oceanic ridge.

§  Right-lateral fault – A strike-slip fault in which the block seen across the fault appears displaced to the right.

§  Rigid zone – Upper part of a glacier in which there is no plastic flow.

§  Rille (Moon) – Elongate trenched or cracklike valley on the lunar surface.

§  Rip current – Narrow currents that flow straight out to sea in the surf zone, returning water seaward that has been pushed ashore by breaking waves.

§  Ripple mark – Any of the small ridges formed on sediment surfaces exposed to moving wind or water. The ridges form perpendicularly to the motion.

§  Rock – Naturally formed, consolidated material composed of grains of one or more minerals. (There are a few exceptions to this definition.)

§  Rock avalanche – A very rapidly moving, turbulent mass of broken-up bedrock.

§  Rock-basin lake – A lake occupying a depression caused by glacial erosion of bedrock.

§  Rock cycle – A theoretical concept relating tectonism, erosion, and various rock-forming processes to the common rock types.

§  Rockfall – Rock falling freely or bouncing down a cliff.

§  Rock flour – A powder of fine fragments of rock produced by glacial abrasion.

§  Rock gypsum – An evaporite composed of gypsum.

§  Rock salt – An evaporite composed of halite.

§  Rockslide – Rapid sliding of a mass of bedrock along an inclined surface of weakness.

§  Rotational slide – In mass wasting, movement along a curved surface in which the upper part moves vertically downward while the lower part moves outward. Also called a slump.

§  Rounded knobs (glacial) – Bedrock that is more resistant to glacial erosion stands out as rounded knobs, usually elongated parallel to the direction of glacier flow. These are also known as roche moutonnιes (French for "rock sheep").

§  Rounding – The grinding away of sharp edges and corners of rock fragments during transportation.

§  Rubble – Angular sedimentary particles coarser than 2 millimeters in diameter.




§  Saltation – A mode of transport that carries sediment downcurrent in a series of short leaps or bounces.

§  Sand – Sediment composed of particles with a diameter between 1/16 and 2 millimeters.

§  Sand dune – A mound of loose sand grains heaped up by the wind.

§  Sandstone – A medium-grained sedimentary rock (grains between 1/16 and 2 millimeters) formed by the cementation of sand grains.

§  Saturated zone – A subsurface zone in which all rock openings are filled with water.

§  Scale – The relationship between distance on a map and the distance on the terrain being represented by that map.

§  Schist – A metamorphic rock characterized by coarse-grained minerals oriented approximately parallel.

§  Schistose – The texture of a rock in which visible platy or needle-shaped minerals have grown essentially parallel to each other under the influence of directed pressure.

§  Scientific method – A means of gaining knowledge through objective procedures.

§  Scoria – A basalt that is highly vesicular.

§  Sea cave – A cavity eroded by wave action at the base of a sea cliff.

§  Sea cliff – Steep slope that retreats inland by mass wasting as wave erosion undercuts it.

§  Seafloor metamorphism – Metamorphism of rock along a mid-oceanic ridge caused by circulating hot water.

§  Seafloor spreading – The concept that the ocean floor is moving away from the mid-oceanic ridge and across the deep-ocean basin, to disappear beneath continents and island arcs.

§  Seamount – Conical mountain rising 1,000 meters or more above the sea floor.

§  Seawall – A wall constructed along the base of retreating cliffs to prevent wave erosion.

§  Sediment – Loose, solid particles that can originate by (1) weathering and erosion of preexisting rocks, (2) chemical precipitation from solution, usually in water, and (3) secretion by organisms.

§  Sedimentary breccia – A coarse-grained sedimentary rock (grains coarser than 2 millimeters) formed by the cementation of angular rubble.

§  Sedimentary facies – Significantly different rock types occupying laterally distinct parts of the same layered rock unit.

§  Sedimentary rock – Rock that has formed from lithification of any type of sediment, precipitation from solution, of (3) consolidation of the remains of plants or animals.

§  Sedimentary structure – A feature found within sedimentary rocks, usually formed during or shortly after deposition of the sediment and before lithification.

§  Seismic gap – A segment of a fault that has not experienced earthquakes for a long time; such gaps may be the site of large future quakes.

§  Seismic profiler – An instrument that measures and records the subbottom structure of the sea floor.

§  Seismic reflection – The return of part of the energy of seismic waves to Earth's surface after the waves bounce off a rock boundary.

§  Seismic refraction – The bending of seismic waves as they pass from one material to another.

§  Seismic sea wave – See tsunami.

§  Seismic wave – A wave of energy produced by an earthquake.

§  Seismogram – Paper record of Earth vibration.

§  Seismograph – A seismometer with a recording device that produces a permanent record of Earth motion.

§  Seismometer – An instrument designed to detect seismic waves or Earth motion.

§  Serpentine – A magnesium silicate mineral. Most asbestos is a variety of serpentine.

§  Shale – A fine-grained sedimentary rock (grains finer than 1/16 millimeter in diameter) formed by the cementation of silt and clay (mud). Shale has thin layers (laminations) and an ability to split (fissility) into small chips.

§  Shear force – In mass wasting, the component of gravitational force that is parallel to an inclined surface.

§  Shearing – Movement in which parts of a body slide relative to one another and parallel to the forces being exerted.

§  Shear strength – In mass wasting, the resistance to movement or deformation of material.

§  Shear stress – Stress due to forces that tend to cause movement or strain parallel to the direction of the forces.

§  Sheet erosion – The removal of a thin layer of surface material, usually topsoil, by a flowing sheet of water.

§  Sheet joints – Cracks that develop parallel to the outer surface of a large mass of expanding rock, as pressure is released during unloading.

§  Sheet-silicate structure – Crystal structure in which each silica tetrahedron shares three oxygen ions.

§  Sheetwash – Water flowing down a slope in a layer.

§  Shield volcano – Broad, gently sloping cone constructed of solidified lava flows.

§  Silica – A term used for oxygen plus silicon.

§  Silicate – A substance that contains silica as part of its chemical formula.

§  Silica tetrahedron – See silicon-oxygen tetrahedron.

§  Silicic rock or magma – Silica-rich igneous rock or magma with a relatively high content of potassium and sodium.

§  Silicon-oxygen tetrahedron – Four-sided, pyramidal object that visually represents the four oxygen atoms surrounding a silicon atom; the basic building block of silicate minerals. Also called a silica tetrahedron or a silicon tetrahedron.

§  Sill – A tabular intrusive structure concordant with the country rock.

§  Silt – Sediment composed of particles with a diameter of 1/256 to 1/16 millimeter.

§  Siltstone – A sedimentary rock consisting mostly of silt grains.

§  Sinkhole – A closed depression found on land surfaces underlain by limestone.

§  Sinter – A deposit of silica that forms around some hot springs and geysers.

§  Slab pull – The concept that subducting plates are pulled along by their dense leading edges.

§  Slate – A fine-grained rock that splits easily along flat, parallel planes.

§  Slaty – Describing a rock that splits easily along nearly flat and parallel planes.

§  Slaty cleavage – The ability of a rock to break along closely spaced parallel planes.

§  Slide – In mass wasting, movement of a relatively coherent descending mass along one or more well-defined surfaces.

§  Slip face – The steep, downwind slope of a dune; formed from loose, cascading sand that generally keeps the slope at the angle of repose (about 34°).

§  Slump – In mass wasting, movement along a curved surface in which the upper part moves vertically downward while the lower part moves outward. Also called a rotational slide.

§  Snow line – See equilibrium line

§  Soil – A layer of weathered, unconsolidated material on top of bedrock; often also defined as containing organic matter and being capable of supporting plant growth. In mass wasting, soil means unconsolidated material, regardless of particle size or composition (also called engineering soil).

§  Soil horizon – Any of the layers of soil that are distinguishable by characteristic physical or chemical properties.

§  Solar nebula – The rotating disk of gas and dust from which the Sun and planets formed.

§  Solar system – The Sun, planets, their moons, and other bodies that orbit the Sun.

§  Solar wind – The outflow of low-density, hot gas from the Sun's upper atmosphere. It is partially this wind that creates the tail of a comet by blowing dust and gas away from the comet's immediate surroundings.

§  Solid solution – The substitution of atoms of one element for those of another element in a particular mineral.

§  Solifluction – Flow of water-saturated soil over impermeable material.

§  Solution – Usually slow but effective process of weathering and erosion in which rocks are dissolved by water.

§  Sorting – Process of selection and separation of sediment grains according to their grain size (or grain shape or specific gravity).

§  Source area – The locality that eroded to provide sediment to form a sedimentary rock.

§  Source rock – A rock containing organic matter that is converted to petroleum by burial and other postdepositional changes.

§  Spatter cone – A small, steep-sided cone built from lava spattering out of a vent.

§  Specific gravity – The ratio of the mass of a substance to the mass of an equal volume of water, determined at a specified temperature.

§  Speleothem – Dripstone deposit of calcite that precipitates from dripping water in caves.

§  Spheroidally weathered boulder – Boulder that has been rounded by weathering from an initial blocky shape.

§  Spit – A fingerlike ridge of sediment attached to land but extending out into open water.

§  Spreading axis (or spreading center) – The crest of the mid-oceanic ridge, where sea floor is moving away in opposite directions on either side.

§  Spring – A place where water flows naturally out of rock onto the land surface.

§  Stable – Describing a mineral that will not react with or convert to a new mineral or substance, given enough time.

§  Stack – A small rock island that is an erosional remnant of a headland left behind as a wave-eroded coast retreats inland.

§  Stalactite – Iciclelike pendant of dripstone formed on cave ceilings.

§  Stalagmite – Cone-shaped mass of dripstone formed on cave floors, generally directly below a stalactite.

§  Standard geologic time scale – A worldwide relative scale of geologic time divisions.

§  Star – A massive, gaseous body held together by gravity and generally emitting light. Normal stars generate energy by nuclear reactions in their interiors.

§  Static pressure – See confining pressure.

§  Stock – A small discordant pluton with an outcropping area of less than 100 square kilometers.

§  Stony-iron meteorite – A meteorite composed of silicate minerals and iron-nickel alloy in approximately equal amounts.

§  Stony meteorite – A meteorite made up mostly of plagioclase and iron-magnesium silicates.

§  Stoping – Upward movement of a body of magma by fracturing of overlying country rock. Magma engulfs the blocks of fractured country rock as it moves upward.

§  Storm surge – High sea level caused by the low pressure and high winds of hurricanes.

§  Strain – Change in size (volume) or shape of a body (or rock unit) in response to stress.

§  Stratigraphy – The field of geology concerning layered rocks and their interrelationships.

§  Stratovolcano – See composite volcano.

§  Streak – Color of a pulverized substance; a useful property for mineral identification.

§  Stream – A moving body of water, confined in a channel and running downhill under the influence of gravity.

§  Stream capture – See stream piracy.

§  Stream channel – A long, narrow depression, shaped and more or less filled by a stream.

§  Stream discharge – Volume of water that flows past a given point in a unit of time.

§  Stream-dominated delta – A delta with fingerlike distributaries formed by the dominance of stream sedimentation; also called a birdfoot delta.

§  Stream gradient – Downhill slope of a stream's bed or the water surface, if the stream is very large.

§  Stream headwaters – The upper part of a stream near the source.

§  Stream mouth – The place where the stream enters the sea, a large lake, or a larger stream.

§  Stream piracy – The natural diversion of the headwaters of one stream into the channel of another.

§  Stream terrace – Steplike landform found above a stream and its flood plain.

§  Stream velocity – The speed at which water in a stream travels.

§  Stress – A force acting on a body, or rock unit, which tends to change the size or shape of that body, or rock unit. Force per unit area within a body.

§  Striations – (1) On minerals, extremely straight, parallel lines; (2) Glacial straight scratches in rock caused by abrasion by a moving glacier.

§  Strike – The compass direction of a line formed by the intersection of an inclined plane (such as a bedding plane) with a horizontal plane.

§  Strike-slip fault – A fault in which movement is parallel to the strike of the fault surface.

§  Strip mine – A mine in which the valuable material is exposed at the surface by removing a strip of overburden.

§  Structural basin – A structure in which the beds dip toward a central point.

§  Structural dome – A structure in which beds dip away from a central point.

§  Structural geology – The branch of geology concerned with the internal structure of bedrock and the shapes, arrangement, and interrelationships of rock units.

§  Structural [or oil] trap – See oil trap.

§  Subduction – The sliding of the sea floor beneath a continent or island arc.

§  Subduction complex – See accretionary wedge.

§  Subduction zone – Elongate region in which subduction takes place.

§  Submarine canyon – V-shaped valleys that run across the continental shelf and down the continental slope.

§  Submergent coast – A coast in which formerly dry land has been recently drowned, either by land subsidence or a rise in sea level.

§  Subsidence – Sinking or downwarping of a part of the Earth s surface.

§  Superposed stream – A river let down onto a buried geologic structure by erosion of overlying layers.

§  Superposition – A principle or law stating that within a sequence of undisturbed sedimentary rocks, the oldest layers are on the bottom, the youngest on the top.

§  Surf – Breaking waves.

§  Surface wave – A seismic wave that travels on Earth's surface.

§  Suspect terrane – A terrane that may not have formed at its present site.

§  Suspended load – Sediment in a stream that is light enough in weight to remain lifted indefinitely above the bottom by water turbulence.

§  S-wave – A seismic wave propagated by a shearing motion, which causes rock to vibrate perpendicular to the direction of wave propagation.

§  S-wave shadow zone – The region on Earth's surface (at any distance more than 103° from an earthquake epicenter) in which S waves from the earthquake are absent.

§  Swelling clay – See expansive clay.

§  Syncline – A fold in which the layered rock usually dips toward an axis.




§  Talus – An accumulation of broken rock at the base of a cliff.

§  Tarn – See rock-basin lake.

§  Tectite – Small, rounded bits of glass formed from rock melting and being thrown into the air due to a meteorite impact.

§  Tectonic forces – Forces generated from within Earth that result in uplift, movement, or deformation of part of Earth's crust.

§  Tectonostratigraphic terrane – See terrane.

§  Temporal - of or pertaining to time.

§  Tensional stress – A stress due to a force pulling away on a body.

§  Tephra – See pyroclastic debris.

§  Terminal moraine – An end moraine marking the farthest advance of a glacier.

§  Terminus – The lower edge of a glacier.

§  Terrain - The character, natural features, and configuration of land. (Dictionary definition)

§  Terrane - any rock formation or series of formations or the area in which a particular formation or group of rocks is predominant. (dictionary definition)

§  Terrane (tectonostratigraphic terrane) – A region in which the geology is markedly different from that in adjoining regions.

§  Terrigenous sediment – Land-derived sediment that has found its way to the sea floor.

§  Texture – A rock's appearance with respect to the size, shape, and arrangement of its grains or other constituents.

§  Theory – An explanation for observed phenomena that has a high possibility of being true.

§  Theory of glacial ages – At times in the past, colder climates prevailed during which significantly more of the land surface of Earth was glaciated than at present.

§  Thermal metamorphism – See contact (thermal) metamorphism.

§  Thrust fault – A reverse fault in which the dip of the fault plane is at a low angle to horizontal.

§  Tidal delta – A submerged body of sediment formed by tidal currents passing through gaps in barrier islands.

§  "Tidal wave" – An incorrect name for a tsunami.

§  Tide-dominated delta – A delta formed by the reworking of sand by strong tides.

§  Till – Unsorted and unlayered rock debris carried by a glacier.

§  Tillite – Lithified till.

§  Time-transgressive rock unit – An apparently continuous rock layer in which different portions formed at different times.

§  “Tink” test - A simple field method of distinguishing between shale (a sedimentary rock composed of Clay the Mineral) and slate (a metamorphic rock composed of interlocking grains of minerals with basal cleavage). Shale, being composed of piles of loose debris, goes "thunk" when dropped, while slate, being harder and more tightly inter-grown, goes "tink." (borrowed from “GeoMan”)

§  Tombolo – A bar of marine sediment connecting a former island or stack to the mainland.

§  Topographic map – A map on which elevations are shown by means of contour lines.

§  Topset bed – In a delta, a nearly horizontal sediment bed of varying grain size formed by distributaries shifting across the delta surface.

§  Trace fossil – Trail, track or burrow resulting from animal movement preserved in sedimentary rock.

§  Traction – Movement by rolling, sliding, or dragging of sediment fragments along a stream bottom.

§  Transform fault – The portion of a fracture zone between two offset segments of a mid-oceanic ridge crest.

§  Transform plate boundary – Boundary between two plates that are sliding past each other.

§  Translational slide – In mass wasting, movement of a descending mass along a plane approximately parallel to the slope of the surface.

§  Transportation – The movement of eroded particles by agents such as rivers, waves, glaciers, or wind.

§  Transported soil – Soil not formed from the local rock but from parent material brought in from some other region and deposited, usually by running water, wind, or glacial ice.

§  Transverse dune – A relatively straight, elongate dune oriented perpendicular to the wind.

§  Travel-time curve – A plot of seismic-wave arrival times against distance.

§  Travertine – A porous deposit of calcite that often forms around hot springs.

§  Trellis pattern – A drainage pattern consisting of parallel main streams with short tributaries meeting them at right angles.

§  Trench – See oceanic trench.

§  Trench suction – The concept that overlying plates move horizontally toward oceanic trenches as subducting plates sink at an angle steeper than their dip.

§  Tributary – Small stream flowing into a large stream, adding water to the large stream.

§  Trough (of wave) – The low point of a wave.

§  Truncated spur – Triangular facet where the lower end of a ridge has been eroded by glacial ice.

§  Tsunami – Huge (not usually tall) ocean wave produced by displacement of the sea floor; also called seismic sea wave.

§  Tufa – A deposition of calcite that forms around a spring, lake, or percolating ground water.

§  Tuff – A rock formed from fine-grained pyroclastic particles (ash and dust).

§  Turbidity current – A flowing mass of sediment-laden water that is heavier than clear water and therefore flows downslope along the bottom of the sea or a lake.

§  Turbulent flow – Eddying, swirling flow in which water drops travel along erratically curved paths that cross the paths of neighboring drops.




§  Ultramafic rock – Rock composed entirely or almost entirely of ferromagnesian minerals.

§  Unconfined aquifer – A partially filled aquifer exposed to the land surface and marked by a rising and falling water table.

§  Unconformity – A surface that represents a break in the geologic record, with the rock unit immediately above it being considerably younger than the rock beneath.

§  Unconsolidated – In referring to sediment grains, loose, separate, or unattached to one another.

§  Underplating – The pooling of magmas at the base of the continental crust.

§  Uniformitarianism – Principle that geologic processes operating at present are the same processes that operated in the past. The principle is stated more succinctly as, "The present is the key to the past." See actualism.

§  Universe – The largest astronomical structure we know of. The Universe contains all matter and radiation and encompasses all space.

§  Unloading – The removal of a great weight of rock.

§  Unpaired terraces – Stream terraces (see definition) that do not have the same elevation on opposite sides of a river.

§  Unsaturated zone – A subsurface zone in which rock openings are generally unsaturated and filled partly with air and partly with water; above the saturated zone.

§  U-shaped valley – Characteristic cross-profile of a valley carved by glacial erosion.




§  Valley glacier – A glacier confined to a valley. The ice flows from a higher to a lower elevation.

§  Varve – Two thin layers of sediment, one dark and the other light in color, representing one year's deposition in a lake.

§  Vein – See hydrothermal vein.

§  Vent – The opening in Earth's surface through which a volcanic eruption takes place.

§  Ventifact – Boulder, cobble, or pebble with flat surfaces caused by the abrasion of wind-blown sand.

§  Vertical exaggeration – An artificial steepening of slope angles on a topographic profile caused by using a vertical scale that differs from the horizontal scale.

§  Vesicle – A cavity in volcanic rock caused by gas in a lava.

§  Viscosity – Resistance to flow.

§  Vitreous luster – See glassy luster.

§  Volcanic breccia – Rock formed from large pieces of volcanic rock (cinders, blocks, bombs).

§  Volcanic dome – A steep-sided, dome- or spine-shaped mass of volcanic rock formed from viscous lava that solidifies in or immediately above a volcanic vent.

§  Volcanic neck – An intrusive structure that apparently represents magma that solidified within the throat of a volcano.

§  Volcanism – Volcanic activity, including the eruption of lava and rock fragments and gas explosions.

§  Volcano – A hill or mountain constructed by the extrusion of lava or rock fragments from a vent.




§  Wastage – See ablation.

§  Water table – The upper surface of the zone of saturation.

§  Wave crest – See crest.

§  Wave-cut platform – A horizontal bench of rock formed beneath the surf zone as a coast retreats because of wave erosion.

§  Wave-dominated delta – A delta formed by the reworking of sand by wave action.

§  Wave height – The vertical distance between the crest (the high point of a wave) and the trough (the low point).

§  Wavelength – The horizontal distance between two wave crests (or two troughs).

§  Wave refraction – Change in direction of waves due to slowing as they enter shallow water.

§  Wave trough – See trough.

§  Weathering – The group of processes that change rock at or near Earth's surface.

§  Welded tuff – A rock composed of pyroclasts welded together.

§  Well  – A hole, generally cylindrical and usually walled or lined with pipe, that is dug or drilled into the ground to penetrate an aquifer below the zone of saturation.

§  Wilson cycle – The cycle of splitting of a continent, opening of an ocean basin, followed by closing of the basin and collision of the continents.

§  Wind ripple – Small, low ridge of sand produced by the saltation of wind-blown sand.

§  Wrinkle ridge (Moon) – Wrinkle on lunar maria surface.




§  Xenolith – Fragment of rock distinct from the igneous rock in which it is enclosed.




§  Zone of ablation That portion of a glacier in which ice is lost.

§  Zone of accumulation – (1) That portion of a glacier with a perennial snow cover; (2) See B horizon (a soil layer).

§  Zone of leaching – See A horizon (a soil layer).

§  Zone of saturation – See saturated zone.

§  Zoning – Orderly variation in the chemical composition within a single crystal.