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caldera	A large circular depression with steep walls and a fairly flat floor, formed after an eruption as the center of the volcano collapses into the drained magma chamber below.
caliche or calcrete	A solid mass created where calcite cements the soil together.
calving	The breaking off of chunks of ice at the edge of a glacier.
Cambrian explosion of life	The remarkable diversification of life, indicated by the fossil record, that occurred at the beginning of the Cambrian Period.
Canadian Shield	A broad, low-lying region of exposed Precambrian rock in the Canadian interior.
canyon	A trough or valley with steeply sloping walls, cut into the land by a stream.
capillary fringe	The thin subsurface layer in which water molecules seep up from the water table by capillary action to fill pores.
carbon-14 dating	A radiometric dating process that can tell us the age of organic material containing carbon originally extracted from the atmosphere.
carbonate rocks	Rocks containing calcite and/or dolomite.
cast	Sediment that preserves the shape of a shell it once filled before the shell dissolved or mechanically weathered away.
catabatic winds	Strong winds that form at the margin of a glacier where the warmer air above ice-free land rises and the cold, denser air from above the glaciers rushes in to take its place.
catastrophic change	Change that takes place either instantaneously or rapidly in geologic time.
catchment	Drainage network.
cement	Mineral material that precipitates from water and fills the spaces between grains, holding the grains together.
cement	Mineral material that precipitates from water and fills the spaces between grains, holding the grains together.
cementation	The phase of lithification in which cement, consisting of minerals that precipitate from groundwater, partially or completely fills the spaces between clasts and attaches each grain to its neighbor.
Cenozoic	The most recent era of the Phanerozoic Eon, lasting from 65 Ma up until the present.
chalk	Very fine-grained limestone consisting of weakly cemented plankton shells.
change of state	The process in which a material changes from one phase (liquid, gas, or solid) to another.
channel	A trough dug into the ground surface by flowing water.
channeled scablands	A barren, soil-free landscape in eastern Washington, scoured clean by a flood unleashed when a large glacial lake drained.
chatter marks	Wedge-shaped indentations left on rock surfaces by glacial plucking.
chemical sedimentary rocks	Sedimentary rocks made up of minerals that precipitate directly from water solution.
chemical weathering	The process in which chemical reactions alter or destroy minerals when rock comes in contact with water solutions and/or air.
chert	A sedimentary rock composed of very fine-grained silica (cryptocrystalline quartz).
Chicxulub crater	A circular excavation buried beneath younger sediment on the Yucut87n peninsula; geologists suggest that a meteorite landed there 65 Ma.
chimney	(1) A conduit in a magma chamber in the shape of a long vertical pipe through which magma rises and erupts at the surface; (2) an isolated column of strata in an arid region.
cinder cone	A subaerial volcano consisting of a cone-shaped pile of tephra whose slope approaches the angle of repose for tephra.
cinders	Fragments of glassy rock ejected from a volcano.
cirque	A bowl-shaped depression carved by a glacier on the side of a mountain.
cirrus cloud	A wispy cloud that tapers into delicate, feather-like curls.
clastic sedimentary rock	Sedimentary rock consisting of cemented-together detritus derived from the weathering of preexisting rock.
cleavage	(1) The tendency of a mineral to break along preferred planes; (2) a type of foliation in low-grade metamorphic rock.
cleavage planes	A series of surfaces on a crystal that form parallel to the weakest bonds holding the atoms of the crystal together.
cliff (or scarp) retreat	The change in the position of a cliff face caused by erosion.
climate	The average weather conditions, along with the range of conditions, of a region over a year.
cloud	A mist of tiny water droplets in the sky.
coal rank	A measurement of the carbon content of coal; higher-rank coal forms at higher temperatures.
coal reserve	The quantities of discovered, but not yet mined,  coal in sedimentary rock of the continents.
coal swamp	A swamp whose oxygen-poor water allows thick piles of woody debris to accumulate; this debris transforms into coal upon deep burial.
coastal plain	Low-relief regions of land adjacent to the coast.
cold front	The boundary at which a cold air mass pushes underneath a warm air mass.
collision	The process of two buoyant pieces of lithosphere converging and squashing together.
columnar jointing	A type of fracturing that yields roughly hexagonal columns of basalt; columnar joints form when a dike, sill, or lava flow cools.
comet	A ball of ice and dust, probably remaining from the formation of the solar system, that orbits the Sun.
compaction	The phase of lithification in which the pressure of the overburden on the buried rock squeezes out water and air that was trapped between clasts, and the clasts press tightly together.
compositional banding	A type of metamorphic foliation, found in gneiss, defined by alternating bands of light and dark minerals.
compressibility	The degree to which a material’s volume changes in response to squashing.
compression	A push or squeezing felt by a body.
compressional waves	Waves in which particles of material move back and forth parallel to the direction in which the wave itself moves.
conchoidal fractures	Smoothly curving, clamshell-shaped surfaces along which materials with no cleavage planes tend to break.
condensation	The process of gas molecules linking together to form a liquid.
condensation nuclei	Preexisting solid or liquid particles, such as aerosols, onto which water condenses during cloud formation.
cone of depression	The downward-pointing, cone-shaped surface of the water table in a location where the water table is experiencing drawdown because of pumping at a well.
confined aquifer	An aquifer that is separated from the Earth’s surface by an overlying aquitard.
conglomerate	Very coarse-grained sedimentary rock consisting of rounded clasts.
contact	The boundary surface between two rock bodies (as between two stratigraphic formations, between an igneous intrusion and adjacent rock, between two igneous rock bodies, or between rocks juxtaposed by a fault).
contaminant plume	A cloud of contaminated groundwater that moves away from the source of the contamination.
continental crust	The crust beneath the continents.
continental divide	A highland separating drainage that flows into one ocean from drainage that flows into another.
continental glacier	A vast sheet of ice that spreads over thousands of square km of continental crust.
continental lithosphere	Lithosphere topped by continental crust; this lithosphere reaches a thickness of 150 km.
continental margin	A continent’s coastline.
continental rift	A linear belt along which continental lithosphere stretches and pulls apart.
continental rifting	The process by which a continent stretches and splits along a belt; if successful, this process separates a larger continent into two smaller continents separated by a divergent boundary.
continental rise	The sloping sea floor that extends from the lower part of the continental slope to the abyssal plain.
continental shelf	A broad, shallowly submerged region of a continent along a passive margin.
continental slope	The slope at the edge of a continental shelf, leading down to the deep sea floor.
continental volcanic arc	A long curving chain of subaerial volcanoes on the margin of a continent adjacent to a convergent plate boundary.
continental-drift hypothesis	The idea that continents have moved and are still moving slowly across the Earth’s surface.
continental-interior desert	An inland desert that develops because by the time air masses reach the continental interior, they have lost all of their moisture.
contour lines	Lines on a map along which a parameter has a constant value; for example, all points along a contour line on a topographic map are at the same elevation.
control rod	Rods that absorb neutrons in a nuclear reactor and thus decrease the number of collisions between neutrons and radioactive atoms.
convection	Heat transfer that results when warmer, less dense material rises while cooler, denser material sinks.
convergence zone	A place where two surface air flows meet so that air has to rise.
convergent margin	Convergent plate boundary.
convergent plate boundary (or consuming boundary)	A boundary at which two plates move toward each other so that one plate sinks (subducts) beneath the other; only oceanic lithosphere can subduct.
coral reef	A mound of coral and coral debris forming a region of shallow water.
core	The dense, iron-rich center of the Earth.
core-mantle boundary	An interface 2,900 km below the Earth’s surface separating the mantle and core.
Coriolis effect	The deflection of objects, winds, and currents on the surface of the Earth owing to the planet’s rotation.
cornice	A huge, overhanging drift of snow built up by strong winds at the crest of a mountain ridge.
correlation	The process of defining the age relations between the strata at one locality and the strata at another.
cosmic rays	Nuclei of hydrogen and other elements that bombard the Earth from deep space.
cosmology	The study of the overall structure of the Universe.
country rock (wall rock)	The preexisting rock into which magma intrudes.
crater	(1) A circular depression at the top of a volcanic mound; (2) a depression formed by the impact of a meteorite.
craton	A long-lived block of durable continental crust commonly found in the stable interior of a continent.
cratonic platform	A province in the interior of a continent in which Phanerozoic strata bury most of the underlying Precambrian rock.
creep	The gradual downslope movement of regolith.
crevasse	A large crack that develops by brittle deformation in the top 60 m of a glacier.
critical mass	A sufficiently dense and large mass of radioactive atoms in which a chain reaction happens so quickly that the mass explodes.
cross section	A diagram depicting the geometry of materials underground as they would appear on an imaginary vertical slice through the Earth.
crude oil	Oil extracted directly from the ground.
crust	The rock that makes up the outermost layer of the Earth.
crustal root	Low-density crustal rock that protrudes downward beneath a mountain range.
crystal	A single, continuous piece of a mineral bounded by flat surfaces that formed naturally as the mineral grew.
crystal form	The geometric shape of a crystal, defined by the arrangement of crystal faces.
crystal habit	The general shape of a crystal or cluster of crystals that grew unimpeded.
crystal lattice	The orderly framework within which the atoms or ions of a mineral are fixed.
crystalline	Containing a crystal lattice.
cuesta	An asymmetric ridge formed by tilted layers of rock, with a steep cliff on one side cutting across the layers and a gentle slope on the other side; the gentle slope is parallel to the layering.
cumulonimbus cloud	A rain-producing puffy cloud.
cumulus cloud	A puffy, cotton-ball-shaped cloud.
current	(1) A well-defined stream of ocean water; (2) the moving flow of water in a stream.
cut bank	The outside bank of the channel wall of a meander, which is continually undergoing erosion.
cutoff	A straight reach in a stream that develops when erosion eats through a meander neck.
cyanobacteria	Blue-green algae; a type of archaea.
cycle	A series of interrelated events or steps that occur in succession and can be repeated, perhaps indefinitely.
cyclone	(1) The counterclockwise flow of air around a low-pressure mass; (2) the equivalent of a hurricane in the Indian Ocean.
cyclothem	A repeated interval within a sedimentary sequence that contains a specific succession of sedimentary beds.