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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

D

data base

A set of words, numbers, locations, or other data put into a computer program. Data bases are set up so that related pieces of information can be easily retrieved and compiled.

daughter product

An isotope produced by decay of a radioactive element.
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debris flow

A type of landslide made up of a mixture of water-saturated rock debris and soil with a consistency similar to wet cement. Debris flows move rapidly downslope under the influence of gravity. Sometimes referred to as earth flows or mud flows.

deflation

Removal of loose material by wind.

deformation

General term for folding, faulting, and other processes resulting from shear, compression, and extension of rocks.

delta

A fan-shaped deposit that forms where a stream enters a lake or ocean and drops its load of sediment.

density

The weight per unit volume of a material.

desert

A region with an average annual rainfall of 10 inches or less.

desert varnish

A gray or reddish-brown layer on rock surfaces that darkens and thickens with increasing age. Desert varnish results from microbial life processes that precipitate thin skins of clays and manganese oxide minerals. The thicknesses can be used to determine the amount of time the surface of a rock has been exposed.
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deposit

Any accumulation of sediment.

desert pavement

A closely-packed surface layer of coarse pebbles and gravel.

desiccate

To dry out, usually by evaporation of water.

diagenesis

A group of processes that cause physical and chemical changes in sediment after it has been deposited and buried under another layer of sediment. Diagenesis may culminate in lithification of sediment, turning it into solid rock.

diapir

Forceful, upward intrusion of a rock mass into overlying rock. In the case of an igneous diapir, the intruding rock may be magma or a crystal-rich mush, either of which is less dense than the surrounding rock.

dike

A sheet-like or tabular-shaped igneous intrusion that cuts across the sedimentary layering, metamorphic foliation, or other texture of a pre-existing rock.
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diorite

Intrusive igneous rock made of plagioclase feldspar and amphibole and/or pyroxene. Similar to gabbro only not as so dark, and containing less iron and magnesium.

dip

A measure of the angle between the flat horizon and the slope of a sedimentary layer, fault plane, metamorphic foliation, or other geologic structure.

disappearing stream

In karst areas, streams often disappear into the ground usually at a sinkhole.

discharge

The amount of water issuing from a spring or in a stream that passes a specific point in a given period of time.

dissolution

The process of chemical weathering of bedrock in which the combination of water and acid slowly removes mineral compounds from solid bedrock and carries them away in liquid solution. Also called chemical solution.

divergent plate boundary

A boundary in which two tectonic plates move apart.
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drainage

Any channel that carries water.

drainage basin

The land area drained by a stream.

doline

See sinkhole

dolomite

A magnesium-rich carbonate sedimentary rock. Also, a magnesium-rich carbonate mineral (CaMgCO3)

dune

A usually asymmetrical hill of wind-deposited sand.
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durable crust

An outer rind or crust formed on a rock. Durable crusts form when rock chemically reacts with water and possibly atmospheric dust, producing a hard outer surface that resists weathering.

 

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