Choose a letter:

a   b   c   d   e   f   g   h   i   j   k   l   m   n   o   p   q   r   s   t   u   v   w   x   y   z

aa	A lava flow with a rubbly surface.
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 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, during which continental slivers accreted to the eastern edge of the North American continent.
accreted terrane	A block of crust 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.
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 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.
air-fall tuff	Tuff formed when ash settles gently from the air.
albedo	The reflectivity of a surface.
Alleghenian orogeny	The 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.
amphibolite facies	A set of metamorphic mineral assemblages formed under intermediate pressures and temperatures.
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.
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.
angularity	The degree to which grains have sharp or rounded edges or corners.
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. A high-rank coal.
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.
arête	A residual knife-edge ridge of rock that separates two adjacent cirques.
archaea or archaeobacteria	A kingdom of “old bacteria,” now commonly found in extreme environments like hot springs.
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.
argillaceous sedimentary rock	Sedimentary rock that contains abundant clay.
arroyo	The channel of an ephemeral stream; dry wash; wadi.
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.
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; this layer 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 gases that surrounds a planet.
atoll	A coral reef that develops around a circular reef surrounding a lagoon.
atomic number	The number of protons in the nucleus of a given element.
atomic weight or atomic mass	The number of protons plus the number of neutrons in the nucleus of a given element.
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.