B Index Glossary geology Science
B Index Glossary geology Science
Backwash
The seaward rush of water down a beach that occurs with a receding wave.
- Water that returns into an ocean or large lake after hitting the shore as swash.
Banded
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.
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.
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
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.
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.
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.
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.
brittle failure
Rupture of rock, a type of permanent strain caused by relatively low stress.
Benioff-Wadati zone
A region where the subduction of oceanic plates causes earthquakes, the foci of which are deeper the farther inland they are.
Basalt
A dark-colored fine-grained extrusive igneous rock composed largely of plagioclase feldspar and pyroxene. Similar in composition to gabbro. Basalt is thought to be one of the main components of oceanic crust.
base flow
streamflow coming from ground-water seepage into a stream.
- 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
The lower limit of erosion by a stream. Sea level is the ultimate base level. However, lakes can serve as a temporary base level in upstream areas. The ultimate base level of all streams is, of course, the sea.
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.
Basic Rock
An igneous rock that has a relatively low silica content. Examples are gabbro and basalt. Also see entries for acid, intermediate and ultrabasic rocks.
Very irregular topography resulting from wind and water erosion of sedimentary rock.
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.
-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.
Bilingual
The ability to use either one of two languages, especially when speaking.
Biological Diversity
A concept recognizing the variety of life forms in an area of the Earth and the ecological interdependence of these life forms.
Biota
The animal and plant life of a region considered as a total ecological entity.
Bituminous
A soft coal that, when heated, yields considerable volatile matter.
Boll Weevil
A small, greyish beetle of the southeastern
boundary
A line indicating the limit of a country, state, or other political jurisdiction.
Break-in-Bulk Point
Commonly, a transfer point on a transport route where the mode of transport (or type of carrier) changes and where large-volume shipments are reduced in size. For example, goods may be unloaded from a ship and transferred to trucks at an ocean port.
Bathymetry
The measurement of ocean depths and the preparation of topographic maps of the ocean floor.
Bauxite
The principal ore of aluminum. A mixture of aluminum oxides and hydroxides that forms from intense chemical weathering of a soil in tropical environments.
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.
Bedding Plane
A distinct surface of contact between two sedimentary rock layers.
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. See also load, suspended load, dissolved load.
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 flood plain 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.
A long, narrow island that parallels a shoreline.
- 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.
basaltic
Of, containing, or composed of basalt.
base
a substance that has a pH of more than 7, which is neutral. A base has less free hydrogen ions (H+) than hydroxyl ions (
Basin
In tectonics, a circular, syncline-like depression of strata. In sedimentology, the site of accumulation of a large thickness of sediments.
- A round or oval depression in the Earth's surface, containing the youngest section of rock in its lowest, central part.
bay
A recess in a shoreline, or an inlet between two headlands.
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.
bed
A layer of sediment or sedimentary rock that can be distinguished from the surrounding layers by such features as chemical composition and grain size.
Benioff 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 backshore by storm waves.
biomass fuel
A renewable fuel derived from a living organism or the byproduct 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 beating. Bitumens can be refined to produce such commercial products as gasoline, fuel oil, and asphalt.
blind valley
A valley formed by and containing sinkholes and disappearmg streams, and therefore dry except during periods of such heavy rainfalls that the sinkholes cannot immediately drain the entire accumulation of water.
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.
Bar-finger sand
An elongated lens of sand deposited during the growth of a distributary in a delta. The bar at the distributary mouth is the growing segment of the bar finger.
Biochemical precipitate
A sediment, especially of limestone or iron, formed from elements extracted from seawater by living organisms.
Block fault
A structure formed when the crust is divided into blocks of different elevation by a set of normal faults.
Bolson
In arid regions, a basin filled with alluvium and intermittent playa lakes and having no outlet.
bornhardt
A large, smooth, round or dome-shaped inselberg.
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.
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 fission able 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.
burial meamorphism
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 lithosta tic pressure
Bedrock
The solid mass of rock that makes up the Earth's crust. Solid rock present beneath any soil, sediment or other surface cover. In some locations it may be exposed at Earth's surface.
Beta-Particle
An electron emitted with high energy and velocity from the nucleus of an atom during radioactive decay.
B-horizon
A layer in the soil, below the A-horizon, where materials leached from above accumulate. Typically enriched in clay and oxides.
Biochemical Rocks
A sedimentary rock that forms from the chemical activities of organisms. Organic (reef and fossiliferous) limestones and bacterial iron ores are examples.
Bioturbated
An adjective used in reference to a sediment or sedimentary rock. Bioturbated sediments have been disturbed by animals (such as burronwing 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
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".
- 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.
Blowout
A shallow, round or trough-shaped depression in sand or dry soil that is formed by wind erosion. The material removed by the wind may also be referred to as "blowout".
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