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CALCIUM CYCLE: The set of processes that carry calcium through Earth's systems.

CALDERA: A large basin-shaped volcanic depression that can form after an eruption if the volcano collapses through the roof of the emptied magma chamber. Potentially catastrophic eruptions of a "resurgent caldera" can occur when fresh magma reenters the collapsed magma chamber.

CAPACITY (STREAM): The amount of sediment and detritus a stream can transport past any point in a given amount of time. (Compare Competence.)

CARBONATE COMPENSATION DEPTH: The ocean depth below which the solution rate of calcium carbonate (CaCO₃) becomes so great that no carbonate organisms or sediments are preserved on the seafloor.

CARBONATE PLATFORM: Broad, shallow area where both biological and nonbiological carbonates are deposited.

CARBONATE SEDIMENT, ROCK: A sediment or sedimentary rock formed from the accumulation of carbonate minerals precipitated organically or inorganically. Rocks are chiefly limestone and dolostone.

CATACLASTIC METAMORPHISM: High-pressure metamorphism occurring primarily by the crushing, shearing, and recrystallization of rock during tectonic movements and resulting in the formation of very fine grained rock.

CATION: Any positively charged ion; the opposite of anion.

CEMENTATION: A lithification process in which minerals are precipitated in the pore space of sediments, often binding the grains.

CENTRAL VENT: The largest vent of a volcano, situated at the center of its cone.

CHANNEL: The trough through which water flows in a stream valley; sometimes reserved for the deepest part of the streambed, in which the main current flows.

CHEMICAL SEDIMENT, ROCK: A sediment or sedimentary rock that is formed at or near its place of deposition by chemical precipitation, usually from seawater.

CHEMICAL WEATHERING: The set of all chemical reactions that can act on rock exposed to water and the atmosphere and so dissolve the minerals or change them to more stable forms.

CHERT: A sedimentary rock made up of chemical or biochemically precipitated silica.

C-HORIZON: The lowest layer of a soil, consisting of fragments of rock and their chemically weathered products.

CINDER CONE: A steep, conical hill built up about a volcanic vent and composed of coarse pyroclastic rock fragments expelled from the vent by escaping gases.

CIRQUE: The head of a glacial valley, usually with the form of one half of an inverted cone. The upper edges have the steepest slopes, approaching the vertical, and the base may be flat or hollowed out. The base is commonly occupied by a small lake or pond after deglaciation.

CLASTIC SEDIMENT, ROCK: A sediment or sedimentary rock formed from particles (clasts) derived from the erosion of preexisting rocks and mechanically transported.

CLAY: Any of a number of hydrous aluminosilicate minerals with sheetlike crystal structure, formed by the weathering and hydration of other silicates. Also, any mineral fragments smaller than 0.0039 mm.

CLEAVAGE (MINERAL): The tendency of a crystal to break along certain preferred planes in the crystal lattice; also, the geometric pattern of such a breakage.

CLEAVAGE (ROCK): The tendency of a rock to break along certain planes induced during deformation or metamorphism, usually in the direction of preferred orientation of the minerals in the rock.

COAL: The metamorphic product of stratified plant remains. It contains more than 50 percent carbon compounds and burns readily.

COAST: The strip of land adjacent to an ocean or sea and extending from low tide landward to the point of major change in landscape features.

COMPACTION: The decrease in volume and porosity of a sediment caused by burial beneath other sediments.

COMPETENCE (STREAM): A measure of the largest particle a stream is able to transport, not the total volume. (Compare Capacity.)

COMPOSITE VOLCANO: A volcanic cone containing layers of both lava flows and pyroclastic rocks. Synonym for stratovolcano.

COMPRESSIVE FORCES: Forces that squeeze together or shorten a body. Compressive forces dominate at convergent plate boundaries.

CONCENTRATION FACTOR: The ratio of the abundance of an element in a mineral deposit to its average abundance in the crust.

CONCORDANT INTRUSION: An intrusive igneous rock having contacts with the country rock that are parallel to bedding or foliation planes. (Compare Discordant intrusion.)

CONDUCTION: See Heat conduction.

CONFINED AQUIFER: An aquifer overlain by relatively impermeable strata (aquicludes), thereby causing the water to be contained under pressure. (Compare Unconfined aquifer.)

CONGLOMERATE: A sedimentary rock, a significant fraction of which is composed of rounded pebbles, cobbles, and boulders. The lithified equivalent of gravel.

CONSOLIDATED MATERIAL: Sediment that is lithified; that is, compacted and bound together by mineral cements.

CONTACT METAMORPHISM: Changes in the mineralogy and texture of rock resulting from the heat and pressure of an igneous intrusion in the near vicinity.

CONTINENTAL DRIFT: The horizontal displacement or rotation of continents relative to one another.

CONTINENTAL GLACIER: A continuous, thick glacier covering more than 50,000 km² and moving independently of minor topographic features. (Compare Valley glacier.)

CONTINENTAL MARGIN: The portion of the ocean floor extending from the shoreline to the landward edge of the abyssal plain and including the continental shelf, slope, and rise.

CONTINENTAL RISE: A broad and gently sloping ramp that rises from an abyssal plain to the continental slope.

CONTINENTAL SHELF: The gently sloping submerged edge of a continent, extending commonly to a depth of about 200 m to the edge of the continental slope.

CONTINENTAL SHELF DEPOSITS: Sediments laid down in a tectonically quiet syncline at a passive continental margin.

CONTINENTAL SLOPE: The region of steep slopes between the continental shelf and continental rise.

CONTINUOUS REACTION SERIES: A reaction series in which the same mineral crystallizes throughout the range of temperatures in question, but in which there is gradual change in the chemical composition of the mineral with changing temperature. (Compare Discontinuous reaction series.)

CONTOUR: A curve on a topographic map that connects points of equal elevation.

CONVECTION: A mechanism of heat transfer in a flowing material in which hot material from the bottom rises because of its lesser density, while cool surface material sinks. (Compare Heat conduction.)

CONVERGENT PLATE BOUNDARY: A boundary at which Earth's plates collide and area is lost either by shortening and crustal thickening or by subduction of one plate beneath the other. The site of volcanism, earthquakes, trenches, and mountain building. (See also Subduction zone.)

CORE: The central part of the Earth below a depth of 2900 km. It is thought to be composed of iron and nickel and to be molten on the outside with a central solid inner core.

COUNTRY ROCK: The rock into which an igneous rock intrudes or a mineral deposit is emplaced.

COVALENT BOND: A bond between atoms in which outer electrons are shared.

CRATER: A bowl-shaped pit at the summit of most volcanoes, around the central vent.

CRATON: A portion of a continent that has not been subjected to major deformation for a prolonged time, typically since Precambrian or early Paleozoic time.

CREEP: Slow, downhill mass movement of soil and regolith under gravitational force.

CREVASSE: Any large vertical crack in the surface of a glacier or snowfield.

CROSS-BEDDING: Inclined beds in a sedimentary rock that were formed at the time of deposition by currents of wind or water in the direction in which the bed slopes downward.

CRUST: The outermost layer of the lithosphere, consisting of relatively light, low-melting temperature materials. Continental crust consists largely of granite and granodiorite. Oceanic crust is mostly basalt.

CRYSTAL: A form of matter, characterized by flat surfaces, in which the atoms, ions, or molecules are arranged in all directions to form a regular, repeating network.

CRYSTAL HABIT: The general shape of a crystal; for example, cubic, prismatic, or fibrous.

CRYSTALLIZATION: The formation of crystalline solids from a gas or liquid, such as in the formation of crystalline minerals in magma.

CUESTA: An asymmetrical ridge with one steep and one gentle face formed where gently dipping beds of erosion-resistant rocks are undercut by erosion of a weaker bed underneath.

CYCLE OF EROSION: A proposed sequence of changes in a landscape that progresses from high, rugged, tectonically formed mountains to low, rounded hills and finally to worn-down, tectonically stable plains.