earth science

Ablation area=The lower portion of a glacier where more snow melts in summer than accumulates in winter so there is a net loss of glacial ice. (syn: zone of wastage)
Abrasion=wearing away of a rock by grinding action.

Drought=Abnormal dry weather for a specific area that is sufficiently prolonged for the lack of water to cause serious hydrological imbalance.

Ablation area=The lower portion of a glacier where more snow melts in summer than accumulates in winter so there is a net loss of glacial ice. (syn: zone of wastage)
Abrasion=wearing away of a rock by grinding action.

The boundary is defined by a global extinction event that caused the abrupt demise of the majority of all life on Earth. see Key bed
Crevasse=A fracture or crack in the upper 40 to 50 meters of a glacier.

Absolute age=Time measured in years.

Absorption nebula=A cold nebula that absorbs light.

Eutrophic lake=A lake characterized by abundant dissolved nitrates, phosphates, and other plant nutrients and by a seasonal deficiency of oxygen in bottom water.

Andesite minerals commonly include plagioclase and hornblende, with lesser amounts of mica, pyroxene, and various accessory minerals.

Cyclone=A low pressure region with its accompanying surface wind.

Accretion=Accumulation of dust and gas into larger bodies.

Ablation area=The lower portion of a glacier where more snow melts in summer than accumulates in winter so there is a net loss of glacial ice. (syn: zone of wastage)
Abrasion=wearing away of a rock by grinding action.

Abyssal fan=A large, fan-shaped accumulation of sediment deposited at the bases of many submarine canyons adjacent to the deep-sea floor. (syn: submarine fan)
Abyssal plain=A flat, level, largely featureless part of the ocean floor between the mid-oceanic ridge and the continental rise.

Acid precipitation=A condition in which natural precipitation becomes acidic after reacting with air pollutants.

(Often called "acid rain.")

Acid precipitation=A condition in which natural precipitation becomes acidic after reacting with air pollutants.

Ablation area=The lower portion of a glacier where more snow melts in summer than accumulates in winter so there is a net loss of glacial ice. (syn: zone of wastage)
Abrasion=wearing away of a rock by grinding action.

Active continental margin=A continental margin characterized by subduction of an oceanic lithospheric plate beneath a continental plate. (syn: Andean margin)
Active volcano=A volcano that is erupting; or one that, while not erupting at the present, has erupted within (geologically) recent time and is considered likely to do so in the (geologically) near future.

Abyssal fan=A large, fan-shaped accumulation of sediment deposited at the bases of many submarine canyons adjacent to the deep-sea floor. (syn: submarine fan)
Abyssal plain=A flat, level, largely featureless part of the ocean floor between the mid-oceanic ridge and the continental rise.

Asthenosphere=The shell within the earth, some tens of kilometers below the surface and of undefined thickness, which is a shell of weakness where plastic movements take place to permit pressure adjustments.

Deposition=The laying-down of rock-forming materials by any natural agent.

An aggregate of cloud droplets forms a visible cloud.

Albedo=Reflectivity of an object; ratio of reflected light to incident light.

Control rod=A column of neutron-absorbing alloys that are spaced among fuel rods to fine-tune nuclear fission in a reactor.

Alluvial fan=A fan-like accumulation of sediment created where a steep stream slows down rapidly as it reaches a relatively flat valley floor.

Alpha Centauri=The closest bright star to our solar system.

Composite cone=A volcano that consists of alternate layers of unconsolidated pyroclastic material and lava flows. (syn: stratovolcano)
Composite Volcano=A steep volcanic cone built by both lava flows and pyroclastic eruptions.

Alternative energy resources=All energy resources other than fossil fuels and nuclear fission; including solar energy; hydroelectric power; geothermal energy; wind energy; biomass energy; tidal, wave, and heat energy from the seas; and nuclear fusion.

Altimeter=An instrument used to determine the altitude of an object with respect to a fixed level.

This movement is amplified by a spring or gear mechanism to indicate changes in air pressure on a scale or recording chart.

Anaerobic=Without oxygen; anaerobic bacteria are bacteria that live without oxygen.

Cost benefit analysis=A system of analysis that attempts to weigh the cost of an act or policy, such as pollution control, directly against the economic benefits.

A horizon=The uppermost layer of soil composed of a mixture of organic matter and leached and weathered minerals. (syn: topsoil)
Aa=Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments.

Anion=An ion that has a negative charge.

Antecedent stream=A stream that was established before local uplift started and cut its channel at the same rate the land wasrising.

The base of cumulus clouds is generally found from 500 to 3000 meters Large cumulonimbus clouds may extend to over 18,000 meters and be topped with anvil-shaped ice clouds.

Aphelion =The point in its orbit where a planet is farthest from the Sun.
Apogee =The point in orbit farthest from the Earth.

Apparent magnitude=The brightness of a star as seen from Earth.

Air mass=A large body of air that has approximately the same temperature and humidity throughout.

Aquifer=A body of rock that contains significant quantities of water that can be tapped by wells or springs.

Ablation area=The lower portion of a glacier where more snow melts in summer than accumulates in winter so there is a net loss of glacial ice. (syn: zone of wastage)
Abrasion=wearing away of a rock by grinding action.

Crystal=A solid element or compound whose atoms are arranged in a regular, orderly, periodically repeated array.

Artesian aquifer=An inclined aquifer that is bounded top and bottom by layers of impermeable rock so the water is under pressure.

Artificial channel=Any channel dredged to modify the natural channel or to alter the course of a stream.

Absolute dating=A technique that geologists use to assign specific dates to rock formations and geologic events.

Asteroid=A rocky space object that can be from a few hundred feet to several hundred km wide.

Constellation=One of 88 groups of stars that astronomers refer to for convenience in referring to the positions of objects in the night sky.

Astronomical unit=(AU) The average distance from the Earth to the Sun; 1 AU is 149,597,870 kilometers (92,960,116 miles).

Astronomy=Astronomy is the study of everything that lies beyond the Earth's atmosphere.

Adiabatic rate=The rate of temperature change in the atmosphere due to the raising or lowering of an air mass.

Atmosphere=One atmosphere is 14.7 pounds per square inch (105 Newtons per square meter); the average atmospheric pressure at sea level on Earth.

Atoll=A circular reef that surrounds a lagoon and is bounded on the outside by deep water of the open sea.

Chemical bond=The linkage between atoms in molecules and between molecules and ions in crystals.

Conduction=The transport of heat by atomic or molecular motion.

Aurora borealis=The Northern Lights caused by the interaction between the solar wind, the Earth's magnetic field and the upper atmosphere; a similar effect happens in the southern hemisphere where it is known as the aurora australis.

Convection=Fluid (gas or liquid) circulation driven by temperature and density differences; the transfer of heat by this automatic circulation (currents).

Avalanche=A large mass of material falling or sliding rapidly due to the force of gravity.

Astronomical unit=(AU) The average distance from the Earth to the Sun; 1 AU is 149,597,870 kilometers (92,960,116 miles).

Eccentricity=A value that defines the shape of an ellipse or planetary orbit; the ratio of the distance between the foci and the major axis.

Anaerobic=Without oxygen; anaerobic bacteria are bacteria that live without oxygen.

Altimeter=A barometer with a scale calibrated in units of elevation rather than pressure.

Barrier island=A long, narrow, low-lying island that extends parallel to the shoreline.

Basal slip=Movement of the entire mass of a glacier along the bedrock.

Basal slip=Movement of the entire mass of a glacier along the bedrock.

Cost benefit analysis=A system of analysis that attempts to weigh the cost of an act or policy, such as pollution control, directly against the economic benefits.

The fragments are clastic, but they are of biological origin.

Alternative energy resources=All energy resources other than fossil fuels and nuclear fission; including solar energy; hydroelectric power; geothermal energy; wind energy; biomass energy; tidal, wave, and heat energy from the seas; and nuclear fusion.

Biome=A community of plants living in a large geographic area characterized by a particular climate.

Biosphere=The zone inhabited by life.

Black hole=An object whose gravity is so strong that the escape velocity exceeds the speed of light.

Blizzard conditions are common in a well-developed Alberta Clipper.

Blowout=A small depression created by wind erosion.

Boiling point=The temperature at which a liquid changes to a vaporous state.

Cobbles=Rounded rock fragments in the 64 to 256 mm size range, larger than pebbles and smaller than boulders.

Brittle fracture=Rupture that occurs when a rock breaks sharply.

Bajada=A broad depositional surface extending outward from a mountain front formed by the merging of alluvial fans.

A butte is smaller and more tower-like than a mesa.

Caldera=A large circular depression caused by an explosive volcanic eruption.

Results in liberation of 80 calories per cubic centimeter.

Calving=A process in which large chunks of ice break off from tidewater glaciers to form icebergs.

Capacity=The maximum quantity of sediment that a stream can carry.

Capillary action=The action by which water is pulled upward through small pores by electrical attraction to the pore walls.

Anemometer=an instrument that measures wind speed
Aneriod Barometer=A barometer whose pressure sensor consists of an aneroid capsule, a thin, hollow disk partially evacuated and sealed.

Argon (A) at 0.93%; and Carbon Dioxide (CO2) 0.033%.

The major volcanic cones of the Cascade Mountains (in Washington, Oregon, and California) are believed to be dormant rather than extinct.

Catastrophism=The model that Earth change occurs through a series of rare but cataclysmic events.

In many cases, water acts as a catalyst and/or lubricant.

Cation=A positively charged ion.

Cavern=An underground cavity or series of chambers created when groundwater dissolves large amounts of rock, usually limestone. (syn: cave)
Celestial sphere=An imaginary sphere around the Earth that all the stars and planets are placed on.

Asteroid number=Asteroids are assigned a serial number when they are discovered; it has no particular meaning except that asteroid N+1 was discovered after asteroid N.
Asteroid=One of the many small celestial bodies in orbit around the Sun. Most asteroids orbit between Mars and Jupiter.

Barchan dune=A crescent shaped dune, highest in the center, with the tips facing downwind.

Cavern=An underground cavity or series of chambers created when groundwater dissolves large amounts of rock, usually limestone. (syn: cave)
Celestial sphere=An imaginary sphere around the Earth that all the stars and planets are placed on.

Antecedent stream=A stream that was established before local uplift started and cut its channel at the same rate the land wasrising.

Active continental margin=A continental margin characterized by subduction of an oceanic lithospheric plate beneath a continental plate. (syn: Andean margin)
Active volcano=A volcano that is erupting; or one that, while not erupting at the present, has erupted within (geologically) recent time and is considered likely to do so in the (geologically) near future.

This movement is amplified by a spring or gear mechanism to indicate changes in air pressure on a scale or recording chart.

Chemical bond=The linkage between atoms in molecules and between molecules and ions in crystals.

Atmospheric chemists divide the atmosphere into the heterosphere, where gases are well mixed, and the homosphere, where gases are found in differentiated layers.

Chert=A hard, dense, sedimentary rock composed of microcrystalline quartz. (syn: flint)
Chlorofluorocarbons=(CFC's); gases containing chlorine, fluorine and carbon atoms that break down the ozone layer and weaken protection from ultraviolet waves.

Biome=A community of plants living in a large geographic area characterized by a particular climate.

Alberta Clipper=A fast moving winter storm originating in the Alberta, Canada region.

Barrier reef=A reef separated from the coast by a deep, wide lagoon.

Degeneracy pressure=The strength of the atomic particles that holds a white dwarf star from further collapse.

Cold front=A front that forms when moving cold air collides with stationary or slower moving warm air.

Column=A dripstone or speleothem formed when a stalactite and a stalagmite meet and fuse together.

Coma=The dust and gas surrounding an active comet's nucleus.

Coma=The dust and gas surrounding an active comet's nucleus.

Competence=A measure of the largest particles that a stream can transport.

Advection=In meteorology, the horizontal component of a convection current in air, i.e.

The principal gases that compose dry air are Nitrogen (N2) 78.09%;

They occur with composite volcanic cones associated with convergent plate margins.

Carbonate=A compound containing carbon and oxygen; an example is calcium carbonate (limestone).

The big bang theory postulates that 10 to 20 billion years ago, all matter exploded from an infinitely compressed state.

Compression Waves=Earthquake waves that move like a slinky.

Banded iron formation=Iron-rich layered sedimentary rocks precipitated from the seas mostly between 2.6 and 1.9 billion years ago, as a result of rising atmospheric oxygen concentrations.

Celestial spheres=A hypothetical series of concentric spheres centered at the center of the Earth.

Current=A continuous flow of water in a concerted direction.

A cloud forms in the atmosphere as a result of condensation of water vapor.

Advection fog=Fog that forms when warm moist air from the sea blows onto cooler land, where the air cools and water vapor condenses at ground level.

Conduction=the movement of heat to an object by its contact with a hotter object.

They occur with composite volcanic cones associated with convergent plate margins.

Artesian Well=A well in an aquifer where the groundwater is confined under pressure and the water level will rise above the top of the confined aquifer.

Confining pressure=See "confining stress."

Conformable=The condition in which sedimentary layers were deposited continuously without interruption.

Contour interval=the difference in elevation between two consecutive contour lines
Contour line=line on a topographic map that connects points on land that have the same elevation.

Andesite=A fine-grained gray or green volcanic rock intermediate in composition between basalt and granite, consisting of about equal amounts of plagioclase feldspar and mafic minerals.

Butte=A conspicuous, isolated, flattop hill with steep slopes.

Altitude=height (number of degrees) of an object above the horizon.In meteorology, the measure of a height of an airborne object in respect to a constant pressure surface or above mean sea level.

Constellation=a group of stars that appears to form a pattern in the sky.

One other important constituents of air is water vapor (H2O) which varies from 0% to about 4%.

Eccentricity=The shape of the ellipse that constitutes the Earth's orbit around the Sun.
Echo sounder=An instrument that emits sound waves and then records them after they reflects off the sea floor.

Consumption=Any process that uses water, and then returns it to the Earth far from its source.

The black color is caused by precipitation of very fine grained metal sulfide minerals as hydrothermal solutions cool by contact with sea water.

Andean margin=A continental margin characterized by subduction of an oceanic lithospheric plate beneath a continental plate. (syn: active continental margin)
Andesite=Intermediate volcanic rocks containing 54 to 62 percent silica and moderate amounts of iron and magnesium.

Accreted terrain=A land mass that originated as an island arc or a microcontinent that was later added onto a continent.

Continental Drift=The theory that horizontal movement of the earth's surface causes slow, relative movements of the continents toward or away from one another.

Continental glacier=A glacier that forms a continuous cover of ice over areas of 50,000 square kilometers or more and spreads outward in all directions under the influence of its own weight. (syn: ice sheet)
Continental margin=The region between the shoreline of a continent and the deep ocean basins including the continental shelf, continental slope, and continental rise.

Contour interval=the difference in elevation between two consecutive contour lines
Contour line=line on a topographic map that connects points on land that have the same elevation.

Advection=In meteorology, the horizontal component of a convection current in air, i.e.

Convergence=The flow of the wind resulting in a horizontal inflow of air into a region.

They occur with composite volcanic cones associated with convergent plate margins.

Anticline=A fold in rock that resembles an arch; the fold is convex upward and the oldest rocks are in the middle Anticyclone: An area of high pressure, also called a High, around which the winds circulate in a clockwise direction in the Northern Hemisphere (and counterclockwise in the Southern Hemisphere).

Core, inner=spherical center of the Earth, about 1200 km in diameter and made of solid iron and nickel.

Corona=The very hot outermost layer of a star's atmosphere.

Corona=The luminous irregular envelope of highly ionized gas outside the chromosphere of the Sun.
Correlation=the process of matching rock layers from different locations.

Cosmic ray=Electromagnetic rays of extremely high frequency and energy; cosmic rays usually interact with the atoms of the atmosphere before reaching the surface of the Earth.

Ash Flow=A turbulent mixture of gas and rock fragments, most of which are ash-sized particles, ejected violently from a crater or fissure.

Creep=The slow movement of unconsolidated material downslope under the influence of gravity.

The boundary is defined by a global extinction event that caused the abrupt demise of the majority of all life on Earth. see Key bed
Crevasse=A fracture or crack in the upper 40 to 50 meters of a glacier.

Advection=In meteorology, the horizontal component of a convection current in air, i.e.

Climate refers to yearly cycles of temperature, wind, rainfall, etc., and not to daily variations (see "weather").

Effluent stream=A stream that receives water from groundwater because its channel lies below the water table. (syn: gaining stream)
El Nino=(Niño, Nin~o) The cyclical warming of East Pacific Ocean sea water temperatures off the western coast of South America that can result in significant changes in weather patterns in the United States and elsewhere.

Anticyclonic=The wind circulation pattern in Highs, or anticyclones, that has a sense of rotation opposite to that of cyclones and the Earth's rotation.

The data is then used to record the topography of the sea floor.

A mixture of these materials is commonly called a debris flow.

Absolute Date=An estimate of the true age of a mineral or rock based on the rate of decay of radioactive minerals.

Bioremediation=The use of microorganisms to decompose an environmental contaminant.

Chemical weathering=breakdown or decomposition of a rock that takes place when minerals are changed into different substances.

Generally, with the passage of a cold front, the temperature and humidity decrease, the pressure rises, and the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere).

Eccentricity=A value that defines the shape of an ellipse or planetary orbit; the ratio of the distance between the foci and the major axis.

Blizzard=Although blizzard is often used to describe any major snow storm with strong winds, the technical definition for a blizzard requires at least 3 hours in duration; low temperatures (usually less than minus 7C or 20F), strong winds (greater than 55 km/h or 35 mph), blowing snow which reduces visibility to less that 1 kilometer (0.6 miles).

Deflation=Erosion by wind.

In the northern hemisphere, the Coriolis effect deflects the diverging winds to the right, forming a pinwheel pattern with the air spiraling clockwise.

Altitude=height (number of degrees) of an object above the horizon.In meteorology, the measure of a height of an airborne object in respect to a constant pressure surface or above mean sea level.

Delta=A fan-shaped deposit of sediment formed at the mouth of a stream or river.

The boundary is defined by a global extinction event that caused the abrupt demise of the majority of all life on Earth. see Key bed
Crevasse=A fracture or crack in the upper 40 to 50 meters of a glacier.

Correlation=Demonstration of the equivalence of rocks or geologic features age from different locations.

Big Bang Hypothesis=The theory that the universe was originally a single dense sphere of hydrogen that exploded into a gigantic expanding cloud that eventually condensed into separate galaxies.

Absolute Humidity=The ratio of the mass of water vapor present in the air to the volume occupied by the gas; the density of water vapor in the air, usually expressed as grams of water vapor per cubic meter of air.

Abyssal fan=A large, fan-shaped accumulation of sediment deposited at the bases of many submarine canyons adjacent to the deep-sea floor. (syn: submarine fan)
Abyssal plain=A flat, level, largely featureless part of the ocean floor between the mid-oceanic ridge and the continental rise.

Deposition=The laying-down of rock-forming materials by any natural agent.

Blowout=A small depression created by wind erosion.

Anticyclone=A system of rotating winds that develop where descending air spreads out over the Earth's surface.

A horizon=The uppermost layer of soil composed of a mixture of organic matter and leached and weathered minerals. (syn: topsoil)
Aa=Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments.

Example: crater
Desert pavement=A continuous cover of stones created as wind erodes fine sediment, leaving larger rocks behind.

Desertification=A process by which semiarid land is converted to desert, often by improper farming or by climate change.

Aa flows commonly developed from pahoehoe flows as they cool and lose gas.

Dew Point=(or Dew Point Temperature) The temperature to which air must be cooled for saturation to occur.

Ash=(volcanic) Fine pyroclastic material less than 2 mm in diameter.

Differential weathering=The process by which certain rocks weathers more rapidly than adjacent rocks usually resulting in an uneven surface.

Chromosphere=A turbulent diffuse gaseous layer of the Sun that lies above the photosphere.

Cretaceous-Tertiary boundary=A major stratigraphic boundary on Earth marking the end of the Mesozoic Era, best known as the age of the dinosaurs.

Anticline=A fold in rock that resembles an arch; the fold is convex upward and the oldest rocks are in the middle Anticyclone: An area of high pressure, also called a High, around which the winds circulate in a clockwise direction in the Northern Hemisphere (and counterclockwise in the Southern Hemisphere).

Discordant=Pertaining to a dike or other feature that cuts across sedimentary layers or other kinds of layering in country rock.

This occurs when warm equatorial waters move in and displace the colder waters of the Peru Current, cutting off the upwelling process.

Cavern=An underground cavity or series of chambers created when groundwater dissolves large amounts of rock, usually limestone. (syn: cave)
Celestial sphere=An imaginary sphere around the Earth that all the stars and planets are placed on.

The dense, cold air distorts into a blunt wedge and pushes under the warmer air, creating a narrow band of violent weather commonly accompanied by cumulus and cumulonimbus clouds.

Epidemiology=The study of the distribution of sickness in a population.

The opposite of convergence is divergence.

Continental rifting=The process by which a continent is pulled apart at a divergent plate boundary.

Diversion=All processes that transfer ground or surface water from its natural place and path in the hydrologic cycle to a new place and path to serve human needs.

Atmospheric chemists divide the atmosphere into the heterosphere, where gases are well mixed, and the homosphere, where gases are found in differentiated layers.

Archean Eon=A division of geologic time 3.8 to 2.5 billion years ago.

Doldrums=the rainy equatorial belt of low pressure and slowly rising air.

Compound Volcano=A volcano that consists of a complex of two or more vents, or a volcano that has an associated volcanic dome, either in its crater or on its flanks.

Dormant volcano=This term is used to describe a volcano which is presently inactive but which may erupt again.

Drainage Basin=The area of land drained by a river system.

Often these snow storms have high winds reaching 100 km/h which reduce visibility through blowing and drifting snow.

They are smaller in size than either drizzle or rain drops.

Drought=Abnormal dry weather for a specific area that is sufficiently prolonged for the lack of water to cause serious hydrological imbalance.

Barchan dune=A crescent shaped dune, highest in the center, with the tips facing downwind.

Blizzard=Although blizzard is often used to describe any major snow storm with strong winds, the technical definition for a blizzard requires at least 3 hours in duration; low temperatures (usually less than minus 7C or 20F), strong winds (greater than 55 km/h or 35 mph), blowing snow which reduces visibility to less that 1 kilometer (0.6 miles).

Air pressure=(barometric pressure) weight of the atmosphere per unit area
Air=The mixture of gases that make up the earth's atmosphere.

Aseismic ridge=A submarine mountain chain with little or no earthquake activity.

Chalk=A very fine grained, soft, earthy, white to gray bioclastic limestone made of the shells and skeletons of marine microorganisms.

Eccentric=Noncircular; elliptical (applied to an orbit).

Eccentricity=A value that defines the shape of an ellipse or planetary orbit; the ratio of the distance between the foci and the major axis.

Our Sun's corona can only be seen during a total solar eclipse.

Disseminated ore deposit=A large low-grade ore deposit in which generally fine-grained metal-bearing minerals are widely scattered throughout a rock body in sufficient concentration to make the deposit economical to mine.

Cold front=the leading edge of an advancing cold air mass that is under running and displacing the warmer air in its path.

In the northern hemisphere, the Coriolis effect deflects the diverging winds to the right, forming a pinwheel pattern with the air spiraling clockwise.

Ash Flow=A turbulent mixture of gas and rock fragments, most of which are ash-sized particles, ejected violently from a crater or fissure.

Effluent stream=A stream that receives water from groundwater because its channel lies below the water table. (syn: gaining stream)
El Nino=(Niño, Nin~o) The cyclical warming of East Pacific Ocean sea water temperatures off the western coast of South America that can result in significant changes in weather patterns in the United States and elsewhere.

Earthquake=A sudden motion or trembling of the Earth caused by the abrupt release of slowly accumulated elastic energy in rocks.

Electron=A fundamental particle which forms a diffuse cloud of negative charge around an atom.

There are three common types of barometers used widely today: mercury barometer; aneroid barometer; electronic barometer.

Compound=A pure substance composed of two or more elements whose composition is constant.

Compound=A pure substance composed of two or more elements whose composition is constant.

Johannes Kepler first discovered that the orbits of the planets are ellipses, not circles; he based his discovery on the careful observations of Tycho Brahe.

Dome=A circular or elliptical anticlinal structure.

Bar=An elongate mound of sediment, usually composed of sand or gravel, in a stream channel or along a coastline.

Emergent coastline=A coastline that was recently under water but has been exposed either because the land has risen or sea level has fallen.

Emission nebula=A glowing cloud of interstellar gas.

Some cosmic rays come from outside the solar system while others are emitted from the Sun and pass through holes in the corona.

Alternative energy resources=All energy resources other than fossil fuels and nuclear fission; including solar energy; hydroelectric power; geothermal energy; wind energy; biomass energy; tidal, wave, and heat energy from the seas; and nuclear fusion.

Basal slip=Movement of the entire mass of a glacier along the bedrock.

See glossary entries for more on these types.

Chemical weathering=The chemical decomposition of rocks and minerals by exposure to air, water, and other chemicals in the environment.

Bioremediation=The use of microorganisms to decompose an environmental contaminant.

Archean Eon=A division of geologic time 3.8 to 2.5 billion years ago.

Epicenter=the point on Earth's surface directly above the focus of an earthquake.

Epidemiology=The study of the distribution of sickness in a population.

Epoch=a subdivision of a geological period on the geologic timetable.

Andesite=A fine-grained gray or green volcanic rock intermediate in composition between basalt and granite, consisting of about equal amounts of plagioclase feldspar and mafic minerals.

Coriolis Effect=the effect of Earth's rotation that causes the deflection of moving objects towards or away from the equator.

The temperature at which the equilibrium vapor pressure between a liquid and its vapor is equal to the external pressure on the liquid.

Autumnal Equinox=see Equinox
Autumnal=Pertaining to the Autumn season.

Correlation=Demonstration of the equivalence of rocks or geologic features age from different locations.

Cenozoic era=The latest of the four eras into which geologic time is subdivided; 65 million years ago to the present.

Base level=The deepest level to which a stream can erode its bed.

Angular unconformity=An unconformity in which younger sediment or sedimentary rocks rest on the eroded surface of tilted orfolded older rocks.

The oldest known rocks formed at the beginning of or just prior to the start of the Archean Eon.
Arête=A sharp narrow ridge between adjacent valleys formed by glacial erosion.

Erratic=(glacial) Large rocks or other debris deposited by a glacier, usually in an area far removed from its source.

Active continental margin=A continental margin characterized by subduction of an oceanic lithospheric plate beneath a continental plate. (syn: Andean margin)
Active volcano=A volcano that is erupting; or one that, while not erupting at the present, has erupted within (geologically) recent time and is considered likely to do so in the (geologically) near future.

Ash flow=A mixture of volcanic ash, larger pyroclastic particles, and gas that flows rapidly along the Earth's surface as a result of an explosive volcanic eruption. (syn: nuee ardente)
Ash=flow tuff-A pyroclastic rock formed when an ash flow solidifies.

Estuary=A bay that formed when a broad river valley was submerged by rising sea level or a sinking coast.

Evacuate=Temporarily move people away from possible danger.

The water evaporates, but the vapor cools and condenses to fog.

Absolute dating=A technique that geologists use to assign specific dates to rock formations and geologic events.

Accumulation area=The upper part of a glacier where accumulation of snow during the winter exceeds melting during the summer, causing a net gain of glacial ice.

Blackbody temperature=The temperature of an object if it is reradiating all the thermal energy that has been added to it; if an object is not a blackbody radiator, it will not reradiate all the excess heat and the leftover will go toward increasing its temperature.

Barometric pressure=The pressure exerted by the atmosphere at a given point.

The difference between the exterior air pressure and the interior pressure causes the disk to expand or contract slightly.

Aspect=The orientation of a slope with respect to the Sun; the geographic orientation or exposure of a slope.

Antarctic Ozone Hole=An extended area of extreme depletion of the ozone layer which forms over the southern polar region eachaustral spring (September through November).

The major volcanic cones of the Cascade Mountains (in Washington, Oregon, and California) are believed to be dormant rather than extinct.

The boundary is defined by a global extinction event that caused the abrupt demise of the majority of all life on Earth. see Key bed
Crevasse=A fracture or crack in the upper 40 to 50 meters of a glacier.

Abyssal fan=A large, fan-shaped accumulation of sediment deposited at the bases of many submarine canyons adjacent to the deep-sea floor. (syn: submarine fan)
Abyssal plain=A flat, level, largely featureless part of the ocean floor between the mid-oceanic ridge and the continental rise.

When the rapidly descending air strikes the ground, it spreads outward in all directions in a circle, like a fast-running faucet hitting the sink bottom.

Correlation=Demonstration of the equivalence of rocks or geologic features age from different locations.

Alternative energy resources=All energy resources other than fossil fuels and nuclear fission; including solar energy; hydroelectric power; geothermal energy; wind energy; biomass energy; tidal, wave, and heat energy from the seas; and nuclear fusion.

Ash Flow=A turbulent mixture of gas and rock fragments, most of which are ash-sized particles, ejected violently from a crater or fissure.

Coal=A flammable organic sedimentary rock formed from partially decomposed plant material and composed mainly of carbon.

Artificial levee=A wall built along the banks of a stream to prevent rising flood water from spilling out of the stream channel onto the flood plain.

Contact metamorphism=Metamorphism caused by heating of country rock, and/or addition of fluids, from a nearby igneous intrusion.

Eccentricity=A value that defines the shape of an ellipse or planetary orbit; the ratio of the distance between the foci and the major axis.

E=mc2 Einstein's famous theory of relativity formula known as the energy-mass relation.

Alternative energy resources=All energy resources other than fossil fuels and nuclear fission; including solar energy; hydroelectric power; geothermal energy; wind energy; biomass energy; tidal, wave, and heat energy from the seas; and nuclear fusion.

Black smoker=A jet of black water spouting from a fracture in the sea floor, commonly near the mid-oceanic ridge.

Abrasion=The mechanical wearing and grinding of rock surfaces by friction and impact.

Alternative energy resources=All energy resources other than fossil fuels and nuclear fission; including solar energy; hydroelectric power; geothermal energy; wind energy; biomass energy; tidal, wave, and heat energy from the seas; and nuclear fusion.

Cross cutting relationships=A fundamental principle of geology, which states that rocks that cut through other rocks are younger than the rocks being cut.

Column=A dripstone or speleothem formed when a stalactite and a stalagmite meet and fuse together.

Alternative energy resources=All energy resources other than fossil fuels and nuclear fission; including solar energy; hydroelectric power; geothermal energy; wind energy; biomass energy; tidal, wave, and heat energy from the seas; and nuclear fusion.

Big Bang Hypothesis=The theory that the universe was originally a single dense sphere of hydrogen that exploded into a gigantic expanding cloud that eventually condensed into separate galaxies.

This movement is amplified by a spring or gear mechanism to indicate changes in air pressure on a scale or recording chart.

Epicycle=a small orbit along which a planet traveled in Ptolemy's geocentric model of the solar system.

Absolute dating=A technique that geologists use to assign specific dates to rock formations and geologic events.

Absolute dating=A technique that geologists use to assign specific dates to rock formations and geologic events.

Cross cutting relationships=A fundamental principle of geology, which states that rocks that cut through other rocks are younger than the rocks being cut.

Alternative energy resources=All energy resources other than fossil fuels and nuclear fission; including solar energy; hydroelectric power; geothermal energy; wind energy; biomass energy; tidal, wave, and heat energy from the seas; and nuclear fusion.

Big Bang Hypothesis=The theory that the universe was originally a single dense sphere of hydrogen that exploded into a gigantic expanding cloud that eventually condensed into separate galaxies.

Ablation area=The lower portion of a glacier where more snow melts in summer than accumulates in winter so there is a net loss of glacial ice. (syn: zone of wastage)
Abrasion=wearing away of a rock by grinding action.

Ablation area=The lower portion of a glacier where more snow melts in summer than accumulates in winter so there is a net loss of glacial ice. (syn: zone of wastage)
Abrasion=wearing away of a rock by grinding action.

The boundary is defined by a global extinction event that caused the abrupt demise of the majority of all life on Earth. see Key bed
Crevasse=A fracture or crack in the upper 40 to 50 meters of a glacier.

On the date of the equinox (literally, meaning "equal nights") nights are of equal length all across the globe from pole to pole.

See glossary entries for more on these types.

C horizon=The lowest soil layer composed of partly weathered bedrock grading downward into unweathered parent rock.

C horizon=The lowest soil layer composed of partly weathered bedrock grading downward into unweathered parent rock.

Andesite=A fine-grained gray or green volcanic rock intermediate in composition between basalt and granite, consisting of about equal amounts of plagioclase feldspar and mafic minerals.

Atmosphere=The envelope of gases that surround a planet's surface held by the planet's gravity.

Anticline=A fold in rock that resembles an arch; the fold is convex upward and the oldest rocks are in the middle Anticyclone: An area of high pressure, also called a High, around which the winds circulate in a clockwise direction in the Northern Hemisphere (and counterclockwise in the Southern Hemisphere).

A horizon=The uppermost layer of soil composed of a mixture of organic matter and leached and weathered minerals. (syn: topsoil)
Aa=Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments.

A cloud-like streamer or trail often seen behind aircraft flying in clear, cold, humid air.

Absolute Humidity=The ratio of the mass of water vapor present in the air to the volume occupied by the gas; the density of water vapor in the air, usually expressed as grams of water vapor per cubic meter of air.

Also a synonym for a tropical cyclone, or hurricane.

Big Bang Hypothesis=The theory that the universe was originally a single dense sphere of hydrogen that exploded into a gigantic expanding cloud that eventually condensed into separate galaxies.

Big Bang Hypothesis=The theory that the universe was originally a single dense sphere of hydrogen that exploded into a gigantic expanding cloud that eventually condensed into separate galaxies.

Celestial spheres=A hypothetical series of concentric spheres centered at the center of the Earth.

Calving=A process in which large chunks of ice break off from tidewater glaciers to form icebergs.

The temperature recorded is considered identical to air temperature.

Contact metamorphism=Metamorphism caused by heating of country rock, and/or addition of fluids, from a nearby igneous intrusion.

Drought=Abnormal dry weather for a specific area that is sufficiently prolonged for the lack of water to cause serious hydrological imbalance.

Abrasion=The mechanical wearing and grinding of rock surfaces by friction and impact.

Artesian aquifer=An inclined aquifer that is bounded top and bottom by layers of impermeable rock so the water is under pressure.

Artesian aquifer=An inclined aquifer that is bounded top and bottom by layers of impermeable rock so the water is under pressure.

Andesite minerals commonly include plagioclase and hornblende, with lesser amounts of mica, pyroxene, and various accessory minerals.

This movement is amplified by a spring or gear mechanism to indicate changes in air pressure on a scale or recording chart.

Continental glacier=A glacier that forms a continuous cover of ice over areas of 50,000 square kilometers or more and spreads outward in all directions under the influence of its own weight. (syn: ice sheet)
Continental margin=The region between the shoreline of a continent and the deep ocean basins including the continental shelf, continental slope, and continental rise.

Biosphere=The zone inhabited by life.

Branching chain reaction=A nuclear fission reaction in which the initial reaction releases two or three neutrons, each of which triggers the fission of additional nuclei.

Recharge could result from reservoirs, storage basins, leaky canals, direct injection of water into an aquifer, or by spreading water over a large land surface.

Altimeter=An instrument used to determine the altitude of an object with respect to a fixed level.

Cosmic ray=Electromagnetic rays of extremely high frequency and energy; cosmic rays usually interact with the atoms of the atmosphere before reaching the surface of the Earth.

Aurora borealis=The Northern Lights caused by the interaction between the solar wind, the Earth's magnetic field and the upper atmosphere; a similar effect happens in the southern hemisphere where it is known as the aurora australis.

The difference between the exterior air pressure and the interior pressure causes the disk to expand or contract slightly.

Andean margin=A continental margin characterized by subduction of an oceanic lithospheric plate beneath a continental plate. (syn: active continental margin)
Andesite=Intermediate volcanic rocks containing 54 to 62 percent silica and moderate amounts of iron and magnesium.

Emission nebula=A glowing cloud of interstellar gas.

Contour interval=the difference in elevation between two consecutive contour lines
Contour line=line on a topographic map that connects points on land that have the same elevation.

Dike=A sheetlike body of igneous rock that cuts across layering or contacts in the rock into which it intrudes.

Batholith=A large plutonic mass of intrusive rock with more than 100 square kilometers of surface exposed.

Atmospheric inversion=See "inversion (atmospheric)."

Eclipse=A phenomenon that occurs when a heavenly body is shadowed by another and therefore rendered invisible.

Anion=An ion that has a negative charge.

Butte=A conspicuous, isolated, flattop hill with steep slopes.

Daughter isotope=An isotope formed by radioactive decay of another isotope.

Aa=A lava flow that has a jagged, sharp, broken surface (like glass).

Black smoker=A jet of black water spouting from a fracture in the sea floor, commonly near the mid-oceanic ridge.

Atoll=A circular reef that surrounds a lagoon and is bounded on the outside by deep water of the open sea.

Some common types of erosion includes landslides, rockfalls, creep, etc.

Dry adiabatic lapse rate=The rate of cooling that occurs when dry air rises without gain or loss of heat.

A horizon=The uppermost layer of soil composed of a mixture of organic matter and leached and weathered minerals. (syn: topsoil)
Aa=Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments.

A horizon=The uppermost layer of soil composed of a mixture of organic matter and leached and weathered minerals. (syn: topsoil)
Aa=Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments.

Banded iron formation=Iron-rich layered sedimentary rocks precipitated from the seas mostly between 2.6 and 1.9 billion years ago, as a result of rising atmospheric oxygen concentrations.

A horizon=The uppermost layer of soil composed of a mixture of organic matter and leached and weathered minerals. (syn: topsoil)
Aa=Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments.

Artificial levee=A wall built along the banks of a stream to prevent rising flood water from spilling out of the stream channel onto the flood plain.

Abyssal fan=A large, fan-shaped accumulation of sediment deposited at the bases of many submarine canyons adjacent to the deep-sea floor. (syn: submarine fan)
Abyssal plain=A flat, level, largely featureless part of the ocean floor between the mid-oceanic ridge and the continental rise.

Results in liberation of 80 calories per cubic centimeter.

Chemical bond=The linkage between atoms in molecules and between molecules and ions in crystals.

Aneroid means "without liquid."

Asthenosphere=The portion of the upper mantle beneath the lithosphere.

Climate=typical weather patterns over a period of years in a given location
Climate=The composite pattern of long-term weather conditions that can be expected in a given region.

Corona=The luminous irregular envelope of highly ionized gas outside the chromosphere of the Sun.
Correlation=the process of matching rock layers from different locations.

When the Earth lies directly between the Sun and the Moon, the Earth's shadow falls on the Moon and we observe a lunar eclipse.

By far the most common variety is vitric ash (glassy particles formed by gas bubbles bursting through liquid magma).

Aurora borealis=The Northern Lights caused by the interaction between the solar wind, the Earth's magnetic field and the upper atmosphere; a similar effect happens in the southern hemisphere where it is known as the aurora australis.

Curie point=The temperature below which rocks can retain magnetism.

Absolute magnitude=The brightness that a star would appear to have if it were 32 light-years (10 parsecs) away.

Blizzard=Although blizzard is often used to describe any major snow storm with strong winds, the technical definition for a blizzard requires at least 3 hours in duration; low temperatures (usually less than minus 7C or 20F), strong winds (greater than 55 km/h or 35 mph), blowing snow which reduces visibility to less that 1 kilometer (0.6 miles).

Asteroid number=Asteroids are assigned a serial number when they are discovered; it has no particular meaning except that asteroid N+1 was discovered after asteroid N.
Asteroid=One of the many small celestial bodies in orbit around the Sun. Most asteroids orbit between Mars and Jupiter.

Active continental margin=A continental margin characterized by subduction of an oceanic lithospheric plate beneath a continental plate. (syn: Andean margin)
Active volcano=A volcano that is erupting; or one that, while not erupting at the present, has erupted within (geologically) recent time and is considered likely to do so in the (geologically) near future.

Chalk=A very fine grained, soft, earthy, white to gray bioclastic limestone made of the shells and skeletons of marine microorganisms.

Breccia=A course-grained rock, composed of angular, broken rock fragments held together by a mineral cement or a fine-grained matrix.

Angle of repose=The maximum slope or angle at which loose material remains stable.

The type normally used by meteorologists measures the altitude with respect to sea level pressure.

Abrasion=The mechanical wearing and grinding of rock surfaces by friction and impact.

This movement is amplified by a spring or gear mechanism to indicate changes in air pressure on a scale or recording chart.

Ablation area=The lower portion of a glacier where more snow melts in summer than accumulates in winter so there is a net loss of glacial ice. (syn: zone of wastage)
Abrasion=wearing away of a rock by grinding action.

A butte is smaller and more tower-like than a mesa.

The black color is caused by precipitation of very fine grained metal sulfide minerals as hydrothermal solutions cool by contact with sea water.

Basement rock=The older granitic and related metamorphic rocks of the Earth's crust that make up the foundations of continents.

The boiling point of pure water at standard pressure is 100°C or 212°F.
Bolide=An exploding meteorite.

Advection=In meteorology, the horizontal component of a convection current in air, i.e.

Absolute Humidity=The ratio of the mass of water vapor present in the air to the volume occupied by the gas; the density of water vapor in the air, usually expressed as grams of water vapor per cubic meter of air.

A horizon=The uppermost layer of soil composed of a mixture of organic matter and leached and weathered minerals. (syn: topsoil)
Aa=Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments.

Because of their semi-plastic condition, bombs are often modified in shape during their flight or upon impact.

Artificial channel=Any channel dredged to modify the natural channel or to alter the course of a stream.

Dew=Moisture condensed onto objects from the atmosphere, usually during the night, when the ground and leaf surfaces become cooler than the surrounding air.

Absolute Zero=Considered to be the point at which theoretically no molecular activity exists or the temperature at which the volume of a perfect gas vanishes.

Aerosol=Any small particle that is larger than a molecule and suspended in air.

Measuring less than 1/10 inch in diameter, ash may be either solid or molten when first erupted.

Conduction=The transport of heat by atomic or molecular motion.

Carbonate platform=An extensive accumulation of limestone such as the Florida Keys and the Bahamas, formed on a continental shelf in warm regions where sediment does not muddy the water and reef-building organisms thrive.

The oldest known rocks formed at the beginning of or just prior to the start of the Archean Eon.
Arête=A sharp narrow ridge between adjacent valleys formed by glacial erosion.

Absorption nebula=A cold nebula that absorbs light.

Anion=An ion that has a negative charge.

Branching chain reaction=A nuclear fission reaction in which the initial reaction releases two or three neutrons, each of which triggers the fission of additional nuclei.

Atmosphere=One atmosphere is 14.7 pounds per square inch (105 Newtons per square meter); the average atmospheric pressure at sea level on Earth.

Alternative energy resources=All energy resources other than fossil fuels and nuclear fission; including solar energy; hydroelectric power; geothermal energy; wind energy; biomass energy; tidal, wave, and heat energy from the seas; and nuclear fusion.

Branching chain reaction=A nuclear fission reaction in which the initial reaction releases two or three neutrons, each of which triggers the fission of additional nuclei.

Eutrophic lake=A lake characterized by abundant dissolved nitrates, phosphates, and other plant nutrients and by a seasonal deficiency of oxygen in bottom water.

Albedo=Reflectivity of an object; ratio of reflected light to incident light.

When the Moon lies directly between the Earth and the Sun, the Moon blocks our view of the Sun and we observe a solar eclipse.

Adiabatic temperature changes=Temperature changes that occur without gain or loss of heat.

Anticyclonic=The wind circulation pattern in Highs, or anticyclones, that has a sense of rotation opposite to that of cyclones and the Earth's rotation.

Aphelion =The point in its orbit where a planet is farthest from the Sun.
Apogee =The point in orbit farthest from the Earth.

It is the principle ore of aluminum.

A horizon=The uppermost layer of soil composed of a mixture of organic matter and leached and weathered minerals. (syn: topsoil)
Aa=Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments.

Bioclastic sedimentary rock=Sedimentary rocks such as most limestone, that are composed of broken shell fragments and similar remains of living organisms.

Aspect=The orientation of a slope with respect to the Sun; the geographic orientation or exposure of a slope.

Crater=A depression formed by the impact of a meteorite. 2) A depression around the orifice of a volcano.

Oxygen (O2) 20.946%;

Antarctic Ozone Hole=An extended area of extreme depletion of the ozone layer which forms over the southern polar region eachaustral spring (September through November).

Antarctic Ozone Hole=An extended area of extreme depletion of the ozone layer which forms over the southern polar region eachaustral spring (September through November).

Barrier island=A long, narrow, low-lying island that extends parallel to the shoreline.

Aerosol=Any small particle that is larger than a molecule and suspended in air.

Usually, the continental shelf will be wider along a passive continental margin, and narrower along an active margin.

In the northern hemisphere, the Coriolis effect deflects the diverging winds to the right, forming a pinwheel pattern with the air spiraling clockwise.

Andean margin=A continental margin characterized by subduction of an oceanic lithospheric plate beneath a continental plate. (syn: active continental margin)
Andesite=Intermediate volcanic rocks containing 54 to 62 percent silica and moderate amounts of iron and magnesium.

Climate=typical weather patterns over a period of years in a given location
Climate=The composite pattern of long-term weather conditions that can be expected in a given region.

Aquifer=A porous and permeable body of rock that can yield economically significant quantities of ground water.

Petroleum, asphalt, natural mineral wax, and asphaltites are all bitumens.

Condensation=The change of state of water from the vapor to the liquid phase.

Energy=Usable heat or power; in physics, it is the capacity of a physical system to perform work.

Named for J. Christian Doppler, an Austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was going away.

Energy=Usable heat or power; in physics, it is the capacity of a physical system to perform work.

In the northern hemisphere, the Coriolis effect deflects the diverging winds to the right, forming a pinwheel pattern with the air spiraling clockwise.

Abyssal fan=A large, fan-shaped accumulation of sediment deposited at the bases of many submarine canyons adjacent to the deep-sea floor. (syn: submarine fan)
Abyssal plain=A flat, level, largely featureless part of the ocean floor between the mid-oceanic ridge and the continental rise.

Aphelion =The point in its orbit where a planet is farthest from the Sun.
Apogee =The point in orbit farthest from the Earth.

Eccentricity=A value that defines the shape of an ellipse or planetary orbit; the ratio of the distance between the foci and the major axis.

The continental drift theory has been replaced by the more complete plate tectonics theory.

Antarctic Ozone Hole=An extended area of extreme depletion of the ozone layer which forms over the southern polar region eachaustral spring (September through November).

Cost benefit analysis=A system of analysis that attempts to weigh the cost of an act or policy, such as pollution control, directly against the economic benefits.

Capillary action=The action by which water is pulled upward through small pores by electrical attraction to the pore walls.

Aquifer=A porous and permeable body of rock that can yield economically significant quantities of ground water.

Ablation area=The lower portion of a glacier where more snow melts in summer than accumulates in winter so there is a net loss of glacial ice. (syn: zone of wastage)
Abrasion=wearing away of a rock by grinding action.

Cation=A positively charged ion.

The big bang theory postulates that 10 to 20 billion years ago, all matter exploded from an infinitely compressed state.

Comet=An interplanetary body composed of loosely bound rock and ice, that forms a bright head and extended fuzzy tail when it enters the inner potion of the solar system.

Caliche=A hard soil layer formed when calcium carbonate precipitates and cements the soil.

Acid precipitation=A condition in which natural precipitation becomes acidic after reacting with air pollutants.

Claystone=A fine-grained clastic sedimentary rock composed predominantly of clay minerals and small amounts of quartz and other minerals of clay size.

Absolute Humidity=The ratio of the mass of water vapor present in the air to the volume occupied by the gas; the density of water vapor in the air, usually expressed as grams of water vapor per cubic meter of air.

Carbonate rocks=Rocks such as limestone and dolomite made up primarily of carbonate minerals.

The principal gases that compose dry air are Nitrogen (N2) 78.09%;

The oldest known rocks formed at the beginning of or just prior to the start of the Archean Eon.
Arête=A sharp narrow ridge between adjacent valleys formed by glacial erosion.

Calving=A process in which large chunks of ice break off from tidewater glaciers to form icebergs.

Disk=The visible surface of the Sun (or any heavenly body) projected against the sky.

Drought=Abnormal dry weather for a specific area that is sufficiently prolonged for the lack of water to cause serious hydrological imbalance.

Doppler Radar=Weather radar that measures direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar.

Electromagnetic radiation=The transfer of energy by an oscillating electric and magnetic field; it travels as a wave and also behaves as a stream of particles.

Absolute Date=An estimate of the true age of a mineral or rock based on the rate of decay of radioactive minerals.

Cobbles=Rounded rock fragments in the 64 to 256 mm size range, larger than pebbles and smaller than boulders.

Also the thinnest layer in sedimentary rocks, commonly ranging in thickness from a centimeter to a meter or two.

Debris Avalanche=A rapid and unusually sudden sliding or flowage of unsorted masses of rock and other material.

Catastrophism=The model that Earth change occurs through a series of rare but cataclysmic events.

Absolute Date=An estimate of the true age of a mineral or rock based on the rate of decay of radioactive minerals.

Absolute Humidity=The ratio of the mass of water vapor present in the air to the volume occupied by the gas; the density of water vapor in the air, usually expressed as grams of water vapor per cubic meter of air.

Acid precipitation=A condition in which natural precipitation becomes acidic after reacting with air pollutants.

Branching chain reaction=A nuclear fission reaction in which the initial reaction releases two or three neutrons, each of which triggers the fission of additional nuclei.

Control rod=A column of neutron-absorbing alloys that are spaced among fuel rods to fine-tune nuclear fission in a reactor.

Active continental margin=A continental margin characterized by subduction of an oceanic lithospheric plate beneath a continental plate. (syn: Andean margin)
Active volcano=A volcano that is erupting; or one that, while not erupting at the present, has erupted within (geologically) recent time and is considered likely to do so in the (geologically) near future.

This movement is amplified by a spring or gear mechanism to indicate changes in air pressure on a scale or recording chart.

Often these snow storms have high winds reaching 100 km/h which reduce visibility through blowing and drifting snow.

Albedo=Reflectivity of an object; ratio of reflected light to incident light.

Alberta Clipper=A fast moving winter storm originating in the Alberta, Canada region.

E=mc2 Einstein's famous theory of relativity formula known as the energy-mass relation.

Branching chain reaction=A nuclear fission reaction in which the initial reaction releases two or three neutrons, each of which triggers the fission of additional nuclei.

Erosion=The removal of weathered rocks by moving water, wind, ice, or gravity.

Elastic deformation=A deformation such that if the stress is removed, the material springs back to its original size and shape.

Angle of repose=The maximum slope or angle at which loose material remains stable.

Anticline=A fold in rock that resembles an arch; the fold is convex upward and the oldest rocks are in the middle Anticyclone: An area of high pressure, also called a High, around which the winds circulate in a clockwise direction in the Northern Hemisphere (and counterclockwise in the Southern Hemisphere).

Alternative energy resources=All energy resources other than fossil fuels and nuclear fission; including solar energy; hydroelectric power; geothermal energy; wind energy; biomass energy; tidal, wave, and heat energy from the seas; and nuclear fusion.

Altimeter=An instrument used to determine the altitude of an object with respect to a fixed level.

They were created to explain retrograde motion.

Equinox=The date when the sun is directly overhead at noon on the equator, occurring on or around both March 21 and September 22, the former is the vernal (spring) equinox and the latter the autumnal (fall) equinox in the Northern Hemisphere and the reverse on the Southern Hemisphere.

Axis of rotation=An imaginary straight line through a planet's poles on which the planet rotates.

Anticyclonic=The wind circulation pattern in Highs, or anticyclones, that has a sense of rotation opposite to that of cyclones and the Earth's rotation.

Brittle fracture=Rupture that occurs when a rock breaks sharply.

Debris Flow=A mixture of water-saturated rock debris that flows downslope under the force of gravity (also called lahar or mudflow).

Condensation Level=The altitude at which a rising air parcel reaches saturation, usually the cloud base height.

Altimeter=A barometer with a scale calibrated in units of elevation rather than pressure.

Abyssal fan=A large, fan-shaped accumulation of sediment deposited at the bases of many submarine canyons adjacent to the deep-sea floor. (syn: submarine fan)
Abyssal plain=A flat, level, largely featureless part of the ocean floor between the mid-oceanic ridge and the continental rise.

Angular unconformity=An unconformity in which younger sediment or sedimentary rocks rest on the eroded surface of tilted orfolded older rocks.

Blue stars=These are the hottest stars, with a surface temperature of more than 37,000°F.
Body wave=A seismic wave that travel through the interior of the Earth.

Anemometer=an instrument that measures wind speed
Aneriod Barometer=A barometer whose pressure sensor consists of an aneroid capsule, a thin, hollow disk partially evacuated and sealed.

Big Bang Hypothesis=The theory that the universe was originally a single dense sphere of hydrogen that exploded into a gigantic expanding cloud that eventually condensed into separate galaxies.

Catastrophism=The model that Earth change occurs through a series of rare but cataclysmic events.

Asteroid=A rocky space object that can be from a few hundred feet to several hundred km wide.

Braided stream=A stream that divides into a network of branching and reuniting shallow channels separated by mid-channel bars.

Abyssal fan=A large, fan-shaped accumulation of sediment deposited at the bases of many submarine canyons adjacent to the deep-sea floor. (syn: submarine fan)
Abyssal plain=A flat, level, largely featureless part of the ocean floor between the mid-oceanic ridge and the continental rise.

Barrier island=A long, narrow, low-lying island that extends parallel to the shoreline.

Aquifer=A body of rock that contains significant quantities of water that can be tapped by wells or springs.

It is a secondary characteristic of an air mass classification, signified by the small "c" before the primary characteristic, which is based on source region.

Alluvium=A general term for clay, silt, sand, gravel or similar unconsolidated material deposited by a stream or other body of running water.

Alluvium=A general term for clay, silt, sand, gravel or similar unconsolidated material deposited by a stream or other body of running water.

Hot fluids rise, cold fluids sink.

Chalk=A very fine grained, soft, earthy, white to gray bioclastic limestone made of the shells and skeletons of marine microorganisms.

Angle of repose=The maximum slope or angle at which loose material remains stable.

A horizon=The uppermost layer of soil composed of a mixture of organic matter and leached and weathered minerals. (syn: topsoil)
Aa=Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments.

Alpha Centauri=The closest bright star to our solar system.

Alpha Centauri=The closest bright star to our solar system.

Measuring less than 1/10 inch in diameter, ash may be either solid or molten when first erupted.

Bitumen=A general name for solid and semi-solid hydrocarbons that are fusible and soluble in carbon bisulfide.

The black color is caused by precipitation of very fine grained metal sulfide minerals as hydrothermal solutions cool by contact with sea water.

Consumption=Any process that uses water, and then returns it to the Earth far from its source.

Devonian=A period of time in the Paleozoic Era that covered the time span between 408 and 360 million years.

Also defined as a region that supports only a sparse plant cover.

Absolute dating=A technique that geologists use to assign specific dates to rock formations and geologic events.

Absorption spectrum=See "spectrum."

Big Bang Hypothesis=The theory that the universe was originally a single dense sphere of hydrogen that exploded into a gigantic expanding cloud that eventually condensed into separate galaxies.

Core, inner=spherical center of the Earth, about 1200 km in diameter and made of solid iron and nickel.

Cleavage=The way crystalline minerals split or break along an even plane (planar surfaces).

Precipitation is generally at and/or behind the front, and with a fast-moving system, a squall line may develop ahead of the front.

Column=A dripstone or speleothem formed when a stalactite and a stalagmite meet and fuse together.

Column=A dripstone or speleothem formed when a stalactite and a stalagmite meet and fuse together.

Cold front=A front that forms when moving cold air collides with stationary or slower moving warm air.

Alluvial fan=A fan-like accumulation of sediment created where a steep stream slows down rapidly as it reaches a relatively flat valley floor.

Drumlin=An elongate hill formed when a glacier flows over and reshapes a mound of till or stratified drift.

Drumlin=An elongate hill formed when a glacier flows over and reshapes a mound of till or stratified drift.

The most common layer designations are: the troposphere, the stratosphere, the mesosphere and the exosphere.

Alluvial fan=A fan-like accumulation of sediment created where a steep stream slows down rapidly as it reaches a relatively flat valley floor.

Compressive stress=Stress that acts to shorten an object by squeezing it.

Cross cutting relationship=Any relationship in which younger rocks or geological structures interrupt or cut across older rocks or structures.

Active continental margin=A continental margin characterized by subduction of an oceanic lithospheric plate beneath a continental plate. (syn: Andean margin)
Active volcano=A volcano that is erupting; or one that, while not erupting at the present, has erupted within (geologically) recent time and is considered likely to do so in the (geologically) near future.

Disseminated ore deposit=A large low-grade ore deposit in which generally fine-grained metal-bearing minerals are widely scattered throughout a rock body in sufficient concentration to make the deposit economical to mine.

A horizon=The uppermost layer of soil composed of a mixture of organic matter and leached and weathered minerals. (syn: topsoil)
Aa=Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments.

Atmosphere=The envelope of gases that surround a planet's surface held by the planet's gravity.

Aerosol=Any small particle that is larger than a molecule and suspended in air.

Aerosol=Any small particle that is larger than a molecule and suspended in air.

Blizzard=Although blizzard is often used to describe any major snow storm with strong winds, the technical definition for a blizzard requires at least 3 hours in duration; low temperatures (usually less than minus 7C or 20F), strong winds (greater than 55 km/h or 35 mph), blowing snow which reduces visibility to less that 1 kilometer (0.6 miles).

Absolute dating=A technique that geologists use to assign specific dates to rock formations and geologic events.

Basin=A low area of the Earth's crust of tectonic origin, commonly filled with sediment.

Absolute Zero=Considered to be the point at which theoretically no molecular activity exists or the temperature at which the volume of a perfect gas vanishes.

Evacuate=Temporarily move people away from possible danger.

End moraine=A moraine that forms at the end, or terminus of a glacier.

Accreted terrain=A land mass that originated as an island arc or a microcontinent that was later added onto a continent.

Big Bang Hypothesis=The theory that the universe was originally a single dense sphere of hydrogen that exploded into a gigantic expanding cloud that eventually condensed into separate galaxies.

Blackbody temperature=The temperature of an object if it is reradiating all the thermal energy that has been added to it; if an object is not a blackbody radiator, it will not reradiate all the excess heat and the leftover will go toward increasing its temperature.

Dry=bulb thermometer=A thermometer used to measure the ambient temperature.

Carbonate platform=An extensive accumulation of limestone such as the Florida Keys and the Bahamas, formed on a continental shelf in warm regions where sediment does not muddy the water and reef-building organisms thrive.

Angular unconformity=An unconformity in which younger sediment or sedimentary rocks rest on the eroded surface of tilted orfolded older rocks.

Contour interval=the difference in elevation between two consecutive contour lines
Contour line=line on a topographic map that connects points on land that have the same elevation.

The data is then used to record the topography of the sea floor.

Delta=The nearly flat, alluvial, fan-shaped tract of land at the mouth of a stream.

Convection=Fluid (gas or liquid) circulation driven by temperature and density differences; the transfer of heat by this automatic circulation (currents).

Convective zone=The subsurface zone in a star where energy is transmitted primarily by convection.

Convective zone=The subsurface zone in a star where energy is transmitted primarily by convection.

Also refers to thin, transparent ice on road surfaces.

Bed load=That portion of a stream's load that is transported on or immediately above the stream bed.

Branching chain reaction=A nuclear fission reaction in which the initial reaction releases two or three neutrons, each of which triggers the fission of additional nuclei.

Also a synonym for a tropical cyclone, or hurricane.

Ash Flow=A turbulent mixture of gas and rock fragments, most of which are ash-sized particles, ejected violently from a crater or fissure.

Climate=typical weather patterns over a period of years in a given location
Climate=The composite pattern of long-term weather conditions that can be expected in a given region.

The ultimate base level is usually sea level, but this is seldom attained.

Drainage basin=The region that is ultimately drained by a single river.

Chert=A hard, dense, sedimentary rock composed of microcrystalline quartz. (syn: flint)
Chlorofluorocarbons=(CFC's); gases containing chlorine, fluorine and carbon atoms that break down the ozone layer and weaken protection from ultraviolet waves.

Asthenosphere=The shell within the earth, some tens of kilometers below the surface and of undefined thickness, which is a shell of weakness where plastic movements take place to permit pressure adjustments.

Bedrock=The solid rock that underlies soil or regolith.

Differential weathering=The process by which certain rocks weathers more rapidly than adjacent rocks usually resulting in an uneven surface.

Big Bang Hypothesis=The theory that the universe was originally a single dense sphere of hydrogen that exploded into a gigantic expanding cloud that eventually condensed into separate galaxies.

Alluvial fan=A fan-like accumulation of sediment created where a steep stream slows down rapidly as it reaches a relatively flat valley floor.

Absolute Humidity=The ratio of the mass of water vapor present in the air to the volume occupied by the gas; the density of water vapor in the air, usually expressed as grams of water vapor per cubic meter of air.

Climate refers to yearly cycles of temperature, wind, rainfall, etc., and not to daily variations (see "weather").

One other important constituents of air is water vapor (H2O) which varies from 0% to about 4%.

Extend varying distances outward from the exposed continental margins.

Black hole=An object whose gravity is so strong that the escape velocity exceeds the speed of light.

Ash=Fine particles of pulverized rock blown from an explosion vent.

Equinox=The date when the sun is directly overhead at noon on the equator, occurring on or around both March 21 and September 22, the former is the vernal (spring) equinox and the latter the autumnal (fall) equinox in the Northern Hemisphere and the reverse on the Southern Hemisphere.

In meteorology, horizontal air flow is called advection, whereas convection is reserved for vertical air flow.

Most of the May 18, 1980 deposits in the upper valley of the North Fork Toutle River and in the vicinity of Spirit Lake are from the debris avalanche.

Often these snow storms have high winds reaching 100 km/h which reduce visibility through blowing and drifting snow.

Cloud=A visible collection of minute particle matter, such as water droplets and/or ice crystals, in the free air.

Andean margin=A continental margin characterized by subduction of an oceanic lithospheric plate beneath a continental plate. (syn: active continental margin)
Andesite=Intermediate volcanic rocks containing 54 to 62 percent silica and moderate amounts of iron and magnesium.

Active continental margin=A continental margin characterized by subduction of an oceanic lithospheric plate beneath a continental plate. (syn: Andean margin)
Active volcano=A volcano that is erupting; or one that, while not erupting at the present, has erupted within (geologically) recent time and is considered likely to do so in the (geologically) near future.

Absolute Humidity=The ratio of the mass of water vapor present in the air to the volume occupied by the gas; the density of water vapor in the air, usually expressed as grams of water vapor per cubic meter of air.

Cone of depression=A cone-like depression in the water table formed when water is pumped out of a well more rapidly than it can flow through the aquifer.

Alternative energy resources=All energy resources other than fossil fuels and nuclear fission; including solar energy; hydroelectric power; geothermal energy; wind energy; biomass energy; tidal, wave, and heat energy from the seas; and nuclear fusion.

Doppler effect=The apparent change in wavelength of sound or light caused by the motion of the source, observer or both.

Petroleum, asphalt, natural mineral wax, and asphaltites are all bitumens.

A horizon=The uppermost layer of soil composed of a mixture of organic matter and leached and weathered minerals. (syn: topsoil)
Aa=Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments.

Clean Water Act=A federal law mandating the cleaning of the nation's rivers, lakes, and wetlands, and forbidding the discharge of pollutants into waterways.

Named for J. Christian Doppler, an Austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was going away.

Cirrus cloud=A wispy, high-level cloud.

Elastic limit=The maximum stress that an object can withstand without permanent deformation.

Absolute Zero=Considered to be the point at which theoretically no molecular activity exists or the temperature at which the volume of a perfect gas vanishes.