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T


talus
A pile of rock fragments lying at the bottom of the cliff or steep slope from which they have broken off.
talus slope
The large pile of rocky boulders that accumulates at the foot of a cliff, typically by the mechanical-weathering process of frost-wedging.
tarn
A deep, typically circular lake that forms when a cirque glacier melts.
tectonic creep
The almost constant movement of certain fault blocks that allows strain energy to be released without major earthquakes.
tension
Stress that stretches or extends rocks, so that they become thinner vertically and longer laterally. Tension may be caused by divergence or rifting.
tephra (plural noun)
Pyroclastic materials that fly from an erupting volcano through the air before cooling, and range in size from fine dust to massive blocks.
terminus
The outer margin of a glacier.
theory
A comprehensive explanation of a given set of data that has been repeatedly confirmed by observation and experimentation and has gained general acceptance within the scientific community but has not yet been decisively proven. See also hypothesis and scientific law.
thermal contraction
A form of mechanical weathering in which cold causes a mineral's crystal structure to contract.
thermal expansion
A form of mechanical weathering in which heat causes a mineral's crystal structure to enlarge.
thermal plume
A vertical column of upwelling mantle material, 100 to 250 kilometers in diameter, that rises from beneath a continent or ocean and can be perceived at the Earth's surface as a hot spot. Thermal plumes carry enough energy to move a plate, and they may be found both at plate boundaries and plate interiors.
thrust fault
A reverse fault marked by a dip of 45 or less.
tidal bore
A turbulent, abrupt, wall-like wave that is caused by a flood tide.
tide
1. The cycle of alternate rising and falling of the surface of an ocean or large lake, caused by the gravitational pull of the Sun and especially Moon in interaction with the Earth's rotation. Tides occur on a regular basis, twice every day on most of the Earth. 2. A single rise or fall within this cycle.
topography
The set of physical features, such as mountains, valleys, and the shapes of landforms, that characterizes a given landscape.
transform motion
The movement of two adjacent lithospheric plates in opposite directions along a parallel line at their common edge. Transform motion often causes earthquakes.
transition zone
The seismic discontinuity located in the upper mantle just beneath the asthenosphere and characterized by a marked increase in the velocity of seismic waves.
translatory motion
the movement of water over a significant distance in the direction of a wave.
transverse dune
One of a series of dunes having an especially steep slip face and a gentle windward slope and standing perpendicular to the prevailing wind direction and parallel to each other. Transverse dunes typically form in arid and semi-arid regions with plentiful sand, stable wind direction, and scarce vegetation. A transverse dune may be as much as 100 kilometers long, 200 meters high, and 3 kilometers wide.
tributary
A stream that supplies water to a larger stream.
trunk stream
A large stream into which tributaries carry water and sediment.
tsunami(plural tsunami)
A vast sea wave caused by the sudden dropping or rising of a section of the sea floor following an earthquake. Tsunami may be as much as 30 meters high and 200 kilometers long, may move as fast as 250 kilometers per hour, and may continue to occur for as long as a few days.

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