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abrasion
A form of mechanical weathering that occurs when loose fragments or particles of rocks and minerals that are being transported, as by water or air, collide with each other or scrape the surfaces of stationary rocks.
accretionary
wedge A mass of sediment and oceanic lithosphere that is transferred from a subducting plate to the less dense, overriding plate with which it converges.
acid rain
Rain that contains such acidic compounds as sulfuric acid and nitric acid, which are produced by the combination of atmospheric water with oxides released when hydrocarbons are burned. Acid rain is widely considered responsible for damaging forests, crops, and human-made structures, and for killing aqua-tic life.
aftershock
A ground tremor caused by the repositioning of rocks after an earthquake. Aftershocks may continue to occur for as long as two years after the initial earthquake. The intensity of an earthquake's aftershocks decreases over time.
aggradation
The process by which a stream's gradient steepens due to increased deposition of sediment.
alloy
A metal that is manufactured by combining two or more molten metals. An alloy is always harder than its component metals. Bronze is an alloy of copper and tin.
alluvial fan
A triangular deposit of sediment left by a stream that has lost velocity upon entering a broad, relatively flat valley.
alluvium
A deposit of sediment left by a stream on the stream's channel or floodplain.
alpine glacier
A mountain glacier that is confined by highlands.
andesite
The dark, aphanitic, extrusive rock that has a silica content of about 60% and is the second most abundant volcanic rock. Andesites are found in large quantities in the Andes Mountains.
angle of repose
The maximum angle at which a pile of unconsolidated material can remain stable.
anthracite
A hard, jet-black coal that develops from lignite and bituminous coal through metamorphism, has a carbon content of 92% to 98%, and contains little or no gas. Anthracite burns with an extremely hot, blue flame and very little smoke, but it is difficult to ignite and both difficult and dangerous to mine.
anticline
A convex fold in rock, the central part of which contains the oldest section of rock. See also syncline.
aquiclude
An impermeable body of rock that may absorb water slowly but does not transmit it.
aquifer
A permeable body of rock or regolith that both stores and transports groundwater.
aquitard
A layer of rock having low permeability that stores groundwater but delays its flow.
arÍte
A sharp ridge of erosion-resistant rock formed between adjacent cirque glaciers.
aridity
index The ratio of a region's potential annual evaporation, as determined by its receipt of solar radiation, to its average annual precipitation.
arroyo
A small, deep, usually dry channel eroded by a short-lived or intermittent desert stream.
artesian
Of, being, or concerning an aquifer in which water rises to the surface due to pressure from overlying water.
asthenosphere
A layer of soft but solid, mobile rock comprising the lower part of the upper mantle from about 100 to 350 kilometers beneath the Earth's surface. See also lithosphere.
atoll
A circular reef that encloses a relatively shallow lagoon and extends from a very great depth to the sea surface. An atoll forms when an oceanic island ringed by a barrier reef sinks below sea level.
atom
The smallest particle that retains all the chemical properties of a given element.
atomic mass
1. The sum of protons and neutrons in an atom's nucleus. 2. The combined mass of all the particles in a given atom.
atomic number
The number of protons in the nucleus of a given atom. Elements are distinguished from each other by their at-omic numbers.

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