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Geological Terms Beginning With "B"



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Backwash

Backwash

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

Banded Iron Ore

Banded Iron Ore

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

Bankfull Stage

Bankfull Stage

A height of water in a stream that completely fills the natural channel. If the water rises any higher it will overtop the banks of the stream and a flood will occur.

Bank Storage

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 slowly seeps into the stream as the high water level subsides - hence the term "bank storage".

Bar

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. The photo shows a sand bar in a river at low water level.

Barchan

Barchan

A sand dune that is crescent-shaped in map view. Barchan dunes form in areas of limited sand supply. They move across the desert floor with their gently sloping convex sides facing upwind and their steeply sloping concave sides facing downwind.

Barrier Island

Barrier Island

A long, narrow island composed of sediment that parallels a shoreline. Shown in the photo are some of the Chandeleur Islands, barrier islands that stand between the Gulf of Mexico and the Louisiana coastline. During storms, the barrier islands absorb some of the wave energy that would otherwise batter the coastline. Their geometry can be significantly changed during these storms.

Basalt

Basalt

A dark-colored fine-grained extrusive igneous rock composed mainly of plagioclase feldspar and pyroxene. Similar in composition to gabbro. Basalt is thought to be one of the main components of oceanic crust and is the most common rock type of lava flows on land.

Base Flow

Base Flow

Water that seeps into a stream through a permeable rock or sediment unit that outcrops in the bottom or banks of the stream.

Base Level

Base Level

The lower limit of erosion by a stream. Sea level is the ultimate base level. However, lakes, dams and other locations of low water flow can serve as a temporary base level in upstream areas.

Basement

Basement

The igneous and metamorphic rocks that exist below the oldest sedimentary cover. In some areas, such as shields, the basement rocks may be exposed at the surface. The rock at river level in the photo is the Vishnu Schist, basement rock in the Phantom Ranch area of the Grand Canyon.

Basalt

Basic Rock

An igneous rock that has a relatively low silica content. Examples are gabbro and basalt. Also see entries for acidic, intermediate and ultrabasic rocks.

Basin

Basin

In tectonics, a circular, syncline-like depression of strata. In sedimentology, the site of accumulation of a large thickness of sediments. Shown in the photo is the Tarim Basin of China, which is occupied by the Taklimakan Desert, the largest desert in China.

Batholith

Batholith

A very large intrusive igneous rock mass that has been exposed by erosion and with an exposed surface area of over 100 square kilometers. A batholith has no known floor. Shown in the photo is El Capitan, a famous granite outcrop in Yosemite National Park. The granites exposed in Yosemite and much of the surrounding area are part of the Sierra Nevada Batholith.

Bathymetry

Bathymetry

The measurement of ocean depths and the preparation of maps that display water depth or the topography of the ocean floor.

Bauxite

Bauxite

The most important ore of aluminum. A mixture of aluminum oxides and hydroxides that forms from intense chemical weathering of a soil in tropical environments.

Bedding

Bedding

The characteristic structure of sedimentary rocks in which layers of different composition, grain size or arrangement are stacked one on top of another in a sequence with oldest at the bottom and youngest at the top. The photo of bedded rock units in the image was acquired by NASA's Mars Rover Curiosity in the Gale Crater of Mars.

Bedding Plane

Bedding Plane

A distinct surface of contact that separates the layers of a sedimentary rock unit.

Bed Load

Bed Load

The larger, heavier particles that are being transported by a stream. Instead of being dissolved or suspended, these are being rolled or bounced along, spending at least part of their time in contact with the stream bottom.

Bedrock

Bedrock

Solid rock present beneath any soil, subsoil, sediment or other surface cover. In some locations it may be exposed at Earth's surface.

Beta-Particle

Beta-Particle

A particle emitted with high energy and velocity from the nucleus of an atom during radioactive decay. It can be equivalent to an electron or a positron.

A-Horizon

B-Horizon

A layer in the soil, below the A-horizon, where materials leached from above accumulate. Typically enriched in iron, clay, aluminum and organic compounds.

Banded Iron Ore

Biochemical Rocks

A sedimentary rock that forms from the chemical activities of organisms. Organic (reef and fossiliferous) limestones and bacterial iron ores are examples. The photo is a specimen of coquina, a variety of limestone composed primarily of shell debris.

Bioturbated

Bioturbated

An adjective used in reference to a sediment or sedimentary rock. Bioturbated sediments have been disturbed by animals (such as burrowing worms or shell fish) or plant roots. These have penetrated the sediment and disturbed any or all original sedimentary laminations and structures. Bioturbated rocks were disturbed in this way while still in the soft sediment phase of their formation.

Bituminous Coal

Bituminous Coal

A rank of coal that falls between anthracite and semi-bituminous. The most abundant rank of coal. Frequently referred to by the layman as "soft coal". It is most often composed of bright and dull bands. The bright bands generally represent well preserved woody materials. The dull bands generally represent degraded woody materials and mineral matter.

Block Fault Mountain

Block Fault Mountain

A linear mountain that is bounded by normal faults. Also known as "fault-block mountain". The photo shows Mount Moran, near Jackson Lake Junction, Wyoming. Mount Moran is part of the Teton Range, a block-faulted mountain range.

Blowout

Blowout Dunes

Small, shallow, round or trough-shaped depressions in sand or dry soil that are formed by wind erosion. The material removed by the wind may also be referred to as "blowout". Sometimes called "blowout dunes".

Breccia

Breccia

A clastic sedimentary rock that is composed of large (over two millimeter diameter) angular clasts. The spaces between the clasts can be a matrix of smaller particles or a mineral cement that binds the rock together. The photo is a piece of chert breccia. A breccia in which the clasts are composed mainly of chert.

Butte

Butte

A conspicuous hill with steep sides and a flat top. The top is usually a cap-rock of resistant material. This structure is frequently an erosional remnant in an area of flat-lying sedimentary rocks. The photo shows the south face of Fajada Butte, Chaco Culture National Historical Park.

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