دیکشنری تخصصی زمین شناسی

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Tableland: A large elevated region with a relatively low relief surface

 

Tar sand: A sandstone containing the densest asphaltic components of petroleum - the end-product of evaporation of volatile components or of some thickening process

 

Talus: A deposit of large angular fragments of physically weathered bedrock, usually at the base of a cliff or steep slope

 

Tectonics: The study of the movements and deformation of the crust on a large scale, including epeirogeny, metamorphism, folding, faulting, and plate tectonics

 

Terminal moraine: A sinuous ridge of unsorted glacial till deposited by a glacier at the line of its farthest advance

Terrestrial planet: A planet similar in size and composition to the Earth; especially Mars, Earth, Venus, and Mercury

 

Terrestrial sediment: A deposit of sediment that accumulated above sea level in lakes, alluvial fans, floodplains, moraines, etc., regardless of its present elevation

 

Texture (rock): The rock characteristics of grain or crystal size, size variability, rounding or angularity, and preferred orientation

 

Thalweg: A sinuous imaginary line following the deepest part of a stream

 

Thermal conductivity: A measure of a rock's capacity for heat conduction

 

Thermal expansion: The property of increasing in volume as a result of an increase in internal temperature

 

Thermonuclear reaction: A reaction in which atomic nuclei fuse into new elements with a large release of heat; especially a reaction that is self-sustaining. Occasionally used to include fission reactions as well

 

Thermoremnent magnetization: A permanent magnetization acquired by igneous rocks in the presence of the Earth's magnetic field as they cool through the Curie point

 

Thrust fault: A dip-slip fault in which the upper block above the fault plane moves up and over the lower block, so that older strata are placed over younger

 

Tidal current: A horizontal displacement of ocean water under the gravitational influence of Sun and Moon, causing the water to pile up against the coast at high tide and move outward at low tide

 

Tidal flat: A broad, flat region of muddy or sandy sediment, covered and uncovered in each tidal cycle

 

Till: An unconsolidated sediment containing all sizes of fragments from clay to boulders deposited by glacial action, usually unbedded

 

Time scale: The division of geologic history into eras, periods, and epochs accomplished through stratigraphy and paleontology

 

Topographic map: See Contour map; also a schematic drawing of prominent landforms indicated by conventionalized symbols, such as hachures or contours

 

Topography: The shape of the Earth's surface, above and below sea level; the set of landforms in a region; the distribution of elevations

 

Topset bed: A horizontal sedimentary bed formed at the top of a delta and overlying the foreset beds

 

Trace element: An element that appears in minerals in a concentration of less than l percent (often less than 0.001 percent

Transform fault: A strike-slip fault connecting the ends of an offset in a mid-ocean ridge. Some pairs of plates slide past each other along transform faults

 

Transgression: A rise in sea level relative to the land which causes areas to be submerged and marine deposition to begin in that region

 

Transition element: Elements of atomic number 21 to 29, 38 to 46, and 71 to 78, whose second outermost electron shell is only partially filled

 

Transpiration: The removal of water from the ground into plants, ultimately to be evaporated into the atmosphere by them

 

Transverse dune: A dune that has its axis transverse to the prevailing winds or to a current. The upwind or upcurrent side has a gentle slope, and the downwind side lies at the angle of repose

 

Trap (oil): A sedimentary or tectonic structure that impedes the upward movement of oil and gas and allows it to collect beneath the barrier

 

Travel-time curve: A curve on a graph of travel time versus distance for the arrival of seismic waves from distant events. Each type of seismic wave has its own curve

 

Travertine: A terrestrial deposit of limestone formed in caves and around hot springs where cooling, carbonate-saturated groundwater is exposed to the air

 

Trellis drainage: A system of streams in which tributaries tend to lie in parallel valleys formed in steeply dipping beds in folded belts

 

Trench: A long and narrow deep trough in the sea floor; interpreted as marking the line along which a plate bends down into a subduction zone

 

Triple junction: A point that is common to three plates and which must also be the meeting place of three boundary features, such as divergence zones, convergence zones, or transform faults

 

Tsunami: A large destructive wave caused by sea-floor movements in an earthquake

 

Tuff: A consolidated rock composed of pyroclastic fragments and fine ash. If particles are melted slightly together from their own heat, it is a "welded tuff."

 

Turbidite: The sedimentary deposit of a turbidity current, typically showing graded bedding and sedimentary structures on the undersides of the sandstones.

 

Turbidity current: A mass of mixed water and sediment that flows downhill along the bottom of an ocean or lake because it is denser than the surrounding water. It may reach high speeds and erode rapidly (see also Density current)

 

Turbulent flow: A high-velocity flow in which streamlines are neither parallel nor straight but curled into small tight eddies (compare Laminar flow)

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Ultramafic rock: An igneous rock consisting dominantly of mafic minerals, containing less than 10 percent feldspar. Includes dunite, peridotite, amphibolite, and pyroxenite

 

Unconformity: A surface that separates two strata. It represents an interval of time in which deposition stopped, erosion removed some sediments and rock, and then deposition resumed (see also Angular unconformity )

 

Unconsolidated material: Nonlithified sediment that has no mineral cement or matrix binding its grains

 

Uniformitarianism, Principle of: The concept that the processes that have shaped the Earth through geologic time are the same as those observable today

 

Unit cell: The smallest contiguous group of atomic structural units in a mineral that can be repeated in three directions to form a crystal

 

Uplift: A broad and gentle epeirogenic increase in the elevation of a region without a eustatic change of sea level

 

Upwelling current: The upward movement of cold bottom water in the sea, which occurs when wind or currents displace the lighter surface water

 

U-shaped valley: A deep valley with steep upper walls that grade into a flat floor, usually eroded by a glacier

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Vadose zone: The region in the ground between the surface and the water table in which pores are not filled with water. Also called the unsaturated zone

 

Valence electron: An electron of the outermost shell of an atom; one of those most active in bonding

 

Valley glacier: A glacier that is smaller than a continental glacier or an icecap, and which flows mainly along well-defined valleys, many with tributaries

 

Van der Waals bond: A bond much weaker than the ionic or covalent, which bonds atoms by small electrostatic attraction

 

Varve: A thin layer of sediment grading upward from coarse to fine and light to dark, found in a lake bed and representing one year's deposition of glacial outwash

 

Vector: A mathematical element that has a direction and magnitude, but no fixed position. Examples are force and gravity

 

Vein: A deposit of foreign minerals within a rock fracture or joint

 

Ventifact: A rock that exhibits the effects of sand-blasting or "snowblasting" on its surfaces, which become fiat with sharp edges in between

 

Vertical exaggeration: The ratio of the horizontal scale (for example, 100,000: 1) to the vertical scale (for example, 500: 1) in an illustration

 

Vesicle: A cavity in an igneous rock that was formerly occupied by a bubble of escaping gas

 

Viscosity: A measure of resistance to flow in a liquid

 

Volcanic ash: A volcanic sediment of rock fragments, usually glass, less than 4 millimeters in diameter that is formed when escaping gases force out a fine spray of magma

 

Volcanic ash fall: A deposit of volcanic ash resting where it was dropped by eruptions and winds

 

Volcanic ash flow : A mixture of vol canic ash and gases that moves downhill as a density current in the atmosphere

 

Volcanic block: A pyroclastic rock fragment ranging from about fist- to car-sized

 

Volcanic bomb: A pyroclastic rock fragment that shows the effects of cooling in flight in its streamlined or "bread-crust" surface

 

Volcanic breccia: A pyroclastic rock in which all fragments are more than 2 millimeters in diameter

 

Volcanic cone: The deposit of lava and pyroclastic materials that has settled close to the volcano's central vent

 

Volcanic dome: A rounded accumulation around a volcanic vent of congealed lava too viscous to flow away quickly; hence usually rhyolite lava. Volcanic dust: See Volcanic ash

 

Volcanic ejecta blanket: A collective term for all the pyroclastic rocks deposited around a volcano, especially by a volcanic explosion

 

Volcanic emanations: Gases, especially steam, emitted from a vent or released from lava

 

Volcanic pipe: The vertical chamber along which magma and gas ascend to the surface; also, a formation of igneous rock that cooled in a pipe and remains after the erosion of the volcano

 

Volcano: Any opening through the crust that has allowed magma to reach the surface, including the deposits immediately surrounding this vent

 

V-shaped valley: A valley whose walls have a more-or-less uniform slope from top to bottom, usually formed by stream erosion

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Wadi: A steep-sided valley containing an intermittent stream in an arid region

 

Warping: In tectonics, refers to the gentle, regional bending of the crust, which occurs in epeirogenic movements

 

Water mass: A mass of water that fills part of an ocean or lake and is distinguished by its uniform physical and chemical properties, such as temperature and salinity

 

Water table: A gently-curved surface below the ground at which the vadose zone ends and the phreatic zone begins; the level to which a well would fill with water

 

Wave-cut terrace: A level surface formed by wave erosion of coastal bedrock to the bottom of the turbulent breaker zone. May appear above sea level if uplifted

 

Wavelength: The distance between two successive peaks, or between troughs, of a cyclic propagating disturbance

 

Wave steepness: The maximum height or amplitude of a wave divided by its wavelength

 

Weathering: The set of all processes that decay and break up bedrock, by a combination of physically fracturing or chemical decomposition

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Xenolith: A piece of country rock found engulfed in an intrusion

 

X-ray diffraction: In mineralogy, the process of identifying mineral structures by exposing crystals to X-rays and studying the resulting diffraction pattern

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Youth (geomorphology): A stage in the geomorphic cycle in which a landscape has just been uplifted and is beginning to be dissected by canyons cut by young streams

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Zeolite: A class of silicates containing H=O in cavities within the crystal structure. Formed by alteration at low temperature and pressure of other silicates, often volcanic glass

 

Zoned crystal: A single crystal of one mineral that has a different chemical composition in its inner and outer parts. Formed from minerals belonging to a solid-solution series, and caused by the changing concentration of elements in a cooling magma that results from crystals settling out