LEXICON

Lexicon: Earth Science

Lexicon: Earth Science

A

Active volcano: Crack in the Earth's crust through which hot magma (molten rock) and gases well up. The magma is termed lava when it reaches the surface. A volcanic mountain, usually cone-shaped with a crater on top, is formed around the opening, or vent, by the build-up of solidified lava and ash (rock fragments). Most volcanoes occur on plate margins (see plate tectonics), where the movements of plates generate magma or allow it to rise from the mantle beneath. However, a number are found far from plate-margin activity, on hot spots where the Earth's crust is thin, for example in Hawaii. There are two main types of volcano: composite volcanoes and shield volcanoes. A volcano that is erupting; or one that, while not erupting at the present, has erupted within (geologically) recent time and is considered likely to do so in the (geologically) near future.

Adiabatic rate: The rate of adiabatic cooling with height above the surface for a rising parcel of air in the troposphere. For a parcel of dry air, the rate of cooling is about 1°C for each 100m of ascent. The value is somewhat lower for moist air because the heat released by condensing water vapour slows the rate of cooling.

Aquifer: A water-bearing layer of rock or sediment capable of holding and transmitting fluid (such as water, gas, or oil).

Aquifer, Confined (or Artesian): An aquifer overlain by a non-permeable layer or layers, in which pressure will force water to rise above the aquifer.

B

Block: Angular chunk of solid rock ejected during a volcanic eruption.

Bomb: The solid unweathered rock that lies beneath the loose surface deposits of soil, alluvium, etc. Fragments of molten or semi-molten rock, several inches to several feet in diameter, which are blown out during an explosive volcanic eruption. Because of their semi-plastic condition, bombs are often modified in shape during their flight or upon impact.

Breccia: Angular fragments of material, commonly formed by physical weathering processes or explosive volcanic activity.

Brittle-Ductile Transition Zone: The location at depth within the earth's crust where the temperature and pressure have risen to such a high level that directed stress results in plastic deformation as opposed to fracturing and faulting.

C

Caldera: The Spanish word for cauldron, a basin-shaped volcanic depression; by definition, at least a mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes. Crater Lake occupies the best-known caldera in the Cascades.

Calorie: A unit of heat energy. The amount of heat required to raise the temperature of 1 gram (cubic Centimeter) of water 1 degree Celsius. Also, the substance which gives food its flavor (Fitzgerald, 2009).

Chattermarks: Erosional features associated with alpine glaciers.

Cinder cone: A volcanic cone built entirely of loose fragmented material (pyroclastics.)

Collection: The accumulation of precipitation into surface and underground areas, including lakes, rivers, and aquifers.

Comet: An object which circles the sun in a non-circular orbit. Commonly made up of a large mass of rock debris and ...