Science / Geology Glossary

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 wate . . . View Full Definition

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 . . . View Full Definition

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

Arroyo: A steep-sided and flat-bottomed gulley in an arid region that is occupied by a stream only intermittently, after rains.

Artesian Well: A well that penetrates an aquiclude to reach an aquifer containing water under pressure. Thus water in the well rises above the surrounding water table.

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 . . . View Full Definition

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.

Atmosphere (Unit): A unit of pressure equal to 101,325 newtons per square meter, or about 14.7 pounds per square inch.

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

B-Horizon: The intermediate layer in a soil, situated below the A-horizon and consisting of clays and oxides. Also called the zone of accumulation.

Backwash: The seaward rush of water down a beach that occurs with a receding wave.

Banded Iron Ore: A rock that consists of alternating layers of chert and iron oxide mineral (usually hematite) with the iron oxide in high enough concentration to be of economic value.

Bank Storage: Water that seeps into the ground along the banks of a stream during a time of high flow. This loss of water into the ground slightly reduces the height that the stream will attain and then s . . . View Full Definition

Bankfull Stage: A height of water in a stream that completely fills the natural channel. If the water rises any higher a flood will occur.

Bar: An underwater ridge, usually of sand and/or gravel, that forms from the deposition and reworking of sediments by currents and/or waves. Bars occur in rivers, river mouths and in offshore waters.

Bar (Stream): An accumulation of sediment, usually sandy, which forms at the borders or in the channels of streams or offshore from a beach.

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