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Geologic Glossary

This glossary contains simplified definitions for technical terms used within this site.
For more complete, technical definitions, click here.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

B

banded gneiss

See gneiss.

basalt

A dark, fine-grained, extrusive (volcanic) igneous rock with a low silica content (40% to 50%), but rich in iron, magnesium and calcium. Generally occurs in lava flows, but also as dikes. Basalt makes up most of the ocean floor and is the most abundant volcanic rock in the Earth's crust.
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base level

The level (elevation) at which a stream or river can erode no more, usually sea level.

basin

A depression in the Earth's surface that collects sediment.

Basin and Range province

This province extends from eastern California to central Utah, and from southern Idaho into the state of Sonora in Mexico. Within the Basin and Range province the Earth's crust and uppermost mantle have been stretched, creating large faults. Along these faults linear mountain ranges were uplifted and flat valleys down-dropped, producing the distinctive topography of the Basin and Range province.
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batholith

Very large mass of intrusive (plutonic) igneous rock that forms when magma solidifies at depth. A batholith must have greater than 100 square kilometers (40 square miles) of exposed area. See pluton, stock.

bed

A layer of sediment or sedimentary rock.

bedding

Parallel layers of sediment or sedimentary rock (beds) that can be distinguished from each other by characteristics such as grain size and chemical composition.

bedding

Sedimentary layers in a rock. The beds are distinguished from each other by grain size and composition, such as in shale and sandstone. Subtle changes, such as beds richer in iron-oxide, help distinguish bedding. Most beds are deposited essentially horizontally.

bedrock

The solid rock that lies beneath soil and other loose surface materials.

billion

In North America, 1,000,000,000
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biotite

A common rock-forming mineral of the mica family. Biotite is a black or dark brown silicate rich in iron, magnesium, potassium, aluminum, and, of course, silica. Like other micas, it forms flat book-like crystals that peal apart into individual sheets on cleavage planes.

blueschist

Metamorphic rock rich in blue amphibole.

borrow pit

A pit or excavation area used for gathering earth materials (borrow) such as sand or gravel.

boulder

Any loose rock (sediment) larger than 256 millimeters (10 inches).
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breccia

Rock made up of angular fragments of other rocks held together by mineral cement or a fine-grained matrix. Volcanic breccia is made of volcanic rock fragments, generally blown from a volcano or eroded from it. Fault breccia is made by breaking and grinding rocks along a fault.
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