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B


backarc basin
A depression landward of a volcanic arc in a subduction zone, which is lined with trapped sediment from the volcanic arc and the plate interior. See also forearc basin.
backarc spreading
The process by which the overriding plate in a subduction zone becomes stretched to the point of rifting, so that magma can then rise into the gap created by the rift. Backarc spreading typically occurs when the subducting plate sinks more rapidly than the overriding plate moves forward.
backshore
The portion of a beach that extends from the high-tide line inland to the sea cliff or vegetation line. Swash reaches the backshore only during major storms.
backswamp
The section of a floodplain where deposits of fine silts and clays settle after a flood. Backswamps usually lie behind a stream's natural levees.
banded iron formation
A rock that is made up of alternating light silica-rich layers and dark-colored layers of iron-rich minerals, which were deposited in marine basins on every continent about 2 billion years ago.
barchan dune
A crescent-shaped dune that forms around a small patch of vegetation, lies perpendicular to the prevailing wind direction, and has a gentle, convex windward slope and a steep, concave leeward slope. Barchan dunes typically form in arid, inland deserts with stable wind direction and relatively little sand.
barrier island
A ridge of sand that runs parallel to the main coast but is separated from it by a bay or lagoon. Barrier islands range from 10 to 100 kilometers in length and from 2 to 5 kilometers in width. A barrier island may be as high as 6 meters above sea level. barrier reef A long, narrow reef that runs parallel to the main coast but is separated from it by a wide lagoon.
basal sliding
The process by which a glacier undergoes thawing at its base, producing a film of water along which the glacier then flows. Basal sliding primarily affects glaciers in warm climates or mid-latitude mountain ranges.
basalt
The dark, dense, aphanitic, extrusive rock that has a silica content of 40% to 50% and makes up most of the ocean floor. Basalt is the most abundant volcanic rock in the Earth's crust.
base level
The lowest level to which a stream can erode the channel through which it flows, generally equal to the prevailing global sea level.
basin
A round or oval depression in the Earth's surface, containing the youngest section of rock in its lowest, central part.
batholith
A massive discordant pluton with a surface area greater than 100 square kilometers, typically having a depth of about 30 kilometers. Batholiths are generally found in elongated mountain ranges after the country rock above them has eroded.
baymouth bar
A narrow ridge of sand that stretches completely across the mouth of a bay. (Also called bay bar and bay barrier.)
beach
The part of a coast that is washed by waves or tides, which cover it with sediments of various sizes and composition, such as sand or pebbles.
beach drift
1. The process by which swash and backwash move sediments along a beach face. 2. The sediments so moved. Beach drift typically consists of sand, gravel, shell fragments, and pebbles. See also longshore drift.
beach face
The portion of a foreshore that lies nearest to the sea and regularly receives the swash of breaking waves. The beach face is the steepest part of the foreshore.
bedding
The division of sediment or sedimentary rock into parallel layers (beds) that can be distinguished from each other by such features as chemical composition and grain size.
bed load
A body of coarse particles that move along the bottom of a stream.
Benioff-Wadati zone
A region where the subduction of oceanic plates causes earthquakes, the foci of which are deeper the farther inland they are.
berm
A low, narrow layer or mound of sediment deposited on a backshore by storm waves.
biogenic chemical sediment
Sedimentary rocks derived from living organisms. Common examples include fossiliferous limestones and coal.
biomass fuel
A renewable fuel derived from a living organism or the by product of a living organism. Biomass fuels include wood, dung, methane gas, and grain alcohol.
bitumen
Any of a group of solid and semi-solid hydrocarbons that can be converted into liquid form by heating. Bitumens can be refined to produce such commercial products as gasoline, fuel oil, and asphalt.
bituminous coal
A shiny black coal that develops from deeply buried lignite through heat and pressure, and that has a carbon content of 80% to 93%, which makes it a more efficient heating fuel than lignite.
body wave
A type of seismic wave that transmits energy from an earthquake's focus through the Earth's interior in all directions. See also surface wave.
bond
To combine, by means of chemical reaction, with another atom to form a compound. When an atom bonds with another, it either loses, gains, or shares electrons with the other atom.
Bowen's reaction series
The sequence of igneous rocks formed from a mafic magma, assuming mineral crystals that have already formed continue to react with the liquid magma and so evolve into new minerals, thereby creating the next rock in the sequence.
braided stream
A network of converging and diverging streams separated from each other by narrow strips of sand and gravel.
breakwater
A wall built seaward of a coast to intercept incoming waves and so protect a harbor or shore. Breakwaters are typically built parallel to the coast.
breccia
A clastic rock composed of particles more than 2 millimeters in diameter and marked by the angularity of its component grains and rock fragments.
breeder reactor
A nuclear reactor that manufactures more fissionable isotopes than it consumes. Breeder reactors use the widely available, nonfissionable uranium isotope U-238, together with small amounts of fissionable U-235, to produce a fissionable isotope of plutonium, Pu-239.
brittle failure
Rupture of rock, a type of permanent strain caused by relatively low stress.
burial metamorphism
A form of regional metamorphism that acts on rocks covered by 5 to 10 kilometers of rock or sediment, caused by heat from the Earth's interior and lithostatic pressure.

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