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Geological Terms Beginning With "A"



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Aa:

A term of Hawaiian origin. Used in reference to a basaltic lava that occurs in flows with a fissured, rough and jagged surface.

Acidic Rock:

An igneous rock that has a relatively high silica content. Examples are granite and rhyolite. Also see entries for basic, intermediate and ultrabasic rocks.

Acquisition:

Obtaining the legal right to test a property for mineral resources and produce any that are discovered. The rights can be obtained by purchasing the entire property (surface and mineral rights), purchasing the mineral rights alone, concession, or leasing the mineral rights.

Acre-Foot:

The volume of water needed to flood one acre of land to a depth of one foot. Equivalent to 43,560 cubic feet, 1,233 cubic meters or 325,851 gallons. One of the most common units of measure used for reservoir capacity. Also used in mineral resource calculations (an acre-foot of coal is a block of coal one acre in area and one foot thick - it weighs approximately 1,800 tons).

Acreage:

An area, measured in acres, that is owned or controlled by one or more owners or lessees. "Gross Acreage" is the entire geographic area under control. "Net Acreage" is the gross acreage multiplied by the fractional share of any individual owner or lessee.

Alkali:

Used in reference to materials that are rich in sodium and/or potassium.

Alluvial Fan:

A fan-shaped wedge of sediment that typically accumulates on land where a stream emerges from a steep canyon onto a flat area. In map view it has the shape of an open fan. Typically forms in arid or semiarid climates.

Alluvium:

An unconsolidated accumulation of stream-deposited sediments, including sands, silts, clays or gravels.

Amphibolite:

Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. It is composed primarily of amphibole and plagioclase, usually with very little quartz. Picture of Amphibolite.

Andesite:

A fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene and biotite. Picture of Andesite.

Angle of Repose:

The maximum angle that a soil, sediment or other loose material can be placed or accumulate and be stable. The angle of repose varies for different types of materials and different moisture conditions.

Angular Unconformity:

An erosional surface that separates rock units of differing dips. The rocks below the surface were deposited, deformed and eroded. The younger rocks above then accumulated upon the erosional surface.

Anthracite:

The highest rank of coal. By definition, a coal with a fixed carbon content of over 91% on a dry ash-free basis. Anthracite coals have a bright luster, break with a conchoidal fracture, a semi-metallic luster and are difficult to ignite. Frequently referred to by the layman as "hard coal".

Aquiclude:

A subsurface rock, soil or sediment unit that does not yield useful quanties of water.

Aquifer:

A subsurface rock or sediment unit that is porous and permeable. To be an aquifer it must have these traits to a high enough degree that it stores and transmits useful quantities of water.

Aquifer (artesian):

An aquifer that is bounded above and below by impermeable rock or sediment layers. The water in the aquifer is also under enough pressure that, when the aquifer is tapped by a well, the water rises up the well bore to a level that is above the top of the aquifer. The water may or may not flow onto the land surface.

Aquifer (confined):

An aquifer that is bounded above and below by impermeable rock or sediment layers. There may or may not be enough pressure in the aquifer to make it an "artesian aquifer".

Aquifer (unconfined):

An aquifer that is not overlain by an impermeable rock unit. The water in this aquifer is under atmospheric pressure and is recharged by precipitation that falls on the land surface directly above the aquifer.

Arkose:

A sandstone that contains at least 25% feldspar. Easily recognized because the feldspar grains are typically pink and angular in shape.

Arroyo:

A flat-bottom gully with steep sides that is a channel for an intermittent stream.

Asthenosphere:

A portion of the upper mantle that is directly below the lithosphere. A zone of low strength in the upper mantle defines the top of the asthenosphere. This weak zone allows the plates of the lithosphere to slide across the top of the asthenosphere.

Astrobleme:

An ancient circular scar on Earth's surface produced by the impact of a meteorite or comet. Use our Google maps page to get close up images of these meteor impact structures.

Atoll:

A ring-shaped group of coral islands that are surrounded by deep ocean water and that enclose a shallow lagoon.

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