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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 Z

S

sand
Loose particles of rock or mineral (sediment) that range in size from 0.0625 - 2.0 millimeters in diameter.
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sandstone
Sedimentary rock made mostly of sand-sized grains.

sanidine
A type of potassium feldspar that forms only at high temperature. Common in potassium-rich volcanic rocks.

scarp
A cliff formed by faulting, erosion, or landslides. (=escarpment)

schist
Metamorphic rock usually derived from fine-grained sedimentary rock such as shale. Individual minerals in schist have grown during metamorphism so that they are easily visible to the naked eye. Schists are named for their mineral constituents. For example, mica schist is conspicuously rich in mica such as biotite or muscovite.

scoria
Very bubbly (vesicular) basalt or andesite. Both scoria and pumice develop their bubbly textures when escaping gas is trapped as lava solidifies. Scoria is more dense and darker than pumice.

sea stack
Sea stacks are blocks of erosion-resistant rock isolated from the land by sea.
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sediment
Loose, uncemented pieces of rock or minerals.
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seismic
Refers to earthquakes.

sedimentary
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth’s surface. Sedimentary rocks often have distinctive layering or bedding.
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serpentine
A family of silicate minerals rich in magnesium and water, derived from low-temperature alteration or metamorphism of the minerals in ultramafic rocks. Rocks made up of serpentine minerals are called serpentinite. Serpentine minerals are light to dark green, commonly varied in hue, and greasy looking; the mineral feels slippery.

shale
Sedimentary rock derived from mud. Commonly finely laminated (bedded). Particles in shale are commonly clay minerals mixed with tiny grains of quartz eroded from pre-existing rocks. Shaley means like a shale or having some shale component, as in shaley sandstone.

sheetwash
Overland flow of water in thin sheets

shrink-swell
Refers to the property of many clays to swell when wetted and shrink when dried.

sillimanite
An aluminum-rich silicate found only in metamorphic rocks that form at high temperature and pressure.

silica
Silicon dioxide (SiO2). One of the most common compounds in the Earth’s crust. Common window glass is made of silica. The building block of the mineral quartz and other silicate minerals.

silicate
Refers to the chemical unit silicon tetroxide, SiO4, the fundamental building block of silicate minerals. Silicate minerals make up most rocks we see at the Earth’s surface.

siliceous
Generally refers to a rock rich in quartz.

sill
See dike.

silt
Loose particles of rock or mineral (sediment) that range in size from 0.002 - 0.0625 millimeters in diameter. Silt is finer than sand, but coarser than clay.
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siltstone
A sedimentary rock made mostly of silt-sized grains.

sinkhole
A depression in the surface commonly found in in karst landscapes. Sinkholes often form where limestone or some other soluble rock is partially dissolved by groundwater, then collapses to form a depression. Sinkholes are often "bowl-shaped" and can be a few to many hundreds of meters in diameter. Also known as dolines.

slump
A type of landslide in which a mass of rock breaks away along a curved surface and rotates more or less intact downslope. The sliding mass of rock is called a slump block.

smectite
Group of clays, those most susceptible to shrink-swell

soil
All loose, unconsolidated earth and organic materials above bedrock that support plant growth.
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speleology
The exploration and study of caves.
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speleothem
A deposit formed in caves when calcium carbonate (CaCO3) or some other mineral precipitates from drips or thin films of water. Stalactites and stalagmites are common speleothems.
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stalactite
A mineral deposit (speleothem) which hangs downwards from a roof or wall of a cave. See stalagmite.
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stalagmite
A mineral deposit (speleothem) which projects upwards from a cave floor. See stalactite.
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stitching pluton
Plutons of roughly the same age which that intruded several tectonic terranes after the terranes were faulted together. The plutons do not really "sew" the terranes together, but they help record when terranes were assembled.

stock
Relatively small globular or columnar-shaped pluton. Like a batholith only smaller.

stope block
Stope blocks form when injection of intrusive igneous rock weakens the solid rock surrounding it, causing blocks to loosen and sink into the molten mass.

stratovolcano
A relatively long-lived volcano built up of both lava flows and pyroclastic material.

stream capture
A process of erosion where one stream erodes headward, diverting some of another stream’s drainage into its own channel. Also called stream piracy.

stringer
A thin, discontinuous mineral vein or rock layer.

subduction
Process of one crustal plate sliding down and below another crustal plate as the two converge. The subduction zone is the area between the two plates, somewhat like a giant reverse fault.
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submarine fan
Fan or cone-shaped accumulation of sedimentary debris--sand, gravel, mud--under the ocean along the edge of the land, either a continent or a volcanic arc. Fans may be a few miles to a hundred or so miles across.

surficial deposit
Any loose, unconsolidated sedimentary deposit lying on bedrock.

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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 slope.

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
A long bench-like surface, often bordering a stream, lake, or sea.

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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U

ultramafic rock
An intrusive igneous rock very rich in iron and magnesium and with much less silicon and aluminum than most crustal rocks. Most come from the Earth’s mantle.

unconformity
The contact between older rocks and younger sedimentary rocks in which at least some erosion has removed some of the older rocks before deposition of the younger. An angular unconformity shows that the older rocks have been deformed and eroded before the younger sedimentary rocks were deposited; there is an angle between the beds of the older and the younger.

unconsolidated
Loose sediment; lacking cohesion or cement.

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V

vein
A mineral-filled fracture or fault in a rock.

vein
Tabular rock or mineral filling of a generally small crack such as a quartz vein. A product of chemical precipitation from a watery solution, in contrast to a dike crystallized from magma, although gradations exist.

veneer
A thin, widespread layer of sediment covering an older surface.

vesicle
A small bubble formed in volcanic rock during solidification.

vitreous
Glassy.

vitrophyre
A volcanic rock with larger crystals (phenocrysts) embedded in a glassy groundmass.

volcanic rock
Igneous rock that cools and solidifies at or very near the Earth’s surface. Volcanoes produce volcanic rock.

volcanic arc
Arcuate chain of volcanoes formed above a subducting plate.

volcanic arc
Arcuate chain of volcanoes formed above a subducting plate. The arc forms where the downgoing descending plate becomes hot enough to release water and gases that rise into the overlying mantle and cause it to melt. Arc rocks are mostly volcanic rocks from the volcanoes and sedimentary rocks made up of eroded debris from the volcanoes. Melted rock in the deeper plumbing of the arc which may crystallizes at depth to become an arc root plutons.
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W

wash
A normally dry stream bed that ocassionally fills with water.

weathering
Weathering includes two surface or near-surface processes that work in concert to decompose rocks. Both processes occur in place. No movement is involved in weathering. Chemical weathering involves a chemical change in at least some of the minerals within a rock. Mechanical weathering involves physically breaking rocks into fragments without changing the chemical make-up of the minerals within it. Mechanical weathering includes processes such as water in cracks freezing and expanding, or changes in temperature that expand and shrink individual minerals enough to break them apart.
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well-sorted
Refers to a sedimentary deposit or rock with grains of the same approximate size.

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X

xenolith
A piece of foriegn rock enclosed within an igneous rock. The foriegn rock is usually picked up from the walls surrounding the igneous rock and is frozen in place before it has a chance to melt. (=inclusion)
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Y


Z

zircon
Mineral of zirconium, silicon, and oxygen (zirconium silicate). Generally glassy-looking, microscopic, four-sided prisms. Most commonly formed in igneous rocks.
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Links to other geologic glossaries.

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This page was last updated on 9/25/00