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USGS Geology in the Parks

Geologic Glossary

This glossary contains simplified definitions for technical terms used within this site.
For more complete, technical definitions, click here.

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

T

talc

Magnesium silicate mineral, with water. Commonly called soapstone. Very soft and platy, like mica. Can be easily carved with a knife. Generally in very fine grained masses.

tarn

Small lake left by the retreat of a glacier. May fill a basin formed by a moraine dam or eroded by the glacier into bedrock.

talus

Pile of rock rubble below a cliff or chute. Talus slope is a common usage although it is redundant because the term "talus" actually includes the concept of a slope.

tectonically active

A term used to describe regions that are strongly affected by movement of Earth's tectonic plates. Earthquakes and volcanoes are common features in these regions.
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tephra

General term for all sizes of particles ejected into the air during volcanic eruptions. Includes particles as tiny as volcanic ash and as large as bombs and blocks (= pyroclastics).
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terrace

Level or near-level area of land, generally above a river or ocean and separated from it by a steeper slope. A river terrace is made by the river at some time in the past when the river flowed at a higher level. It A terrace may be made of river deposits such as gravel or sand, or it could be cut by the river on bedrock. A glacial terrace or outwash terrace is similar but is formed by a stream or river from a glacier upstream.

terrane

A rock formation or assemblage of rock formations that share a common geologic history. A geologic terrane is distinguished from neighboring terranes by its different history, either in its formation or in its subsequent deformation and/or metamorphism. Terranes are separated by faults. An exotic terrane is one that has been transported into its present setting from some distance.

Tertiary Period

The earliest Period of the Cenozoic Era, beginning about 66.4 million years ago and ending 1.6 million years ago.
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thermal aureole

Zone of rock around an igneous intrusion that has been altered or metamorphosed by heat from the hot magma. The rock in the zone is baked.

till

Unsorted, unstratified rock rubble or debris carried on and/or deposited by the ice of a glacier

thrust fault

See fault.

thrust plate

Slab of rock, generally on the scale of a mountain or more, bounded by two thrust faults.

tonalite

Intrusive igneous rock made of plagioclase feldspar, quartz, and amphibole or biotite. May be similar to diorite but contains considerable quartz and is not as dark, and chemically has less calcium, iron and magnesium.

topography

The shape of the land surface. See relief.

trench

Trenches are deep, linear zones that form where an oceanic plate sinks (subducts) beneath another plate.
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tuff

Volcanic rock made up of rock and mineral fragments in a volcanic ash matrix. Tuffs commonly are composed of much shattered volcanic rock glass--chilled magma blown into the air and then deposited. If volcanic particles fall to the ground at a very high temperature, they may fuse together, forming a welded tuff.

 

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