A

AA: A blocky and fragmented form of basaltic lava occurring in flows with fissured and angular surfaces.

ABLATION: The annual amount of ice and snow lost from a glacier by the processes of melting, sublimation, wind erosion, and iceberg calving.

ABSOLUTE AGE: The age in years of a particular geologic event or feature, generally obtained with radiometric dating techniques. (Compare Relative age.)

ABYSSAL PLAIN: A flat, sediment-covered province of the seafloor.

ACTIVE MARGIN: A continental margin characterized by earthquakes, igneous activity, and/or uplifted mountains resulting from convergent or transform plate motion.

A-HORIZON: The uppermost layer of a soil, containing organic material and leached minerals.

ALLUVIAL FAN: A low, cone-shaped deposit of terrigenous sediment formed where a stream undergoes an abrupt widening as it leaves a mountain front for an open valley.

AMPHIBOLITE: A mostly nonfoliated metamorphic rock consisting primarily of amphibole and plagioclase feldspar.

ANDESITE: A volcanic rock type intermediate in composition between rhyolite and basalt; the extrusive equivalent of diorite.

ANGLE OF REPOSE: The steepest slope angle at which a particular sediment will lie without cascading down.

ANGULAR UNCONFORMITY: An unconformity in which the bedding planes of the rocks above and below are not parallel.

ANION: Any negatively charged ion; the opposite of cation.

ANTECEDENT STREAM: A stream that existed before the present topography was created, thereby maintaining its original course despite changes in the structure of the underlying rocks and in topography.

ANTICLINE: A large upfold of strata, usually from 100 m to 300 km in width, whose limbs are lower than its center. (Compare Syncline.)

AQUICLUDE: A stratum with low permeability that acts as a barrier to the flow of groundwater. Also called "confining layer."

AQUIFER: A permeable formation that stores and transmits groundwater in sufficient quantity to supply wells.

ARETE: The sharp, jagged crest along the divide between glacial cirques, resulting from the headward erosion of the walls of adjoining cirques.

ARGILLITE: A low-grade metamorphic rock made from a shaly sedimentary rock, characterized by irregular fracture and lack of foliation.

ARTESIAN FLOW: Flow in a confined aquifer, in which the groundwater is at a greater pressure than in an unconfined aquifer at similar depths, thereby causing water in a well that penetrates a confined aquifer (an artesian well) to rise above the level of the aquiclude.

ASEISMIC RIDGE: A submarine ridge of volcanic origin, far from present plate boundaries and therefore characterized by the absence of seismic activity. (Compare Mid-ocean ridge, which is seismically active.)

ASTHENOSPHERE: The weak layer below the lithosphere that is marked by low seismic wave velocities and high seismic wave attenuation. Movement in the asthenosphere occurs by plastic deformation.

ASYMMETRICAL FOLD: A fold of strata in which the dips of the two limbs are unequal.

ATOLL: A continuous or broken circle of coral reefs and low coral islands surrounding a central lagoon.

ATOM: The smallest unit of an element that retains the element's physical and chemical properties.

ATOMIC NUMBER: The number of protons in the nucleus of an atom.

ATOMIC WEIGHT: The sum of the masses of the protons and neutrons in the atomic nucleus of an element.

AXIAL PLANE: In folds, the plane that most nearly separates two symmetrical limbs. In a simple anticline, it is vertical; in complex folding, it is perpendicular to the direction of compression.