Geology Cafe

Geologic Glossary

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



Occurs when more glacier ice is lost by melting and evaporation each year than is added by snowfall.

absolute age

The approximate age of a geologic event, feature, fossil, or rock in years. 'Absolute' ages are determined by using natural radioactive 'clocks'. The preferred term is radiometric age.


A process that adds part of one tectonic plate to a larger plate along a convergent (collisional) plate boundary.

accretionary wedge

A large wedge-shaped mass of sediment and oceanic lithosphere. An accretionary wedge forms along a convergent plate boundary when material is scraped from a sinking, subducting plate and is added to (accreted) another larger plate along a convergent plate boundary.


A bright to gray-green member of the amphibole mineral family. In addition to silica, it contains calcium, magnesium, and iron. Actinolite is a non-hazardous relative of asbestos and is a common mineral in metamorphic rocks.

active volcano

A volcano that has erupted within historical time and is likely to do so again in the future.

A horizon

The top layer of soil. Plant and other organic debris builds up in this layer. This is the part of the soil generally referred to as 'top soil'.

alluvial fan

A fan-shaped pile of sediment that forms where a rapidly flowing mountain stream enters a relatively flat valley. As water slows down, it deposits sediment (alluvium) that gradually builds a fan.
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Sand, gravel, and silt deposited by rivers and streams in a valley bottom.


A family of silicate minerals forming prism or needlelike crystals. Amphibole minerals generally contain iron, magnesium, calcium and aluminum in varying amounts, along with water. Hornblende always has aluminum and is a most common dark green to black variety of amphibole; it, forms forming in many igneous and metamorphic rocks. Actinolite has no aluminum; it and is needle-shaped and light green. Blue amphibole contains sodium and, of course, is bluish in color.
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A rock made up mostly amphibole and plagioclase feldspar . Although the name amphibolite usually refers to a type of metamorphic rock, an igneous rock composed dominantly of amphibole can be called an amphibolite too.


Fine-grained, generally dark colored, igneous volcanic rock with more silica than basalt . Commonly with visible crystals of plagioclase feldspar. Generally occurs in lava flows, but also as dikes. The most common rock in volcanic arcs.


Literally, "without water". Refers to minerals or other materials which do not have water as an primary constituent.


A downward-curving (convex) fold in rock that resembles an arch. The central part, being the most exposed to erosion, display the oldest section of rock. See syncline .

annual snowline

A term used by glaciologists (scientists who study glaciers) for the boundary where the amount of snow loss from melting equals the amount of snow accumulation from snowfall .
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An igneous rock texture in which individual mineral grains are too small to be distinguished with the naked eye.


A light-colored igneous rock with the same mineral composition as granite: quartz, plagioclase feldspar, and potassium feldspar, but with a fine-grained, almost sugary texture.

Archean Eon

The time interval between 3800-2500 million years ago. The Archean is one of the Precambrian time intervals.
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The science that focuses on the study of past human cultures.

arc rocks

See Volcanic arc rocks.


A term used to describe clay -rich rocks.

argillic horizon

A clay -rich layer of soil. Clay often forms in overlying soil layers from the decomposition of feldspars and other minerals. The extremely fine clay particles are gradually carried down by water to accumulate into the argillic horizon.


name used for unusually hard, fine-grained sedimentary rocks, such as shale , mudstone , siltstone , and claystone . Commonly black.


A region without earthquakes (seismic activity).


Fine particles of volcanic rock and glass blown into the atmosphere by a volcanic eruption.
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The uppermost layer of the mantle , located below the lithosphere. This zone of soft, easily deformed rock exists at depths of 100 kilometers to as deep as 700 kilometers.
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Augen are relatively large, eye-shaped mineral grains in certain types of metamorphic rocks , especially schist and gneiss . (Augen means eyes in German)


Masses of rock or ice that fall or slide suddenly under the force of gravity.


banded gneiss

See gneiss.


A dark, fine-grained, extrusive (volcanic) igneous rock with a low silica content (40% to 50%), but rich in iron, magnesium and calcium. Generally occurs in lava flows, but also as dikes . Basalt makes up most of the ocean floor and is the most abundant volcanic rock in the Earth's crust.
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base level

The level (elevation) at which a stream or river can erode no more, usually sea level.


A depression in the Earth's surface that collects sediment .

Basin and Range province

This province extends from eastern California to central Utah, and from southern Idaho into the state of Sonora in Mexico. Within the Basin and Range province the Earth's crust and uppermost mantle have been stretched, creating large faults . Along these faults linear mountain ranges were uplifted and flat valleys down-dropped, producing the distinctive topography of the Basin and Range province.
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Very large mass of intrusive (plutonic) igneous rock that forms when magma solidifies at depth. A batholith must have greater than 100 square kilometers (40 square miles) of exposed area. See pluton , stock .


A layer of sediment or sedimentary rock.


Parallel layers of sediment or sedimentary rock (beds) that can be distinguished from each other by characteristics such as grain size and chemical composition.


Sedimentary layers in a rock. The beds are distinguished from each other by grain size and composition, such as in shale and sandstone . Subtle changes, such as beds richer in iron-oxide, help distinguish bedding. Most beds are deposited essentially horizontally.


The solid rock that lies beneath soil and other loose surface materials.


In North America, 1,000,000,000
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A common rock-forming mineral of the mica family. Biotite is a black or dark brown silicate rich in iron, magnesium, potassium, aluminum, and, of course, silica. Like other micas, it forms flat book-like crystals that peal apart into individual sheets on cleavage planes.


Metamorphic rock rich in blue amphibole .

borrow pit

A pit or excavation area used for gathering earth materials ( borrow ) such as sand or gravel.


Any loose rock (sediment) larger than 256 millimeters (10 inches).
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Rock made up of angular fragments of other rocks held together by mineral cement or a fine-grained matrix. Volcanic breccia is made of volcanic rock fragments, generally blown from a volcano or eroded from it. Fault breccia is made by breaking and grinding rocks along a fault .



A descriptive term used for rocks and other earth materials that have an abundance of calcium carbonate (CaCO 3 ). 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 (CaCO 3 ).


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


Mineral made of calcium carbonate (CaCO 3 ). 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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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.


A sedimentary rock made mainly of calcium carbonate (CaCO 3 ). 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 ).


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.


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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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.


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.


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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A fragment of a pre-existing rock or fossil embedded within another rock.
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A sedimentary rock composed of fragments (clasts) of pre-existing rock or fossils. (=Detrital sedimentary rocks)


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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The tendency of a mineral to break along weak planes.


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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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 .


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


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


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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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.


The rocky, relatively low density, outermost layer of the Earth.
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Growth of minerals (crystalline solids) from a liquid or gas.


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.


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.


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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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 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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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.

divergent plate boundary

A boundary in which two tectonic plates move apart.
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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 (CaMgCO 3 )


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 .



A sudden ground motion or vibration of the Earth. Produced by a rapid release of stored-up energy along an active fault .
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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.
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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.
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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 pluton s that crystallize very deep in the crust.


Removal of material by water, wind, or ice.
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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 .


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



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 .
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fault scarp

A steep slope or cliff formed when movement along a fault exposes the fault surface.
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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.
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A term used to describe light-colored igneous rocks with an abundance of light-colored minerals , especially feldspars and quartz .
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Partially compacted snow that survives the summer melting season.
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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).
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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. more details...


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 cave s.
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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.
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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.
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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.


A volcanic vent that emits hydrogen sulfide or other gases.



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.
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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.
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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 .
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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.
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An intrusive igneous rock similar to granite, but contains more plagioclase than potassium feldspar.
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All sedimentary particles larger than two millimeters is called gravel. Gravel is subdivided into pebbles, cobbles, and boulders.
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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
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A very small channel formed by running water. Gullies hold water for brief periods of time after a rain storm or snow melt.



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


Headlands are projections of land that stick out into a sea or lake.
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An epoch of the Quaternary Period beginning 10,000 years ago and continuing today.
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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.


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


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


igneous rock

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

The innermost layer of Earth. Consists of solid iron and nickel.
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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 .
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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)
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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.



A narrow crack in rock along which there has been no significant movement of either side. Joints commonly form in parallel sets.



A distinctive landscape (topography) that can develop where the underlying bedrock, often limestone or marble , is partially dissoved by surface or ground water .


An aluminum-rich, blue to light green silicate mineral. Kyanite forms in metamorphic rocks at moderate temperature and high pressure.



A type of mudflow that originates on the slopes of volcanoes when volcanic ash and debris becomes saturated with water and flows rapidly downslope.


Very thin layers of less than 1 cm thickness.


Downslope movement of rock, soil, and mud.


Present and historical uses of land, such as for agriculture, mining, recreation and grazing.


Magma that reaches the Earth's surface through a volcanic eruption. When cooled and solidified, forms extrusive (volcanic) igneous rock .


A metamorphic mineral that forms only under very high pressure. It is a calcium aluminum silicate and usually forms microscopic crystals.


A dating method that uses the growth rate of certain lichen species as an indicator of the age of the surface the lichen is growing on.


A sedimentary rock made mostly of the mineral calcite (calcium carbonate). Limestone is usually formed from shells of once-living organisms or other organic processes, but may also form by inorganic precipitation.


A mineral composed of iron oxides and water. Rust. Very common in many rocks after weathering at the Earth's surface. Imparts brown or yellow colors to many rocks.


A linear (relatively straight) topographic feature or features such as a fault, line of dense vegetation, or a chain of aligned volcanoes.


Parallel arrangement of elongate minerals or groups of minerals. To envision lineation, imagine packages of spaghetti or pencils.


"Made of stone". Refers to pieces of rock within other rocks such as tuff and conglomerate.


The conversion of loose sediment into solid sedimentary rock. Several processes, including compaction of grains, filling of spaces between grains with mineral cement , and crystallization act to solidify sediment.


The outer layer of solid rock that includes the crust and uppermost mantle . This layer, up to 100 kilometers (60 miles) thick, forms the Earth's tectonic plates. Tectonic plates float above the more dense, flowing layer of mantle called the asthenosphere .
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A wind-blown deposit of sediment made mostly of silt-sized grains.


The appearance of the reflection of light from the surface of a mineral. Luster is described as metallic, glassy, dull, etc.



A term used to describe minerals or igneous rocks that are rich in iron and/or magnesium. Mafic igneous rocks have a high percentage of dark-colored (mafic) minerals.


Molten rock. Magma may be completely liquid or a mixture of liquid rock, dissolved gases and crystals. Molten rock that flows out onto the Earth's surface is called lava .

magma chamber

A body of molten rock and solid crystal mush beneath the Earth's surface. When this chamber cools and solidifies, it is called a pluton .

magnetic anomalies

See magnetic reversals .

magnetic reversals

Earth's magnetic field occassionally "flips"or reverses polarity. This means that, if a polarity reversal happened today, your compass would point south instead of north!


Iron oxide mineral (Fe 3 O 4 ). Usually tiny black, metallic crystals. Magnetite will attract a magnet and sometimes, in a rock, a hiker's compass needle.


A measure of the total amount of energy released by an earthquake .
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Device for measuring magnetism.


The layer of the Earth below the crust and above the core . The uppermost part of the mantle is rigid and, along with the crust, forms the 'plates' of plate tectonics. The mantle is made up of dense, iron and magnesium rich ( ultramafic ) rock such as dunite and peridotite.
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A metamorphic rock of made of calcium carbonate. Marble forms from limestone by metamorphic recrystallization.

mass wasting

Movement of rock and soil downslope under the influence of gravity.


Fine-grained material surrounding larger grains in a sedimentary rock.
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mechanical weathering

The mechanical break-up or disintegration of rock into smaller fragments.
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Mixture of rocks formed by tectonic disruption, such as multiple faulting , which brings disparate rock types together. Usually consists of a matrix of weak material, like shale , with hard pieces of exotic rocks, such as gneiss or igneous rocks.


Metamorphosed conglomerate .

metamorphic rock

A rock that has undergone chemical or structural changes produced by increase in heat or pressure, or by replacement of elements by hot, chemically active fluids.


Group of silicate minerals composed of varying amounts of aluminum, potassium, magnesium, iron and water. All micas form flat, plate-like crystals. Crystals cleave into smooth flakes. Biotite is dark, black or brown mica; muscovite is light-colored or clear mica.
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A general term for mica -rich rocks.


In geology, a mound of organic debris or organic-rich soil created by an animal. In archeology, a mound of human refuse.


"Mixed rock". A metamorphic rock that forms in one of two ways. The metamorphic rock may be heated enough to partially melt, but not completely. The molten minerals resolidify within the metamorphic rock, producing a rock that incorporates both metamorphic and igneous features. Migmatites can also form when metamorphic rock experiences multiple injections of igneous rock that solidify to form a network of cross-cutting dikes .

Migmatites are usually conspicuously mixed light- and dark-colored rocks, generally formed of dark-colored gneiss and/or schist and lighter igneous intrusive dikes and sills intruded during deformation; they look to have been mushed around while still hot and plastic, and display prominent folding.


A naturally occurring chemical compound or limited mixture of chemical compounds. Minerals generally form crystals and have specific physical and chemical properties which can be used to identify them.
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The formation of minerals. New minerals may be added to fractures and empty spaces in a rock or by replacing preexisting minerals with different ones.


The study of minerals.


An Epoch that includes the time interval of about 23.7 to 5.3 million years ago.
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The boundary separating the base of the Earth's crust and the top of the mantle . The Moho occurs at a depth of 5-10 kilometers beneath oceanic crust and about 35-65 kilometers below continental crust . The term moho is an abbreviation for Mohorovicic discontinuity, named for Andrija Mohorovicic, a Croatian seismologist.


A hill-like pile of rock rubble located on or deposited by a glacier. An end moraine forms at the terminus of a glacier. A terminal moraine is an end moraine at the farthest advance of the glacier. A lateral moraine forms along the sides of a glacier. See till.


The study of shape or form. See geomorphology .


Wet clay and silt-rich sediment.


Same as debris flow .


A very fine-grained sedimentary rock formed from mud.


One of the mica family of minerals. Muscovite is light-colored or clear mica, sometimes called isingglass.
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An type of unconformity in which young sedimentary rocks lie on top of older metamorphic or intrusive igneous rocks.

nonsilicate minerals

A mineral without silicon (Si). See silicate minerals .

normal fault

A fault that drops rock on one side of the fault down relative to the other side.

nuee ardente

This is a type of volcanic activity whose name is french, meaning firey cloud. Associated with pyroclastic flows, they are very hot, sometimes glowing clouds of gas and volcanic debris that is forcefully ejected down the sides of a volcano.

normal polarity

A magnetic field produced by the Earth that is the same as exists today.



Dark-colored volcanic glass. Usually has the same chemical composition as the extrusive igneous rock, rhyolite.

oceanic crust

The relatively thin, dense crust that forms the ocean basins.

oceanic rocks

Rocks formed in the deep ocean. Includes sedimentary rocks deposited on the deep ocean floor as well as the basalt of the oceanic crust. Commonly include some slices of the underlying mantle ( ultramafic rocks ) as well.


Silicate mineral containing iron and magnesium. A green glassy mineral formed at high temperature. Common in basalt , especially ocean-floor basalt, and in ultramafic rocks. Gem-quality olivine is called peridote . Rock made up entirely of olivine is called dunite .


A Period in the Paleozoic Era that includes the time interval from about 505 to 438 million years ago.
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A mineral deposit that can be mined at a profit.


An episode of mountain building and/or intense rock deformation.


Gneiss formed by squeezing (deformation and usually some recrystallization) of a granitic igneous plutonic rock .


A mass of rock that appears at the Earth surface.

outer core

The liquid outer layer of the core that lies directly beneath the mantle .
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Glacial outwash is the deposit of sand, silt, and gravel formed below a glacier by meltwater streams and rivers. An outwash plain is an extensive, relatively flat area of such deposits.

overbank deposits

Silt and clay deposited on a flood plain by a flooding stream.


Removal of electrons from an atom or ion. Usually by combining with oxygen ions. Minerals exposed to air may oxidize as a form of chemical weathering .



A lava flow with a smooth, ropy surface.


The science that studies the past distribution of plants and animals.


The study of how global climate has changed through time.


Natural magnetism is acquired by some rocks, especially igneous rocks that are rich in iron, as they solidify. As the rocks cool, iron-bearing minerals are 'frozen' into position. The orientation of iron-bearing magnetic minerals record the location of the Earth's magnetic poles and the latitude of the rock at the time of cooling.


The study of ancient seismic ( earthquake ) events.

Paleozoic Era

Includes the time from about 570-245 million years ago.
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The study of pollen, living and fossil.


The supercontinent which formed at the end of the Paleozoic Era and began breaking up about 200 million years ago to form today's continents.
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parent isotope

A radioactive isotope that changes to a different, daughter isotope when its nucleus decays.

parent rock

The preexisting rock from which a metamorphic rock forms.

passive margin

A tectonically inactive continental margin characterized by a lack of earthquakes and volcanic activity.


A bare rock surface that provides a protective rock cover over the material beneath it.


A sloping bedrock surface at the base of a mountain, formed when erosion removes much of the mountain's mass.


Loose particles of rock or mineral ( sediment ) that range in size from 2 - 64 millimeters in diameter. Pebbles are the smallest type of gravel.
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A very coarse-grained igneous rock , commonly with a granitic composition. Usually forms from molten rock rich in water or other volatiles that facilitate the growth of large crystals. Forms sills and dikes .


A fine-grained sedimentary rock consisting mostly of clay and/or silt . Mudstone, shale, siltstone, and claystone are all pelitic.

perennial stream

A stream that runs continuously throughout the year.


The ability of a rock or other material to allow water to flow through its interconnected spaces. Permeable bedrock makes a good aquifer, a rock layer that yields water to wells. See porosity . (3 MB porosity animation available)


The study of rocks.


Lover of rocks. May be a petrologist, a stone mason, a rock climber.


An igneous rock texture in which the mineral grains are large enough to be seen with the unaided eye and are of approximately equal size.

Phanerozoic Eon

The eon beginning about 570 million years ago and continuing to the present. The portion of Earth history with rocks containing abundant fossils.
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A term used to describe large crystals embedded in a mass of finer crystals (groundmass) in an igneous rock . See ' porphyritic '.


A magnesium-rich member of the mica mineral family. Phlogopite is a yellowish-brown to coppery-colored mica. Like all micas, phlogopite forms flat, plate-like crystals that cleave into smooth flakes.


A very fine-grained, foliated metamorphic rock, generally derived from shale or fine-grained sandstone. Phyllites are usually black or dark gray; the foliation is commonly crinkled or wavy. Differs from less recrystallized slate by its sheen, which is produced by barely visible flakes of muscovite ( mica ).


A member of the feldspar mineral family. Plagioclase feldspars are silicates that contain considerable sodium and calcium. Feldspar crystals are stubby prisms, generally white to gray and a glassy luster .
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Generally tiny animals or plants that live floating in water.

plastic deformation

Permanent deformation (change in size or shape) of soft, but solid rock by folding or flowing without fracturing.


A slab of rigid lithosphere ( crust and uppermost mantle ) that moves over the asthenosphere .
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plate tectonics

The theory that the Earth's outer shell is made up of about a dozen lithospheric plates that move about and interact at their boundaries.
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Playas are shallow, short-lived lakes that form where water drains into basins with no outlet to the sea and quickly evaporates. Playas are common features in arid (desert) regions and are among the flattest landforms in the world.
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Pleistocene Epoch

The earliest Epoch of the Quaternary Period, beginning about 1.6 million years ago and ending 10,000 years ago. Commonly known as the ' Ice Age ', a time with episodes of widespread continental glaciation.
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The latest Epoch of the Tertiary Period, beginning about 5.3 million years ago and ending 1.6 million years ago.
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A large body of intrusive igneous rock that solidified within the crust. Batholiths and Stocks are types of plutons.

plutonic rock

Any igneous rock that cools beneath the surface. (=intrusive rock).

pluvial lake

A lake formed in a land-locked basin during a period of increased rainfall associated with glacial advance elsewhere.


The percentage of open spaces (pores) in rock or soil. When these spaces are interconnected, water, air, or other fluids can migrate from space to space. Interconnected spaces make the soil or bedrock permeable .


An igneous rock texture characterized by larger crystals (phenocrysts) in a matrix of distinctly finer crystals (groundmass).


Large mineral grains that grow during metamorphism .


An igneous rock , usually a dike or sill , with larger, generally conspicuous, early-formed crystals contained within a matrix of much smaller crystals.


The 'unofficial' time period that encompasses all time from the Earth's formation, 4.55 billion years ago to 570 million years ago, the beginning of the Paleozoic Era.
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precarious boulder

A large rock resting on another in an unstable position. Precarious boulders may remain in place for thousands of years until an earthquake or human-caused tremor dislodges them.

precipitate (verb)

The process that separates solids from a solution.

precipitate (noun)

Mineral precipitate . A mineral deposited from a water solution in pores or other openings in rocks. Chemical reaction with the surrounding rock, changes in pressure or temperature, or just drying up (evaporation) can cause a mineral to precipitate out of solution. Quartz veins are common products of mineral precipitation.

Proterozoic Eon

The 'Precambrian' time interval from 2.5 billion to 570 million years ago.
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A light-colored, frothy, glassy volcanic rock . The texture is formed by rapidly expanding gas in erupting lava.


Iron sulfide mineral (FeS). Forms silvery to brassy metallic cubes or masses. Common in many rocks. Known as fool's gold. Weathered pyrite produces limonite (iron oxide) that stains rock brown. or yellow.


An igneous rock texture produced from consolidation of fragmented volcanic material ejected during a violent eruption. Also used to describe ash, bombs and other material forcefully ejected during a volcanic eruption (=tephra)
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pyroclastic eruption

A volcanic eruption that produces a large volume of solid volcanic fragments ( pyroclastics ) rather than fluid lava. This type of eruption is typical of volcanoes with high silica , viscous, gas-rich magma.

pyroclastic flow

An extremely hot mixture of gas, ash and pumice fragments, that travels down the flanks of a volcano or along the surface of the ground at speeds of 50 to 100 miles per hour.


Family of silicate minerals containing iron, magnesium, and calcium in varying amounts. Differ from amphibole family by lack of water in the crystals. The most common variety, augite , contains aluminum as well. Generally forms very dark green to black stubby prisms.



One of the most common minerals in the Earth's crust (and in some new-age boutiques). Made up of silicon dioxide (SiO 2 ),it is also called silica. Commonly found in white masses. Crystals are clear, glassy 6-sided prisms.
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Hard, somewhat glassy-looking rock made up almost entirely of quartz. Metamorphosed quartz sandstone and chert are quartzites.


The most recent Period of the Cenozoic Era. Encompasses the time interval of 1.6 million years ago through today.


radiocarbon dating

The age of organic material determined by the amounts of carbon isotopes 12, 13 and 14. The ratio of 12 to 14 is about the same in all living things but when a plant or animal dies, no more carbon is taken on. Carbon 12 and 13 are stable isotopes and the amounts remain the same even in dead material. Carbon 14 is an radioactive isotope that decays radioactively until none is left; . Thus, the ratio records the time elapsed since death. Since carbon 14 decays relatively rapidly, the method is only reliable for the last 40,000 years. See radiometric age .

radiolarian chert

A rock made up of the spherical siliceous shells of radiolarians which are single-celled planktonic animals (protozoans).
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radiometric age

The approximate age of a geologic event, feature, fossil, or rock in years. Radiometric ages, sometimes termed 'absolute' ages, are determined by using natural radioactive 'clocks'. See radiocarbon dating .
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radiometric dating

A dating method that uses measurements of certain radioactive isotopes to calculate the ages in years (absolute age) of rocks and minerals.
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regional metamorphism

Metamorphism affecting a large region that is associated with mountain building events.


Refers to differences in elevation of different points in a region.

relative dating

The process of placing rocks and geologic structures in the correct chronological order. This process does not yield ages in number of years. See radiometric dating.


A volcanic rock chemically equivalent to granite Usually light colored, very fine-grained or glassy-looking. May have tiny visible crystals of quartz and/or feldspar dispersed in a glassy white, green, or pink groundmass.

ribbon chert

Chert and shale in thin alternating beds The beds resemble parallel ribbons stretched over an outcrop.

rift zone

A region of Earth's crust along which divergence is taking place. A linear zone of volcanic activity and faulting usually associated with diverging plates or crustal stretching.

ring of fire

A zone of volcanoes, earthquakes, and mountain-building encircling the Pacific Ocean formed where plates collide.


Rocks are made of different kinds of minerals , or broken pieces of crystals, or broken pieces of rocks. Some rocks are made of the shells of once-living animals, or of compressed pieces of plants. Rocks are divided into three basic types, igneous , sedimentary and metamorphic , depending upon how they were formed.
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Falling, bouncing, and rolling of debris down slope.

root of a volcano

Plutonic igneous rock formed from magma that crystallized beneath the volcano it once fed.



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


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


A cliff formed by faulting, erosion, or landslides. (Also called escarpment).


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.


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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Loose, uncemented pieces of rock or minerals.
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Refers to earthquakes.


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.


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.


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.


Overland flow of water in thin sheets


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


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


Silicon dioxide (SiO 2 ). 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.


Refers to the chemical unit silicon tetroxide, SiO 4 , the fundamental building block of silicate minerals . Silicate minerals make up most rocks we see at the Earth's surface.


Generally refers to a rock rich in quartz .


See dike .


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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A sedimentary rock made mostly of silt-sized grains.


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.


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.


Group of clays , those most susceptible to shrink-swell


All loose, unconsolidated earth and organic materials above bedrock that support plant growth.
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The exploration and study of caves.
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A deposit formed in caves when calcium carbonate (CaCO 3 ) or some other mineral precipitates from drips or thin films of water. Stalactites and stalagmites are common speleothems.
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A mineral deposit ( speleothem ) which hangs downwards from a roof or wall of a cave. See stalagmite .
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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.


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.


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 .


A thin, discontinuous mineral vein or rock layer.


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.


A upward-curving (concave) fold in rock that resembles a trough. The central part contains the youngest section of rock.



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.


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.


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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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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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.


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.


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.


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.


The shape of the land surface. See relief .


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


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 .


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.


Loose sediment ; lacking cohesion or cement .



A mineral-filled fracture or fault in a rock.


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.


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


A small bubble formed in volcanic rock during solidification.




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 . 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 .



A normally dry stream bed that ocassionally fills with water.


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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Refers to a sedimentary deposit or rock with grains of the same approximate size.



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. (another term also used is inclusion)
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Mineral of zirconium, silicon, and oxygen (zirconium silicate). Generally glassy-looking, microscopic, four-sided prisms. Most commonly formed in igneous rocks .