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Geologic Glossary

This glossary contains simplified definitions for technical terms used within this site.
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C

calcareous

A descriptive term used for rocks and other earth materials that have an abundance of calcium carbonate (CaCO3). For example, a calcareous sandstone has up to 50% calcium carbonate.

calcic horizon

A soil layer at least 15 cm thick that has been enriched with calcium carbonate (CaCO3).

caliche

A hard, white soil horizon, rich in calcium carbonate, that commonly forms in arid and semi-arid areas.

calcite

Mineral made of calcium carbonate (CaCO3). Generally white, easily scratched with knife. Most seashells are made of calcite or related minerals. This is the lime of limestone.
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caldera

Large, generally circular, fault-bounded depression caused by the withdrawal of magma from below a volcano or volcanoes. Commonly, the magma erupts explosively as from a giant volcano and, falling back to Earth as volcanic ash, fills the caldera so formed.

carbonate

A sedimentary rock made mainly of calcium carbonate (CaCO3). Limestone and dolomite are common carbonate sedimentary rocks.

carbonic acid

A mild acid formed when water and carbon dioxide chemically combine in the atmosphere and soil.This acid is a very important component in the development of cave decorations (speleothems).

cave

A natural opening in the ground extending beyond the zone of light and large enough to permit the entry of an average human.
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cave system

A cave or caves having a complex network of interconnected chambers and passages that constitute an underground drainage system.
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cavernous weathering

A combination of chemical and mechanical weathering processes act on rock surfaces to produce hollows and caverns. This is also called honeycomb weathering.

cementation

One of the processes that work together to turn sediment into sedimentary rock (lithification). Mineral-laden water percolates through sediment with open pore spaces. The spaces are gradually filled by minerals precipitating from the water, binding the grains together.

Cenozoic Era

The time span between 66.4 million years ago to the present.
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chemical sedimentary rock

Sedimentary rock composed of minerals that were precipitated from water. This process begins when water traveling through rock dissolves some of the minerals, carrying them away from their source. Eventually these minerals are redeposited, or precipitated, when the water evaporates away or when the water becomes over-saturated.

chemical weathering

The process that changes the chemical makeup of a rock or mineral at or near the Earth's surface. Chemical weathering alters the internal structure of minerals by the removing and/or adding elements. Compare with mechanical weathering.
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chert

A very fine-grained sedimentary rock made of quartz. Usually made of millions of globular siliceous skeletons of tiny marine plankton called radiolarians. Black chert is called flint.

chlorite

Family of platy silicate minerals containing various amounts of magnesium, iron, aluminum, water, and small amounts of other elements. Some mineralogists include chorites in the mica family because the crystals form small flakes. Commonly green.

cinder

A bubbly (vesicular) volcanic rock fragment that forms when molten, gas-filled lava is thrown into the air, then solidifies as it falls.
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cinder cone

A volcanic cone built almost entirely of loose volcanic fragments, ash, and pumice (pyroclastics or tephra)
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clast

A fragment of a pre-existing rock or fossil embedded within another rock.
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clastic

A sedimentary rock composed of fragments (clasts) of pre-existing rock or fossils. (=Detrital sedimentary rocks)

clay

A family of platy silicate minerals that commonly form as a product of rock weathering. Also, any particle smaller than 1/256 of a millimeter in diameter.
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cleavage

The tendency of a mineral to break along weak planes.

cobble

Loose particles of rock or mineral (sediment) that range in size from 64 - 256 millimeters in diameter. Cobbles are a size of gravel larger than pebbles, but smaller than boulders.
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compaction

Occurs when the weight of overlying material compresses more deeply buried sediment. Along with cementation, this process converts sediments to solid rock.

composite volcano

See stratovolcano.

conformable

Rock layers that were deposited in sequence without episodes of erosion between deposition of layers. .

conglomerate

A sedimentary rock rock made of rounded rock fragments, such as pebbles, cobbles, and boulders, in a finer-grained matrix. To call the rock a conglomerate, some of the consituent pebbles must be at least 2 mm (about 1/13th of an inch) across.
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contact metamorphism

Metamorphism caused by heat from an igneous intrusion.

continental collision

Convergence of two continental plates. Such a convergence between the Indian and Eurasian plates is responsible for producing the Himalayas.
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continental crust

The rigid, outer layer of relatively low density rock that makes up the continents.

continental drift

A hypothesis proposed by Alfred Wegener suggesting that the continents are not stationary, but have 'drifted' through time. Plate tectonics is the name for the theory that provided the evidence necessary to support Wegener's hypothesis.
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convection cell

The theorized movement of the asthenosphere. Heated material from close to the earth's core becomes less dense and rises toward the solid lithosphere. At the lithosphere-asthenosphere boundary heated asthenosphere material begins to move horizontally until it cools and eventually sinks down lower into the mantle, where it is heated and rises up again, repeating the cycle.

Cordilleran ice sheet

Ice cap that grew in western North America during the Pleistocene Epoch. It began growing first in Canada, eventually covering much of British Columbia, Alaska, the northern U.S., and parts of several western states.

convergent plate boundary

A boundary in which two plates collide. The collision can be between two continents (continental collision), an relatively dense oceanic plate and a more buoyant continental plate (subduction zone) or two oceanic plates (subduction zone).
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core

The innermost layer of the Earth, made up of mostly of iron and nickel. The core is divided into a liquid outer core and a solid inner core. The core is the most dense of the Earth's layers.
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crater

The depression produced by a meteorite impact or at the summit of a volcano.

craton

The relatively stable nucleus of a continent. Cratons are made up of a shield-like core of Precambrian Rock and a buried extension of the shield.
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crossbedding

A rock or sediment structure formed by currents of wind or water. It is characterized by relatively thin layers of sediment that are inclined at an angle to the dominant bedding.
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cross cutting

A principle of relative dating. Simply stated: a rock or fault is younger than any rock (or fault) through which it cuts.

crust

The rocky, relatively low density, outermost layer of the Earth.
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crystallization

Growth of minerals (crystalline solids) from a liquid or gas.


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