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Geological Terms Beginning With "A"



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Aa

Aa

A term of Hawaiian origin. Used in reference to a basaltic lava flow that with a fissured, rough, clinkery or jagged surface. They are very difficult to walk on.

abandoned mine lands

Abandoned Mine Lands

Areas where past mining activity has scarred the land surface, streams or ground water. These lands might be owned by persons, companies, organizations or government agencies. Abandoned mine lands are dangerous places. Every year in the United States accidents at abandoned mines and quarries claim numerous lives.

ablation

Ablation (glaciers)

The loss of ice and snow from a glacier through any process, including: melting, sublimation, evaporation, wind erosion and calving.

ablation

Ablation (meteorites)

The removal of surface material from a meteoroid as it passes through Earth's atmosphere. The removal is caused by air molecules impacting the meteoroid, which is probably traveling at a speed of at least 20 kilometers per second. Trails of these ablated particles are often visible from Earth as a bright streak of light sometimes called a "shooting star" or a meteoroid trail.

ablation

Ablation (sediment and soil)

The removal of small-size soil and sediment particles from a land surface by the persistent action of wind. Eventually all of the particles that can be carried by the wind will be removed, leaving a rocky surface known as a "desert pavement". The desert pavement surface above is on an alluvial fan in the Providence Mountains State Recreational Area of California.

Acidic Rock

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. The photo is a specimen of white granite about two inches across.

acid mine drainage

Acid Mine Drainage (AMD)

Acidic water discharged from a mining operation. The acid is usually produced by the reaction of sulfide minerals newly exposed to oxygen during the mining process. The acid waters usually carry dissolved metals which precipitate downstream as the acid is neutralized. Coal mines and metal mines are the typical sources of acid mine drainage. Today, active mines are required to have treatment systems that prevent the release of acid waters. Abandoned mines are typically the source of acid discharge and can continue to produce acid discharge for decades without remediation. The photo at right is of Cement Creek, a stream in Colorado degraded by metal mining.

Acre

Acre

An acre is a unit of land measure that represents 43,560 square feet or 1/640 of a square mile. A square property that is 208.71 feet long and 208.71 feet wide is approximately one acre.

Acre-Foot

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. The acre-foot is 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).

Acre

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.

active fault

Active Fault

A fault that has slipped in historic time and which is likely to slip again in the future. Strain accumulates on active faults and some creep slowly over time. The image shows the surface exposure of the Denali fault with approximately 5 meters of offset near the Delta River in Alaska.

active volcano

Active Volcano

A volcano that has erupted within historic time or one that is currently erupting. The photo is of Pavlof Volcano on the Alaska Peninsula, one of the most active volcanoes in North America.

A-Horizon

A-Horizon

A soil layer immediately below the surficial organic material. It consists of a mixture of organics and mineral matter. The majority of soil organisms live within this layer and it may be heavily bioturbated. As water moves from the surface down through this layer, soluble constituients are removed and carried deeper into the ground.

plagioclase

Alkali

In chemistry, an "alkali" is a strongly basic substance such as sodium hydroxide or sodium carbonate. These substances have the ability to neutralize acids to form salts. In geology, "alkali" is an adjective used in reference to silicate minerals or rocks that are rich in alkali metals such as sodium or potassium. Orthoclase, plagioclase and microcline would be "alkali feldspars". The specimen in the photo is a cleavage fragment of plagioclase.

Alluvial Fan

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. Alluvial fans typically form in arid or semiarid climates. Shown in the photo is the Badwater Alluvial Fan of Death Valley.

Alluvium

Alluvium

An unconsolidated accumulation of stream-deposited sediments, including sands, silts, clays or gravels. Shown in the photo is an outcrop of alluvium.

Amphibolite

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.

Andesite

Andesite

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

Angle of Repose

Angle of Repose

The maximum angle that a soil, sediment or other loose, cohseionless material can be placed or accumulate and be stable from down-slope movement. The angle of repose varies for different types of materials and different moisture conditions. Image © Barcin iStockphoto.com.

Angular Unconformity

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. Shown in the photo is a section of "The Great Unconformity" of the Grand Canyon.

Anthracite

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".

anticline

Anticline

A fold in rock strata with a convex upward shape. The rocks in the core of an anticline are the oldest. The anticline in the photo is along New Jersey Route 23 near Butler, NJ.

Aquiclude

Aquiclude

The opposite of an aquifer. An aquiclude or aquitard is a subsurface rock, soil or sediment unit that does not yield useful quanties of water. It may be porous and capable of containing water, but the transmission rate is so poor that it can not be considered to be a water source. Clay and shale are typical aquicludes.

aquifer

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.

artesian aquifer

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.

confined aquifer

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".

unconfined 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

Arkose

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

Arroyo

Arroyo

A flat-bottom gully with steep sides that is cut into alluvium. It serves as a channel for an intermittent or ephemeral stream. This term is commonly used in the arid and semiarid areas of the southwestern United States.

volcanic ash

Ash (volcanic)

Rock, mineral and volcanic glass fragments smaller than 2 millimeters in size that are blown from the vent of an erupting volcano. It is produced by the shattering of rocks during an eruption and by magma being ejected as a fine spray - propelled by volcanic gas escaping from the vent.

Asthenosphere

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

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

Atoll

A ring-shaped island or group of coral islands that are surrounded by deep ocean water and that enclose a shallow lagoon. The satellite image at left shows Rose Atoll. The island is about 1.5 miles across and the central lagoon has a maximum depth of about 60 feet. A narrow passage on the northern corner of the island is the only surface connection of the lagoon to the ocean.

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