Geologic Glossary

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- D -

Dacite lava is most often light gray, but can be dark gray to black. Dacite lava consists of about 63 to 68 percent silica (SiO2). Common minerals include plagioclase feldspar, pyroxene, and amphibole. Dacite generally erupts at temperatures between 800 and 1000° C. It is one of the most common rock types associated with enormous Plinian-style eruptions. When relatively gas-poor dacite erupts onto a volcano's surface, it typically forms thick rounded lava flow in the shape of a dome.

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.

debris avalanche
A rapid and unusually sudden sliding or flowage of unsorted masses of rock and other material. As applied to the major avalanche involved in the eruption of Mount St. Helens (1980), a rapid mass movement that included fragmented cold and hot volcanic rock, water, snow, glacier ice, trees, and some hot pyroclastic material. Most of the May 18 deposits in the upper valley of the North Fork Toutle River and in the vicinity of Spirit Lake are from the debris avalanche.

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 a lahar (originating at a volcano), or as earth flows or mud flows.

Removal of loose material by wind.

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

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

The weight per unit volume of a material.

density current
A gravity-induced flow of one current through, over, or under another fluid media, owing to density differences. Factors affecting density differences include temperature, salinity, and concentration of suspended particles.

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

Any accumulation of sediment.

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

To dry out, usually by evaporation of water.

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.

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.

A general term for a volcanic vent or pipe drilled through enclosing rocks (usually flat-lying sedimentary rocks) by the explosive energy of gas-charged magmas. The diamond-bearing kimberlite pipes of South Africa are diatremes.

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

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.

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.

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

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.

directed blast
A hot, low-density mixture of rock debris, ash, and gases that moves at high speed along the ground surface. Directed blasts are generated by explosions.

divergent plate boundary
A boundary in which two tectonic plates move apart.

A steep-sided mass of viscous (doughy) lava extruded from a volcanic vent, often circular in plan view and spiny, rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome. Domes are formed by andesite, dacite, and rhyolite lavas.

dormant volcano
A volcano that is not presently erupting but that is considered likely to erupt in the future.

Any channel that carries water.

drainage basin
The land area drained by a stream.

See sinkhole

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

A usually asymmetrical hill of wind-deposited sand.

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.

- E -

A sudden ground motion or vibration of the Earth. Produced by a rapid release of stored-up energy along an active fault.
Material that is thrown out by a volcano, including pyroclastic material (tephra) and, from some volcanoes, lava bombs.
An epoch of the Tertiary period, spanning the time between 54.8 and 33.7 million years ago.

Term describing the process of wind erosion, transport, and deposition, and wind-created deposits and structures such as sand dunes.

The largest time unit on the geologic time scale.

ephemeral stream
A stream drainage that is usually dry and fills with water only during brief episodes of rainfall. Many desert streams ephemeral.

The point on the Earth's surface located directly above the focus of an earthquake.

Family of silicate minerals containing mostly calcium, aluminum, iron and magnesium along with water. Epidote is apple green and generally forms very small, stubby, prismatic crystals. It often occurs in veins or as a green coating on fracture surfaces. Most common in metamorphic rocks, but occasionally forms in igneous plutons that crystallize very deep in the crust.

Removal of material by water, wind, or ice.

Occurs when solid, liquid, or gaseous volcanic materials are ejected into the Earth's atmosphere or surface by volcanic activity. Eruptions may occur as quiet lava flows or violent explosive events.

In geology, the process of stretching the Earth's crust. Usually cracks (faults) form, and some blocks sink, forming sedimentary basins.

extinct volcano
A volcano that is not expected to erupt again.

Igneous rocks that cool and solidify rapidly at or very near the Earth's surface. Also known as volcanic rocks.

- F -

A fan-shaped sedimentary deposit that forms where rapidly flowing water enters a relatively open, flat area. As water slows down, it deposits sediment and gradually builds a fan. See alluvial fan.

A fracture in the Earth along which one side has moved in relative to the other. Sudden movements on faults cause earthquakes.

fault scarp
A steep slope or cliff formed when movement along a fault exposes the fault surface.

Family of silicate minerals containing varying amounts of potassium, sodium and calcium along with aluminum, silicon and oxygen. Potassium feldspars contain considerable potassium. Plagioclase feldspars contain considerable sodium and calcium. Feldspar crystals are stubby prisms, generally white, gray, or pink.

A term used to describe light-colored igneous rocks with an abundance of light-colored minerals, especially feldspars and quartz.

Partially compacted snow that survives the summer melting season.

firn limit
A term used by glaciologists (scientists who study glaciers) for the boundary where the amount of snow loss from melting and evaporation equals the amount of snow accumulation from snowfall (also called the annual snowline).

fission tracks
Microscopic tunnels made in crystals by escaping nuclear particles emitted by radioactive elements. Most commonly studied are fission tracks in zircon crystals made by the radioactive decay of uranium, present as an impurity.

Elongate, narrow fractures.

Term used to describe sedimentary or metamorphic rocks that tend to split into layers that are 1-10 cm thick.

A lake, stream, or other body of water that flows over its natural confining boundaries. During a flood, water flows out over land not normally covered with water.

flood plain
A relatively flat surface next to a stream. During floods, when the stream overflows its banks, water flows over the flood plain. Streams construct flood plains that accommodate their maximum flood capacity.

A general term for a type of cave decoration or speleothem that encrusts floors or walls of caves.

Term used to describe river or stream-related features or processes. Fluvial deposits are sediments deposited by the flowing water of a stream.

The location where an earthquake begins. Rock ruptures at this spot, then seismic waves radiate outward in all directions.

Aligned layers of minerals characteristic of some metamorphic rocks. Foliation forms in metamorphic rocks when pressure squeezes flat or elongates minerals so that they become aligned. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied.

A rock formation is a body of rock of considerable extent with distinctive characteristics that allow geologists to map, describe, and name it.

Mineralized remains or traces of organisms.

fossil fuel
General term for any hydrocarbon used as fuel, including coal, oil, natural gas, and oil shale.

Any break in rock along which no significant movement has occurred.

freeze-thaw cycle
In colder temperate regions, water trapped in fractures and between grains of rocks repeatedly freezes, then thaws during the winter months. In some areas this occurs on a daily basis as water freezes at night, then melts in warmer daytime temperatures.

Only in the coldest regions does water remain frozen throughout the winter.

frost wedging
A process that mechanically breaks apart rock caused by expansion of water as it freezes in cracks and crevices.

An opening at the Earth's surface that emits water vapor, hydrogen sulfide, and other gases, often at high temperature.

fumarole activity
Volcanic gas emissions, that may be accompanied by a change in the temperature of the gases or fluids emitted.

- G -

A dark, coarse-grained intrusive igneous rock. Gabbro is made of calcium-rich plagioclase, with amphibole and/or pyroxene, and is chemically equivalent to basalt.

Family of silicate minerals containing varying amounts of aluminum, iron, magnesium, and calcium. Schist and gneiss often have tiny, glassy red garnet dodecahedrons.

A branch of geology that focuses on the chemical composition of Earth materials.

geothermal energy
Power generation using natural steam derived from the Earth's internal heat.

A branch of geology and geography that studies the development of landforms.

glass (volcanic)
Natural glass (obsidian) that forms when molten lava cools too rapidly to permit crystal growth.

A long-lived sheet or mass of ice made of recrystallized snow. Glaciers move downhill due to the stress of their own weight.

glacier outburst flood
A sudden release of melt water from a glacier or glacier-dammed lake sometimes resulting in a catastrophic flood, formed by melting of a channel or by subglacial volcanic activity.

A coarse-grained, foliated metamorphic rock that commonly has alternating bands of light and dark-colored minerals.

A continent formed in the Southern Hemisphere during the Late Paleozoic. It included most of South America, Africa, India, Austrailia, and Antarctica.

An elongate block of rock down-dropped along roughly parallel faults.

graded bed
A sediment layer with a gradation of grain size from large grains to finer grains.

grain size
Refers to the size of individual mineral crystals or particles within a rock or sediment deposit.

A coarse-grained intrusive igneous rock with at least 65% silica. Quartz, plagioclase feldspar and potassium feldspar make up most of the rock and give it a fairly light color. Granite has more potassium feldspar than plagioclase feldspar. Usually with biotite, but also may have hornblende.

A general term for intrusive igneous rocks that look similar to granite but may range in composition from quartz-diorite to granite. All granitic rocks are light colored; feldspar and quartz are visible in hand specimen.

An intrusive igneous rock similar to granite, but contains more plagioclase than potassium feldspar.

All sedimentary particles larger than two millimeters is called gravel. Gravel is subdivided into pebbles, cobbles, and boulders.

A metamorphic rock derived from basalt or chemically equivalent rock such as gabbro. Greenstones contain sodium-rich plagioclase feldspar, chlorite, and epidote, as well as quartz. The chlorite and epidote make greenstones green.

ground water
Water found beneath the Earth's surface where all empty space in the rock is completely filled with water.

Coarse sand and gravel that forms from weathering of granitic rocks

A very small channel formed by running water. Gullies hold water for brief periods of time after a rain storm or snow melt.

- H -

The time required for one-half of the atoms of a radioactive substance to decay.

A measure of a mineral's resistance to scratching. The hardness of a mineral is measured by scratching it against another substance of known hardness.

harmonic tremor
Continuous rhythmic earthquakes in the Earth's upper lithosphere that can be detected by seismographs. Harmonic tremors often precede or accompany volcanic eruptions.

hawaiian eruption
Hawaiian" eruptions may occur along fissures or fractures that serve as linear vents, such as during the eruption of Mauna Loa Volcano in Hawaii in 1950, or they may occur at a central vent such as during the 1959 eruption in Kilauea Iki Crater of Kilauea Volcano, Hawaii. In fissure-type eruptions, molten, incandescent lava spurts from a fissure on the volcano's rift zone and feeds lava streams that flow downslope. In central-vent eruptions, a fountain of fiery lava spurts to a height of several hundred feet or more. Such lava may collect in old pit craters to form lava lakes, or form cones, or feed radiating flows.

Headlands are projections of land that stick out into a sea or lake.

An epoch of the Quaternary Period beginning 10,000 years ago and continuing today.

An elongate block of rock uplifted along roughly parallel faults.

See amphibole.

hornblende schist
A schist rich in hornblende. Generally with abundant plagioclase feldspar as well. Grades into amphibolite.

A dark, very fine-grained metamorphic rock produced by the recrystallization of a fine-grained rock by heat from a nearby igneous intrusion. From the German, meaning horn rock.

hot spot
An area of concentrated heat in the mantle that produces magma that rises to the Earth's surface to form volcanic islands. The volcanic activity of the Hawaiian Islands is one example. Hot spots generally persist for millions of years.

hummocky ground
A ground surface that has lots of small hills and swales; uneven ground.

The science that deals with water on and beneath the Earth surface.

Pertains to hot water or the action of heated water, often considered heated by magma or in association with magma.

thdrothermal alteration
Alteration of rocks or minerals by the reaction of hot water (and other fluids) with pre-existing rocks. The hot water is generally heated groundwater and dissolved minerals.

Literally, "with water". Refers to minerals or other materials which have water as a primary constituent.

- I -

igneous rock
Rock formed when molten rock (magma) that has cooled and solidified (crystallized). See intrusive (plutonic) and extrusive (volcanic) igneous rock.

A potassium-rich clay mineral

Rock or sediment that does not allow passage of water.

A term used to describe down-cutting (downward erosion) by a stream. Incision deepens and often steepens the stream channel.

inner core
The innermost layer of Earth. Consists of solid iron and nickel.

One way to measure the strength of an earthquake. Intensity measures of the effects of an earthquake on buildings and the reactions of people. Compare with magnitude.

internal drainage
An area in which surface water cannot reach the ocean. Any water that falls into an area with internal drainage as rain or snow does not escape out of it; not one of the streams that originate within these basins ever find an outlet to the ocean. (see drainage basin)

Emplacement of magma (molten rock) into preexisting rock. Dikes, sills, and batholiths are intrusions.

intrusive rock
Igneous rock that cools and solidifies beneath the Earth's surface. (= plutonic rock)

island arc
An arc-shaped chain of volcanic islands produced where an oceanic plate is sinking (subducting) beneath another.

Different forms of a single element that have the same number of protons but different numbers of neutrons in their nuclei. Some radioactive isotopes are unstable and shed nuclear particles over time until they become stable. For instance, unstable isotopes of uranium break down to become lead.

Continue to glossary entries J through N

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