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Geological Glossary - B

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 horizon The subsoil. This layer lies below the topsoil and contains more clay and iron oxides than either the A-horizon or the C-horizon.
B-horizon The intermediate layer in a soil, situated below the A-horizon and consisting of clay and oxides. Also called the zone of accumulation.
B-type earthquake Events with weak P-waves and no S-waves a low-frequency content and occurring under volcanoes at depths < 1 km.
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 fault. Along these faults linear mountain ranges were uplifted and flat valleys down-dropped, producing the distinctive topography of the Basin and Range province.
Benioff zone A narrow zone defined by earthquake foci and tens of kilometers thick dipping from the surface under the Earth's crust. Zone of earthquake foci produced near the surface of and within the subducted lithosphere.
Brownian coagulation The process by which a particle grows by the rapid condensation of gases and fine particles.
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
backwash The return flow of water down a beach after a wave has broken.
bajada Multiple alluvial fans joining together to form an undulating landform.
banded iron formation An iron formation that consists of alternating iron-rich and iron-poor layer. Most rocks of this type are older than about 2 billion years.
banded iron ore A sediment consisting of layer of chert alternating with bands of ferric iron oxides (hematite and limonite) in valuable concentrations.
bankfull stage The height of water in a stream that just corresponds to the level of the surrounding floodplain.
bar " A unit of pressure equal to 10 to the sixth dynes/square centimeter; approximately one atmosphere. "
bar (stream) An accumulation of sediment, usually sandy, which form at the borders or in the channels of streams or offshore from a beach.
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.
barchan dune A crescent-shaped sand dune moving across a clean surface with its convex face upwind and its concave slip face downwind.
barrier island A long, narrow island parallel to the shore, composed of sand and built by wave action.
barrier reef An elongate organic reef that parallels a coastline and is large enough to dissipate ocean wave, leaving a quiet-water lagoon on its landward side.
basal sliding The movement or speed of movement of a glacier on its bed. Also called basal slip.
basalt A fine-grained, dark, mafic igneous rock composed largely of plagioclase feldspar and pyroxene. Volcanic rock (or magma) that is generally dark in color, contains 45 to 54 percent silica, and is rich in iron and magnesium. An eruption of basaltic magma is generally quiet and results in flows (both vesicular and non-vesicular) and breccia. Undersea eruptions commonly result in the formation of pillow lava. Basalt represents the initial differentiated material erupted by the earth at spreading center.
base-level " The level below which a stream cannot erode; usually sea level sometimes locally the level of a lake or resistant formation. "
basement The oldest rocks recognized in a given area, a complex of metamorphic and igneous rocks that underlies all the sedimentary formation. Usually Precambrian or Paleozoic in age.
basement complex A series of rocks generally with complex structure beneath the dominantly sedimentary rocks. In many places, these are igneous and metamorphic rocks of either Early or Late Precambrian, but in some places these may be much younger, as Paleozoic, Mesozoic, or even Cenozoic
basement rocks Rocks beneath a large geologic feature (such as the fold-and-thrust belt of a mountain system) which are genetically unrelated to the overlying feature.
basic rock Any igneous rock containing mafic mineral rich in iron and magnesium, but containing no quartz and little sodium rich plagioclase feldspar.
basin In tectonics, a circular, syncline-like depression of strata. In sedimentology, the site of accumulation of a large thickness of sediments.
batholith A great irregular mass of coarse-grained igneous rock with an exposed surface of more than 100 square kilometers, which has either intruded the country rock or been derived from it through metamorphism.
bathymetry The study and mapping of sea-floor topography.
bauxite " A rock composed primarily of hydrous aluminum oxides and formed by weathering in tropical areas with good drainage; a major ore of aluminum. "
bay A wide area of water extending into land from a sea or lake.
beach A narrow strip of land bordering a large body of water.
bed A distinct sedimentary layer (stratum) thicker than 1 centimeter.
bed-load The sediment that a stream moves along the bottom of its channel by rolling and bouncing.
bedding Sedimentary layer in a rock. The bed 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. A characteristic of sedimentary rocks in which parallel planar surfaces separating different grain sizes or compositions indicate successive depositional surfaces that existed at the time of sedimentation.
belemnite Extinct invertebrate animal (cephalopod) known from cigar-shaped fossil.
benthic foraminifera Benthic protozoans (i.e., live on the seafloor) which may form carbonate tests. The tests are preserved in sediment and form an important basis for paleoceanographic reconstructions.
bentonite A clay material composed principally of the mineral montmorillonite. It has a great affinity for fresh water and when hydrated will increase its volume more than seven times. Water/bentoninte suspensions are essentially impermeable. Commonly used as a sealant for ponds.
beta-particle An electron emitted with high energy and velocity from a nucleus undergoing radioactive decay.
bimodal distribution A size or mass spectrum that has two peaks of concentration.
biochemical precipitate A sediment, especially of limestone or iron, formed from elements extracted from sea water by living organism.
biogeochemistry The chemical interactions among the atmosphere, biosphere, hydrosphere and lithosphere. Study of the cycling transformations and transport of chemicals in and between landscapes and ecosystems.
biogeography The study of the distribution of organisms on a geographic scale.
biosphere All the area occupied or favorable for occupation by living organism. It includes parts of the lithosphere, hydrosphere, and atmosphere
biostratigraphic unlt A body of rock uch as a zone defined on the basis of its fossil content and having approximately time-parallel upper and lower boundaries.
biostratigraphy Relative (not absolute, i.e. number of years) age dating and time correlation of stratified rocks (mostly sedimentary plus some volcanics) by means of fossil contained in those rocks based on our knowledge of their time sequence locally regionally and globally -- all of this ultimately being a function of evolutionary change.
biostratigraphy The study and classification of rocks and their history based on their fossil content.
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 crystal that peal apart into individual sheets on cleavage planes.
bioturbation Disturbance of oceanic sediment near the water interface by bottom-dwelling (benthic), organisms, for example, burrowing worms.
bird's-eye limestone Layered limestone that is full of holes some of which may have been secondarily filled by cementing mineral. The holes were produced by burrowing animals or by gas bubbles. Most of these limestones are of intertidal or supratidal origin.
bituminous coal A soft coal formed by an intermediate degree of metamorphism and containing 15 to 20 percent volatile. The most common grade of coal.
black smokers "Seafloor hydrothermal fluids jetting from chimneys typically about 10 cm in radius. Flow rates are ~ 1-5 m/s; temperature is 350ø-400ø C. Black coloration is due to sulfide mineral precipitates."
block fault A structure formed when the crust is divided into blocks of different elevation by a set of normal fault.
blowout A shallow circular or elliptical depression in sand or dry soil formed by wind erosion.
blueschist Metamorphic rock rich in blue amphibole.
body wave A seismic wave that can travel through the interior of the earth. P-waves and S-waves are body waves.
bolide An extraterrestrial body in the 1-10-km size range, which impacts the earth at velocities of literally faster than a speeding bullet (20-70 km/sec = Mach 75), explodes upon impact, and creates a large crater. Bolide is a generic term, used to imply that we do not know the precise nature of the impacting body . . . whether it is a rocky or metallic asteroid, or an icy comet, for example.
bolson In arid regions, a basin filled with alluvium and intermittent playa lakes and having no outlet.
bomb Fragments of molten or semi-molten rock several inches to several feet in diameter which are blown out during an explosive volcanic eruption. Because of their semi-plastic condition bomb are often modified in shape during their flight or upon impact.
bond The force that holds together two atom in a compound. It may be derived from the sharing of electron (covalent) or from electrostatic attraction between ions.
borehole geophysics Science of (seismic or electrical) physical measurement between boreholes or between a borehole and the surface.
boulder A piece of gravel larger than 256 millimeters (~10 inches) in size.
brackish water Water that is lower in salinity than normal sea water and higher in salinity than freshwater ranging from 30 to 0.5 parts salt per 1000 parts water.
braided stream A stream that has many intertwining channels separated by bar of coarse sediment. Braided streams develop where sediment is supplied to the stream system at a very high rate on an alluvial fan, for example, or in front of a melting glacier.
breccia A rock that resembles conglomerate in consisting of clast of gravel surrounded by sand, but in breccia the clasts are angular, whereas in conglomerate they are rounded.
brittle-ductile transition zone The location at depth within the earth's crust where the temperature and pressure have risen to such a high level that directed stress results in plastic deformation as opposed to fracturing and faulting.
butte A steep sided and flat topped hill formed by erosion of flat laying strata where remnants of a resistant layer protect the softer rocks underneath.
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