Dictionary of Geology Definitions - Geologic Glossary - A

A

ablation

Occurs when more glacier ice is lost by melting and evaporation each year than is added by snowfall.

absolute age

The approximate age of a geologic event, feature, fossil, or rock in years. 'Absolute' ages are determined by using natural radioactive 'clocks'. The preferred term is radiometric age.

accretion

A process that adds part of one tectonic plate to a larger plate along a convergent (collisional) plate boundary.

accretionary wedge

A large wedge-shaped mass of sediment and oceanic lithosphere. An accretionary wedge forms along a convergent plate boundary when material is scraped from a sinking, subducting plate and is added to (accreted) another larger plate along a convergent plate boundary.

actinolite

A bright to gray-green member of the amphibole mineral family. In addition to silica, it contains calcium, magnesium, and iron. Actinolite is a non-hazardous relative of asbestos and is a common mineral in metamorphic rocks.

active volcano

A volcano that has erupted within historical time and is likely to do so again in the future.

A horizon

The top layer of soil. Plant and other organic debris builds up in this layer. This is the part of the soil generally referred to as 'top soil'.

alluvial fan

A fan-shaped pile of sediment that forms where a rapidly flowing mountain stream enters a relatively flat valley. As water slows down, it deposits sediment (alluvium) that gradually builds a fan.

alluvium

Sand, gravel, and silt deposited by rivers and streams in a valley bottom.

amphibole

A family of silicate minerals forming prism or needlelike crystals. Amphibole minerals generally contain iron, magnesium, calcium and aluminum in varying amounts, along with water. Hornblende always has aluminum and is a most common dark green to black variety of amphibole; it, forms forming in many igneous and metamorphic rocks. Actinolite has no aluminum; it and is needle-shaped and light green. Blue amphibole contains sodium and, of course, is bluish in color.

amphibolite

A rock made up mostly amphibole and plagioclase feldspar. Although the name amphibolite usually refers to a type of metamorphic rock, an igneous rock composed dominantly of amphibole can be called an amphibolite too.

andesite

Fine-grained, generally dark colored, igneous volcanic rock with more silica than basalt. Commonly with visible crystals of plagioclase feldspar. Generally occurs in lava flows, but also as dikes. The most common rock in volcanic arcs.

anhydrous

Literally, "without water". Refers to minerals or other materials which do not have water as an primary constituent.

anticline

A downward-curving (convex) fold in rock that resembles an arch. The central part, being the most exposed to erosion, display the oldest section of rock. See syncline.

annual snowline

A term used by glaciologists (scientists who study glaciers) for the boundary where the amount of snow loss from melting equals the amount of snow accumulation from snowfall (also called firn limit).

aphanitic

An igneous rock texture in which individual mineral grains are too small to be distinguished with the naked eye.

aplite

A light-colored igneous rock with the same mineral composition as granite: quartz, plagioclase feldspar, and potassium feldspar, but with a fine-grained, almost sugary texture.

Archean Eon

The time interval between 3800-2500 million years ago. The Archean is one of the Precambrian time intervals.

archeology

The science that focuses on the study of past human cultures.

arc rocks

See Volcanic arc rocks.

argillaceous

A term used to describe clay-rich rocks.

argillic horizon

A clay-rich layer of soil. Clay often forms in overlying soil layers from the decomposition of feldspars and other minerals. The extremely fine clay particles are gradually carried down by water to accumulate into the argillic horizon.

argillite

name used for unusually hard, fine-grained sedimentary rocks, such as shale, mudstone, siltstone, and claystone. Commonly black.

aseismic

A region without earthquakes (seismic activity).

ash

Fine particles of volcanic rock and glass blown into the atmosphere by a volcanic eruption.

asthenosphere

The uppermost layer of the mantle, located below the lithosphere. This zone of soft, easily deformed rock exists at depths of 100 kilometers to as deep as 700 kilometers.

augen

Augen are relatively large, eye-shaped mineral grains in certain types of metamorphic rocks, especially schist and gneiss. (Augen means eyes in German)

avalanche

Masses of rock or ice that fall or slide suddenly under the force of gravity.