Glossary
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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
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.
capacity
a measure of the total quantity of sediment a stream can carry.
capillary fringe
The thin subsurface layer in which water molecules seep up from the water table by capillary action to fill pores.
carbonate rocks
Rocks containing calcite and/or dolomite.
carbon-14 dating
A radiometric dating process that can tell us the age of organic material containing carbon originally extracted from the atmosphere.
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.
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 Yucután peninsula; geologists suggest that a bolide 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.
cirque glaciers
Glaciers that fill the bowl-shaped depressions high on mountains in which glaciers originate.
cirrus cloud
A wispy cloud that tapers into delicate, feather-like curls.
clastic (detrital) 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 movement of 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.
coal
a black, combustible sedimentary rock consisting of over 50% carbon.
coal rank
A measurement of the carbon content of coal; higher-rank coal formed at higher temperatures.
coal reserve
The quantities of discovered 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
A flat stretch of coastal land that merges with the continental shelf along a passive margin.
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.
competence
a measure of the maximum particle size a stream can carry.
composite volcano
Stratovolcano.
compositional banding
A type of metamorphic foliation 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.
condononts
small teeth-like fossils of early Paleozoic times.
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.
consuming boundary
Convergent plate boundary.
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).
contact metamorphism
Thermal metamorphism.
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-drift hypothesis
The hypothesis that continents have moved and are still moving slowly across the Earth’s surface.
continental glacier
A vast sheet of ice that spreads over thousands of square km of continental crust.
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.
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, rifting 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.
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
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.
corilleran ice sheet
The Pleistocene ice sheet that covered the mountains of western Canada and the southern third of Alaska.
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 at the crest of a mountain ridge built up by strong winds.
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.
cosmogenic dating
A dating technique that allows geologists to determine how long a boulder has been stable by measuring the concentration of isotopes formed by the impact of cosmic rays on minerals in the rock surface.
cosmology
The study of the overall structure of the Universe.
coulees
Large vertical-sided valleys in basalt bedrock of Washington State.
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 bolide.
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.
cross beds
thin bands, tilted at an angle to the boundary of the overall bedding in a sequence of strata, that develop during the deposition of sediment in a current.
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 Archaeobacteria.
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.