Glossary
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aa
A lava flow with a rubbly surface.
abandoned meander
A meander that dries out after it gets cut off.
ablation
The removal of ice at the toe of a glacier by melting, sublimation (the evaporation of ice into water vapor), and/or calving.
abrasion
The process in which one material (such as sand-laden water) grinds away at another (such as a stream channel’s floor and walls).
absolute age
Numerical age (the age specified in years).
absolute plate velocity
The movement of a plate relative to a fixed point in the mantle.
abyssal plain
A broad, relatively flat region of the ocean that lies at least 4.5 km below sea level.
Acadian orogeny
A convergent mountain-building event that occurred around 400 million years ago in which continental slivers accreted to the eastern edge of the North American continent.
accrete
339.
accreted terrane
A block of land that collided with a continent at a convergent margin and stayed attached to the continent.
accretionary coast
A coastline that receives more sediment than erodes away.
accretionary orogen
An orogen formed by the attachment of numerous buoyant slivers of crust to an older, larger continental block.
accretionary prism
A wedge-shaped mass of sediment and rock scraped off the top of a downgoing plate and accreted onto the overriding plate at a convergent plate margin.
acid mine runoff
A dilute solution of sulfuric acid, produced when sulfur-bearing minerals in mines react with rainwater, that flows out of a mine.
acid rain
Precipitation in which air pollutants react with water to make a weak acid that then falls from the sky.
active continental margin
A continental margin that coincides with a plate boundary.
active fault
A fault that has moved recently or is likely to move in the future.
active sand
The top layer of beach sand, which moves daily because of wave action.
active volcano
A volcano that has erupted within the past few centuries and will likely erupt again.
adiabatic cooling
The cooling of a body of air or matter without the addition or subtraction of thermal energy (heat).
adiabatic heating
The warming of a body of air or matter without the addition or subtraction of heat.
adits
A network of tunnels into an ore body, created by drilling and blasting.
aerosols
Tiny solid particles or liquid droplets that remain suspended in the atmosphere for a long time.
aftershocks
The series of smaller earthquakes that follow a major earthquake.
air
The mixture of gases that make up the Earth’s atmosphere.
air-fall tuff
Tuff formed when ash settles gently from the air.
air mass
A body of air, about 1,500 km across, that has recognizable physical characteristics.
air pressure
The push that air exerts on its surroundings.
albedo
The reflectivity of a surface.
Alleghenian orogeny
The convergent orogenic event that occurred about 270 million years ago when Africa collided with North America.
alloy
A metal containing more than one type of metal atom.
alluvial fan
A gently sloping apron of sediment dropped by an ephemeral stream at the base of a mountain in arid or semi-arid regions.
alluvium
Sorted sediment deposited by a stream.
alluvium-filled valley
A valley whose floor fills with sediment.
amber
Hardened (fossilized) ancient sap or resin.
ambphibolite
dark colored metamorphic rock containing amphibole, plagioclase, and garnet.
amplitude
The height of a wave from crest to trough.
Ancestral Rockies
The late Paleozoic uplifts of the Rocky Mountain region; they eroded away long before the present Rocky Mountains formed.
angiosperm
A flowering plant.
angle of repose
The angle of the steepest slope that a pile of uncemented material can attain without collapsing from the pull of gravity.
angularity
The degree to which grains have sharp or rounded edges or corners.
angular unconformity
An unconformity in which the strata below were tilted or folded before the unconformity developed; strata below the unconformity therefore have a different tilt than strata above.
anhedral grains
Crystalline mineral grains without well-formed crystal faces.
Antarctic bottom water mass
The mass of cold, dense water that sinks along the coast of Antarctica.
antecedent stream
A stream that cuts across an uplifted mountain range; the stream must have existed before the range uplifted and must then have been able to downcut as fast as the land was rising.
anthracite coal
Shiny black coal formed at temperatures between 200° and 300°C.
anthropogenic
the kind of global change that results from human activity.
anticline
A fold with an arch-like shape in which the limbs dip away from the hinge.
anticyclone
The clockwise flow of air around a high-pressure mass.
Antler orogeny
The Late Devonian mountain-building event in which slices of deep-marine strata were pushed eastward, up and over the shallow-water strata on the western coast of North America.
anvil cloud
A large cumulonimbus cloud that spreads laterally at the tropopause to form a broad, flat top.
aphanitic
A textural term for fine-grained igneous rock.
Apparent polar-wander path
A path on the globe along which a magnetic pole appears to have wandered over time; in fact, the continents drift, while the magnetic pole stays fairly fixed.
aquiclude
Sediment or rock that transmits no water.
aquifer
Sediment or rock that transmits water easily.
aquitard
Sediment or rock that does not transmit water easily and therefore retards the motion of the water.
Archaeobacteria
A kingdom of “old bacteria,” now commonly found in extreme environments like hot springs. (Also called “Archaea.”)
Archean
The middle Precambrian eon.
Archimedes’ principle
The mass of the water displaced by a block of material equals the mass of the whole block of material.
aręte
A residual knife-edge ridge of rock that separates two adjacent cirques.
argillaceous sedimentary rock
Sedimentary rock that contains abundant clay.
arkose
a sediment formed when a mixture of quartz and feldspar grains is buried and lithified.
arroyo
The channel of an ephemeral stream; dry wash; wadi.
artesian springs
A place where the ground surface intersects a joint that taps a confined aquifer, and the water pressure is great enough to push groundwater through the joint up to the surface.
artesian well
A well in which water rises on its own.
ash fall
Ash that falls to the ground out of an ash cloud.
ash flow
An avalanche of ash that tumbles down the side of an explosively erupting volcano.
assay
Testing rock to determine how much metal can be economically extracted from it.
assimilation
The process of magma contamination in which blocks of wall rock fall into a magma chamber and dissolve.
asthenosphere
The layer of the mantle that lies between 100–150 km and 350 km deep; the asthenosphere is relatively soft and can flow when acted on by force.
atm
A unit of air pressure that approximates the pressure exerted by the atmosphere at sea level.
atmosphere
A layer of a mixture of gases (air) that surrounds the Earth.
atoll
A coral reef that develops as a circular reef surrounding a lagoon.
atomic number
The number of protons in the nucleus of a given element.
atomic weight
The number of protons plus the number of neutrons in the nucleus of a given element. (Also known as atomic mass.)
aurora australis
The same phenomenon as the aurora borealis, but in the Southern Hemisphere.
aurora borealis
A ghostly curtain of varicolored light that appears across the night sky in the Northern Hemisphere when charged particles from the Sun interact with the ions in the ionosphere.
avalanche
A turbulent cloud of debris mixed with air that rushes down a steep hill slope at high velocity; the debris can be rock and/or snow.
avalanche chute
A downslope hillside pathway along which avalanches repeatedly fall, consequently clearing the pathway of mature trees.
avulsion
The process in which a river overflows a natural levee and begins to flow in a new direction.
axial plane
The imaginary surface that encompasses the hinges of successive layers of a fold.
axial trough
A narrow depression that runs along a mid-ocean ridge axis.