Glossary

Aa flows: A type of lava flow that has a jagged, blocky surface.

Abrasion: The grinding and scraping of a rock surface by the friction and impact of rock particles carried by water, wind, or ice.

Absolute instability: The condition of air that has an environmental lapse rate that is greater than the dry adiabatic rate (1°C per 100 meters).

Absolute magnitude: The apparent brightness of a star if it were viewed from a distance of 10 parsecs (32.6 light-years). Used to compare the true brightness of stars.

Absolute stability: The condition of air that has an environmental lapse rate that is less than the wet adiabatic rate.

Abyssal plains: A very level area of the deep-ocean floor, usually lying at the foot of the continental rise.

Accretionary wedge: A large wedge-shaped mass of sediment that accumulates in subduction zones. Here sediment is scraped from the subducting oceanic plate and accreted to the overriding crustal block.

Active continental margins: A margin that is usually narrow and consists of highly deformed sediments. Such margins occur where oceanic lithosphere is being subducted beneath the margin of a continent.

Adiabatic temperature changes: Cooling or warming of air that occurs because air is allowed to expand or is compressed, not because heat is added or subtracted.

Advection fog: A fog formed when warm, moist air is blown over a cool surface.

Aerosols: Tiny solid and liquid particles suspended in the atmosphere.

Air: The mixture of gases and particles that make up Earth’s atmosphere. Nitrogen and oxygen are most abundant.

Air mass: A large body of air that is characterized by a sameness of temperature and humidity.

Air pressure: Force exerted by the weight of the air above.

Air-mass weather: The conditions experienced in an area as an air mass passes over it. Because air masses are large and fairly homogeneous, air-mass weather will be fairly constant and may last for several days.

Albedo: The reflectivity of a substance, usually expressed as a percentage of the incident radiation reflected.

Alluvial fan: A fan-shaped deposit of sediment formed when a stream’s slope is abruptly reduced.

Alluvium: Unconsolidated sediment deposited by a stream.

Alpine glaciers: A glacier confined to a mountain valley, which in most instances was previously a stream valley.

Altocumulus: White to gray clouds, often made up of separate globules; “sheepback” clouds.

Altostratus: Stratified veil of clouds that is generally thin and may produce very light precipitation. When thin, the Sun or Moon may be visible as a “bright spot,” but no halos are produced.

Ambiguous properties: A property of a mineral that is not diagnostic because it varies among different specimens of the mineral.

Andesite: A gray, fine-grained igneous rock, primarily of volcanic origin and commonly exhibiting a porphyritic texture.

Andesitic composition: A compositional group of igneous rocks, in which the rock contains at least 25 percent dark silicate minerals. The other dominant mineral is plagioclase feldspar.

Aneroid barometer: An instrument for measuring air pressure that consists of evacuated metal chambers that are very sensitive to variations in air pressure.

Angle of repose: The steepest angle at which loose material remains stationary without sliding downslope.

Angular unconformity: An unconformity in which the strata below dip at an angle different from that of the beds above.

Annual temperature range: The difference between the highest and lowest monthly means.

Anticlines: A fold in sedimentary strata that resembles an arch.

Anticyclones: A high-pressure center characterized by a clockwise flow of air in the Northern Hemisphere.

Apparent magnitude: The brightness of a star when viewed from Earth.

Aquifers: Rock, sediment, or soil through which groundwater moves easily.

Aquitards: An impermeable bed that hinders or prevents groundwater movement.

Arctic (A) air masses: Air mass that originates at high polar latitudes. Generally very cold and dry.

Arêtes: A narrow knifelike ridge separating two adjacent glaciated valleys.

Artesian: A situation in which groundwater under pressure rises above the level of the aquifer

Asteroid belt: The region in which most asteroids orbit the Sun between Mars and Jupiter.

Asteroids: One of thousands of small planetlike bodies, ranging in size from a few hundred kilometers to less than 1 kilometer, whose orbits lie mainly between those of Mars and Jupiter.

Asthenosphere: A subdivision of the mantle situated below the lithosphere. This zone of weak material exists below a depth of about 100 kilometers and in some regions extends as deep as 700 kilometers. The rock within this zone is easily deformed.

Astronomical unit (AU): The average distance from Earth to the Sun; 1.5 x 10 to the eighth power kilometers (93 x 10 to the 6th power miles).

Astronomy: The scientific study of the universe; it includes the observation and interpretation of celestial bodies and phenomena.

Atmosphere: The gaseous portion of a planet; the planet’s envelope of air. One of the traditional subdivisions of Earth’s physical environment.

Atomic number: The number of protons in the nucleus of an atom.

Atoms: The smallest particle that exists as an element.

Augite: A black, opaque silicate mineral of the pyroxene group that is a dominant component of basalt.

Autumnal: See Equinox.

Back swamps: A poorly drained area on a floodplain that results when natural levees are present.

Bajada: An apron of sediment along a mountain front created by the coalescence of alluvial fans.

Barometric: See Pressure tendency.

Barred spiral galaxies: A galaxy that has straight arms extending from its nucleus.

Barrier islands: A low, elongated ridge of sand that parallels the coast.

Bars: In a stream channel, a common term for deposits of sand and gravel. In meteorology, a unit of air pressure, where standard sea level pressure is defined as 1 bar.

Basalt: A fine-grained igneous rock of mafic composition.

Basalt plateaus: Vast accumulations of basaltic lava resulting in a flat, broad plain of great thickness. See also Flood basalts.

Basaltic composition: A compositional group of igneous rocks in which the rock contains substantial dark silicate minerals and calcium-rich plagioclase feldspar.

Base level: The level below which a stream cannot erode.

Basins: A circular downfolded structure.

Batholiths: A large mass of igneous rock that formed when magma was emplaced at depth, crystallized, and subsequently exposed by erosion.

Bathymetry: The measurement of ocean depths and the charting of the topography of the ocean floor.

Baymouth bar: A sandbar that completely crosses a bay, sealing it off from the open ocean.

Beach: An accumulation of sediment found along the landward margin of the ocean or a lake.

Beach drift: The transport of sediment in a zigzag pattern along a beach caused by the uprush of water from obliquely breaking waves.

Beach nourishment: The addition of large quantities of sand to a beach system to offset losses caused by wave erosion.

Bed load: Sediment that is carried by a stream along the bottom of its channel.

Bergeron process: A theory that relates the formation of precipitation to supercooled clouds, freezing nuclei, and the different saturation levels of ice and liquid water.

Big Bang theory: A theory which proposes that the universe originated as a single mass, which subsequently exploded.

Biochemical sediments: A sedimentary rock composed of material that was extracted from water by organisms to create hard parts such as shells.

Biogenous: Seafloor sediment consisting of material of marine-organic origin.

Biosphere: The totality of life on Earth; the parts of the lithosphere, hydrosphere, and atmosphere in which living organisms can be found.

Biotite: A dark, iron-rich mineral and a member of the mica family with excellent cleavage.

Black holes: A massive star that has collapsed to such a small volume that its gravity prevents the escape of all radiation.

Blowouts: A depression excavated by the wind in easily eroded deposits.

Body waves: Seismic waves that travel through Earth’s interior.

Bowen’s reaction series: A concept, proposed by N. L. Bowen, illustrating the relationships between magma and the minerals crystallizing from it during the formation of igneous rocks.

Braided channels: Type of alluvial channel consisting of a broad network of diverging and converging channels. Forms where the stream’s load includes abundant coarse material and the discharge is highly variable.

Breakwater: A structure that protects a near-shore area from breaking waves.

Breccia: A sedimentary rock composed of angular fragments that were lithified.

Brittle deformation: The loss of strength by a material, usually in the form of sudden fracturing.

Calcite: Calcium carbonate (CaCO3), one of the two most common carbonate minerals.

Calderas: A large depression typically caused by collapse or ejection of the summit area of a volcano.

Calorie: The amount of energy needed to raise the temperature of 1 gram of water through 1 degree Centigrade.

Capacity: The total amount of sediment a stream is able to transport.

Carbonic acid: A weak acid formed when carbon dioxide is dissolved in water. It plays an important role in chemical weathering.

Catastrophism: The concept that Earth was shaped by catastrophic events of a short-term nature.

Caverns: A naturally formed underground chamber or series of chambers most commonly produced by solution activity in limestone.

Celestial sphere: An imaginary hollow sphere on which the ancients believed the stars were hung and were carried around Earth.

Cementation: One way in which sedimentary rocks are lithified. As material precipitates from water that percolates through the sediment, open spaces are filled and particles are joined into a solid mass.

Cenozoic: A time span on the geologic calendar beginning about 66 million years ago, following the Mesozoic era.

Chemical bond: A strong attractive force that exists between atoms in a substance. It involves the transfer or sharing of electrons that allows each atom to attain a full valence shell.

Chemical compounds: A substance formed by the chemical combination of two or more elements in definite proportions and usually having properties different from those of its constituent elements.

Chemical sedimentary rocks: Sedimentary rock consisting of material that was precipitated from water by either inorganic or organic means.

Chemical weathering: The processes by which the internal structure of a mineral is altered by the removal and/or addition of elements.

Chinooks: A wind that blows down the leeward side of a mountain and that warms by compression.

Cinder cones: A rather small volcano built primarily of pyroclastics ejected from a single vent.

Circle of illumination: The great circle that separates daylight from darkness.

Circulation: Movements of ocean water caused by density differences brought about by variations in temperature and salinity.

Circum-Pacific belt: The zone of intense seismic activity that encompasses the coastal regions of Chile, Central America, Indonesia, Japan, and Alaska, including the Aleutian Islands.

Cirque: An amphitheater-shaped basin at the head of a glaciated valley produced by frost wedging and plucking.

Cirrocumulus: Thin, white, ice-crystal clouds that take the form of ripples, waves, or globular masses all in a row. May produce a “mackerel sky.” Least common of high clouds.

Cirrostratus: Thin sheet of white, ice-crystal clouds that may give the sky a milky look. Sometimes produce halos around the Sun and Moon.

Cirrus: As one of the three basic cloud forms, refers to high, thin ice-crystal clouds. As one of the three high cloud types, refers to thin, delicate, fibrous, ice-crystal clouds, which sometimes take the form of hooked filaments called “mares’ tails.”

Clay: A group of light-colored silicates that typically form as products of chemical weathering of igneous rocks. Major components of soil and sedimentary rocks. Kaolinite is a common clay mineral derived from the weathering of feldspar.

Cleavage: The tendency of a mineral to break along planes of weak bonding.

Climate: A description of aggregate weather conditions; the sum of all statistical weather information that helps describe a place or region.

Clouds: A form of condensation best described as a dense concentration of suspended water droplets or tiny ice crystals.

Clouds of vertical development: Clouds having bases in the low height range but extending upward into the middle or high altitudes.

Coal: A sedimentary rock consisting primarily of organic matter, formed in stages from accumulations of large quantities of undecayed plant material. Used as a fossil fuel.

Coarse-grained texture: An igneous rock texture in which the crystals are roughly equal in size and large enough that individual minerals can be identified with the unaided eye.

Cold front: A front along which a cold air mass thrusts beneath a warmer air mass.

Collision–coalescence process: A theory of raindrop formation in warm clouds (above 0°C [32°F]) in which large cloud droplets (giants) collide and join together with smaller droplets to form raindrops. Opposite electrical charges may bind the cloud droplets together.

Color: An obvious mineral characteristic that is often unreliable as a diagnostic property.

Columnar jointing: A pattern of cracks that form during cooling of molten rock, resulting in columns that are generally six sided.

Coma: The fuzzy, gaseous component of a comet’s head.

Comet: A small body that generally revolves around the Sun in an elongated orbit.

Compaction: A type of lithification in which the weight of overlying material compresses more deeply buried sedi-ment. It is most important in the fine-grained sedimentary rocks such as shale.

Competence: A measure of the largest particle a stream can transport; a factor dependent on velocity.

Composite volcanoes: A volcano composed of both lava flows and pyroclastic material.

Concordant: A term used to describe intrusive igneous masses that form parallel to the bedding of the surrounding rock.

Condensation: The change of state from a gas to a liquid.

Conditional instability: The condition of moist air with an environmental lapse rate between the dry and wet adiabatic rates.

Conduction: The transfer of heat through matter by molecular activity. Energy is transferred through collisions from one molecule to another.

Conduit: A pipelike opening through which magma moves toward Earth’s surface. It terminates at a surface opening called a vent.

Cone of depression: A cone-shaped depression in the water table immediately surrounding a well.

Confined aquifer: An aquifer that has impermeable layers (aquitards) both above and below.

Confining pressure: Stress that is applied uniformly in all directions.

Conformable: Having layers of rock deposited without interruption.

Conglomerate: A sedimentary rock composed of rounded, gravel-size particles.

Contact metamorphism: Changes in rock caused by the heat from a nearby magma body. Also known as thermal metamorphism.

Continental (c) air masses: An air mass that forms over land; it is normally relatively dry.

Continental drift hypothesis: A theory which originally proposed that the continents are rafted about. It has essentially been replaced by the plate tectonics theory.

Continental margins: The portion of the seafloor adjacent to the continents. It may include the continental shelf, continental slope, and continental rise.

Continental rift: A linear zone along which continental lithosphere stretches and pulls apart. Its creation may mark the beginning of a new ocean basin.

Continental rise: The gently sloping surface at the base of the continental slope.

Continental shelf: The gently sloping submerged portion of the continental margin that extends from the shoreline to the continental slope.

Continental slope: The steep gradient that leads to the deep-ocean floor and marks the seaward edge of the continental shelf.

Continental volcanic arc: Mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent. Examples include the Andes and the Cascades.

Continental volcanic arcs: Mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent. Examples include the Andes and the Cascades.

Convection: The transfer of heat by the movement of a mass or substance. It can take place only in fluids.

Convergence: The condition that exists when the distribution of winds within a given area results in a net horizontal inflow of air into the area. Since convergence at lower levels is associated with an upward movement of air, areas of convergent winds are regions favorable to cloud formation and precipitation.

Convergent plate boundaries: A boundary in which two plates move together, causing one of the slabs of lithosphere to be consumed into the mantle as it descends beneath an overriding plate.

Coquina: A coarse rock composed of loosely cemented shells and shell fragments.

Core: The innermost layer of Earth, based on composition. It is thought to be largely an iron–nickel alloy, with minor amounts of oxygen, silicon, and sulfur.

Coriolis effect: The deflective force of Earth’s rotation on all free-moving objects, including the atmosphere and oceans. Deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Also called Coriolis force.

Correlation: The establishment of the equivalence of rocks of similar age in different areas.

Cosmological redshifts: Changes in the spectra of galaxies which indicate that they are moving away from the Milky Way as a result of the expansion of space.

Cosmology: The study of the universe, including its properties, structure, and evolution.

Country rock: Pre-existing crustal rocks intruded by magma. Host rock may be displaced or assimilated by magmas.

Covalent bonds: A chemical bond produced by the sharing of electrons.

Crater: The depression at the summit of a volcano, or a depression that is produced by a meteorite impact.

Crevasses: A deep crack in the brittle surface of a glacier.

Cross beds: Relatively thin layers that are inclined at an angle to the main bedding. Formed by currents of wind or water.

Crust: The very thin outermost layer of Earth.

Cryovolcanism: The eruption of magmas derived from the partial melting of frozen volatiles (ices) instead of silicate rocks.

Crystal settling: The downward movement of minerals during the crystallization of magma. The earlier-formed minerals are denser than the liquid portion and settle to the bottom of the magma chamber.

Crystal shape: See Habit.

Cumulonimbus: Towering clouds, sometimes spreading out on top to form an “anvil head.” Associated with heavy rainfall, thunder, lightning, hail, and tornadoes.

Cumulus: As one of the three basic cloud forms, refers to clouds that consist of globular masses that are often described as cottonlike in appearance. As one of the two types of clouds of vertical development, refers to dense, billowy clouds often characterized by flat bases, which may occur as isolated clouds or may be closely packed.

Cup anemometer: An instrument used to determine wind speed.

Curie point: The temperature above which a material loses its magnetization.

Cut bank: The area of active erosion on the outside of a meander.

Cutoff: A short channel segment created when a river erodes through the narrow neck of land between meanders.

Cyclones: A low-pressure center characterized by a counterclockwise flow of air in the Northern Hemisphere.

Daily mean: A statistic that is determined by averaging the hourly readings or, more commonly, by averaging the maximum and minimum temperatures for a day.

Daily temperature range: The difference between the maximum and minimum for a day (for instance, of temperature).

Dark energy: A hypothetical form of energy that produces a force that opposes gravity and is thought to be the cause of the accelerating expansion of the universe.

Dark matter: Undetected matter that is thought to exist in great quantities in the universe.

Dark silicate minerals: A silicate mineral that contains ions of iron and/or magnesium in its structure. It is dark in color and has a higher specific gravity than a light silicate mineral.

Decompression melting: Melting that occurs as rock ascends due to a drop in confining pressure.

Deep-ocean basin: The portion of the seafloor that lies between the continental margin and the oceanic ridge. This region comprises almost 30 percent of Earth’s surface.

Deep-ocean trenches: A narrow, elongated depression on the floor of the ocean.

Deep-sea fan: A cone-shaped deposit at the base of the continental slope. The sediment is transported to the fan by turbidity currents that follow submarine canyons.

Deflation: The lifting and removal of loose material by wind.

Deformation: General term for the processes of folding, faulting, shearing, compression, or extension of rocks.

Delta: An accumulation of sediment formed where a stream enters a lake or an ocean.

Density: The weight per unit volume of a particular material.

Deposition: The process by which water vapor is changed directly to a solid, without passing through the liquid state.

Desert: One of the two types of dry climate; the driest of the dry climates.

Dew-point temperature: The temperature to which air has to be cooled in order to reach saturation. Often shortened to dew point.

Diagnostic properties: A property of a mineral that aids in mineral identification. Taste or feel, crystal shape, and streak are examples of diagnostic properties.

Differential stress: Forces that are unequal in different directions.

Diffused light: Solar energy is scattered and reflected in the atmosphere and reaches Earth’s surface in the form of diffuse blue light from the sky.

Dikes: A tabular-shaped intrusive igneous feature that cuts through the surrounding rock.

Diorite: A coarse-grained, intrusive igneous rock primarily composed of plagioclase feldspar and amphibole minerals.

Dip-slip faults: A fault in which the movement is parallel to the dip of the fault.

Discharge: The quantity of water in a stream that passes a given point in a period of time.

Disconformity: A type of unconformity in which the beds above and below are parallel.

Discordant: A term used to describe plutons that cut across existing rock structures, such as bedding planes.

Dissolved load: The portion of a stream’s load that is carried in solution.

Distributaries: A section of a stream that leaves the main flow.

Divergence: The condition that exists when the distribution of winds within a given area results in a net horizontal outflow of air from the region. In divergence at lower levels, the resulting deficit is compensated for by a downward movement of air from aloft; hence, areas of divergent winds are unfavorable to cloud formation and precipitation.

Divergent plate boundaries: A region where the rigid plates are moving apart, typified by the mid-ocean ridges.

Divide: An imaginary line that separates the drainage of two streams; often found along a ridge.

Dolomite: Calcium/magnesium carbonate, CaMg(CO3)2, one of the two most common carbonate minerals.

Dome: A roughly circular upfolded structure similar to an anticline.

Doppler radar: A new generation of radar that can handle the tasks performed by conventional radar and also detect motion directly. It greatly improves tornado and severe storm warnings.

Drainage basin: The land area that contributes water to a stream. Also called a watershed.

Drawdown: The difference in height between the bottom of a cone of depression and the original height of the water table.

Drizzle: Precipitation from stratus clouds consisting of tiny droplets.

Drumlins: A streamlined asymmetrical hill composed of glacial till. The steep side of the hill faces the direction from which the ice advanced.

Dry adiabatic rate: The rate of adiabatic cooling or warming in unsaturated air. The rate of temperature change is 1°C per 100 meters.

Dry climate: A climate in which yearly precipitation is less than the potential loss of water by evaporation.

Ductile deformation: A type of solid-state flow that produces a change in the size and shape of a rock body without fracturing. Occurs at depths where temperatures and confining pressures are high.

Dunes: A hill or ridge of wind-deposited sand.

Dwarf galaxies: A very small galaxy, usually elliptical and lacking spiral arms.

Dwarf planets: A celestial body that orbits stars, massive enough to be spherical but has not cleared its neighboring region of planetesimals.

Earth science: The name for all the sciences that collectively seek to understand Earth. It includes geology, oceanography, meteorology, and astronomy.

Earth system: Earth viewed as a dynamic system of interacting parts and processes, including the geosphere, hydrosphere, biosphere, and atmosphere.

Earth system science: An interdisciplinary study that seeks to examine Earth as a system composed of numerous interacting parts or subsystems.

Earthquake: Vibration of Earth produced by the rapid release of energy.

Echo sounders: An instrument used to determine the depth of water by measuring the time interval between emission of a sound signal and the return of its echo from the bottom.

Economic minerals: A mineral used extensively in the manufacture of products.

EF scale: A scale originally developed by Theodore Fujita for classifying the severity of a tornado, based on the correlation of wind speed and the degree of destruction.

Elastic deformation: Rock deformation in which the rock will return to nearly its original size and shape when the stress is removed.

Elastic rebound: The sudden release of stored strain in rocks that results in movement along a fault.

Electromagnetic radiation: Transfer of energy in the form of light and related types of radiation, including gamma rays, x rays, ultraviolet light, infrared light, microwaves, and radio waves.

Electrons: A negatively charged subatomic particle that has a negligible mass and is found outside an atom’s nucleus

Element: A substance that cannot be decomposed into simpler substances by ordinary chemical or physical means.

Elliptical galaxies: A galaxy that is round or elliptical in outline. It contains little gas and dust, no disk or spiral arms, and few hot, bright stars.

Emergent coasts: A coast where land that was formerly below sea level has been exposed either because of crustal uplift or a drop in sea level or both.

End moraine: A ridge of till that marks a former position on the front of a glacier.

Enhanced Fujita intensity scale: A scale originally developed by Theodore Fujita for classifying the severity of a tornado, based on the correlation of wind speed and the degree of destruction.

Environmental lapse rate: The rate of temperature decrease with increasing height in the troposphere.

Eons: The largest time unit on the geologic time scale, next in order of magnitude above era.

Ephemeral streams: A stream that is usually dry because it carries water only in response to specific episodes of rainfall. Most desert streams are of this type.

Epicenter: The location on Earth’s surface that lies directly above the forces of an earthquake.

Epochs: A unit of the geologic calendar that is a subdivision of a period.

Equatorial low: A belt of low pressure that lies near the equator and between the subtropical highs.

Equinox: See Equinox.

Era: A major division on the geologic calendar; eras are divided into shorter units called periods.

Eras: A major division on the geologic calendar; eras are divided into shorter units called periods.

Eruption columns: A buoyant plume of hot, ash-laden gases that can extend thousands of meters into the atmosphere.

Escape velocity: The initial velocity an object needs to escape from the surface of a celestial body.

Eskers: A sinuous ridge composed largely of sand and gravel deposited by a stream flowing in a tunnel within or beneath a glacier near its terminus.

Estuaries: A funnel-shaped inlet of the sea that formed when a rise in sea level or subsidence of land caused the mouth of a river to be flooded.

Evaporation: The process of converting a liquid to a gas.

Evaporite deposits: A sedimentary rock formed of material deposited from solution by evaporation of the water.

Evapotranspiration: The combined effect of evaporation and transpiration.

Exfoliation domes: A large, dome-shaped structure, usually composed of granite, formed by sheeting.

Extrusive rocks: Igneous rock formed when magma solidifies at Earth’s surface.

Eye: A zone of scattered clouds and calm averaging about 20 kilometers in diameter at the center of a hurricane.

Eye wall: The doughnut-shaped area of intense cumulonimbus development and very strong winds that surrounds the eye of a hurricane.

Fault creep: Gradual displacement along a fault. Such activity occurs relatively smoothly and with little noticeable seismic activity.

Fault scarps: A cliff created by movement along a fault. It represents the exposed surface of the fault prior to modification by weathering and erosion.

Fault-block mountains: A mountain formed by the displacement of rock along a fault.

Faults: A break in a rock mass along which movement has occurred.

Felsic: A compositional group of igneous rocks in which the rock is made up almost entirely of light-colored silicates.

Fetch: The distance that the wind has traveled across the open water.

Fine-grained texture: A texture of igneous rocks in which the crystals are too small for individual minerals to be distinguished with the unaided eye.

Fiords: A steep-sided inlet of the sea that formed when a glacial trough was partially submerged.

Fissure: A crack in rock along which there is a distinct separation.

Fissure eruptions: An eruption in which lava is extruded from narrow fractures or cracks in the crust.

Flood: The overflow of a stream channel that occurs when discharge exceeds the channel’s capacity. A flood is the most common and destructive geologic hazard.

Flood basalts: Flows of basaltic lava that issue from numerous cracks or fissures and that commonly cover extensive areas to thicknesses of hundreds of meters.

Floodplain: A flat, low-lying portion of a stream valley that is subject to periodic inundation.

Focus: The zone within Earth where rock displacement produces an earthquake. Also known as a hypocenter.

Fog: A cloud with its base at or very near Earth’s surface.

Folds: A bent rock layer or series of layers that were originally horizontal and subsequently deformed.

Foliation: A texture of metamorphic rocks that gives the rock a linear or layered appearance.

Footwall block: The rock surface below a fault.

Fossil assemblage: A set of fossil organisms found together in a given layer; used in relative dating.

Fossils: The remains or traces of an organism preserved from the geologic past.

Fracture: Any break or rupture in rock along which no appreciable movement has taken place.

Fracture zones: A linear zone of irregular topography on the deep-ocean floor that follows transform faults and their inactive extensions.

Fragmental texture: An igneous rock texture resulting from the consolidation of individual rock fragments that are ejected during a violent eruption.

Freezing nuclei: Solid particles that serve as cores for the formation of ice crystals.

Freezing rain: A coating of ice on objects formed when supercooled rain freezes on contact.

Frontal (precipitation) fog: Fog formed when rain evaporates as it falls through a layer of cool air.

Frontal lifting: Lifting of air that results when cool air acts as a barrier over which warmer, lighter air will rise.

Frontal wedging: Lifting of air that results when cool air acts as a barrier over which warmer, lighter air will rise.

Fronts: The boundary between two adjoining air masses that have contrasting characteristics.

Frost wedging: The mechanical breakup of rock caused by the expansion of freezing water in cracks and crevices.

Fumaroles: A vent in a volcanic area from which fumes or gases escape.

Gabbro: A dark-green to black intrusive igneous rock composed of dark silicate minerals. Gabbro makes up a significant percentage of oceanic crust.

Galaxies: A collection of interstellar matter, stars, and star remains that are gravitationally bound to one another.

Garnet: A dark silicate mineral with a glassy luster, lacking cleavage and of varying colors; can be used as a gemstone.

Geocentric: The concept of an Earth-centered universe.

Geologic time: The span of time since the formation of Earth.

Geologic time scale: The division of Earth history into blocks of time, such as eons, eras, periods, and epochs. The time scale was originally created using relative dating principles.

Geology: The science that examines Earth, its form and composition, and the changes it has undergone and is undergoing.

Geosphere: The solid Earth; one of Earth’s four basic spheres.

Geostrophic winds: A wind, usually above a height of 600 meters (2000 feet), that blows parallel to the isobars.

Geothermal gradient: The gradual increase in temperature with depth in the crust. The average is 30°C per kilometer in the upper crust.

Giants: In astronomy: a luminous star of large radius.

Glacial budget: The balance, or lack of balance, between ice formation at the upper end of a glacier and ice loss in the zone of wastage.

Glacial drift: An all-embracing term for sediments of glacial origin, no matter how, where, or in what shape they were deposited. Also known as drift.

Glacial erratics: An ice-transported boulder that was not derived from bedrock near its present site.

Glacial striations: A scratch or groove on bedrock caused by glacial abrasion.

Glacial trough: A mountain valley that has been widened, deepened, and straightened by a glacier.

Glacier: A thick mass of ice that originates on land from the compaction and recrystallization of snow and that shows evidence of past or present flow.

Glassy texture: A term used to describe the texture of certain igneous rocks, such as obsidian, that contain no crystals.

Glaze: See freezing rain. A coating of ice on objects formed when supercooled rain freezes on contact.

Gneiss: Medium- to coarse-grained banded metamorphic rocks in which granular and elongated minerals dominate.

Grabens: A valley formed by the downward displacement of a fault-bounded block.

Gradient: The slope of a stream; generally measured in feet per mile.

Granite: A coarse-grained igneous rock of approximately 10–20 percent quartz and 50 percent potassium feldspar that forms where large masses of magma solidify at depth. Used as a building material.

Granitic composition: A compositional group of igneous rocks in which the rock is made up almost entirely of light-colored silicates.

Greenhouse effect: The transmission of short-wave solar radiation by the atmosphere coupled with the selective absorption of longer-wavelength terrestrial radiation, especially by water vapor and carbon dioxide.

Groin: A short wall built at a right angle to the shore to trap moving sand.

Ground moraine: An undulating layer of till deposited as the ice front retreats.

Groundmass: The matrix of smaller crystals within an igneous rock that has porphyritic texture.

Groundwater: Water in the zone of saturation.

Guyots: A submerged flat-topped seamount. Also known as a tablemount.

Gypsum: Nonsilicate mineral commonly found in sedimentary rocks used to manufacture plaster and similar building materials.

Gyres: The large, circular surface-current pattern found in each ocean.

Habit: The common or characteristic shape of a crystal or aggregate of crystals. Also known as crystal shape.

Hail: Nearly spherical ice pellets having concentric layers and formed by the successive freezing of layers of water.

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

Halite: Mineral name for common table salt (NaCl); a nonsilicate mineral commonly found in sedimentary rocks.

Hanging valleys: A tributary valley that enters a glacial trough at a considerable height above its floor.

Hanging wall block: The rock surface above a fault.

Hard stabilization: An artificial structure built to protect a coast or to prevent the movement of sand along a beach. Examples include groins, jetties, breakwaters, and seawalls.

Hardness: The resistance a mineral offers to scratching.

Heat: The kinetic energy of random molecular motion.

Heliocentric: The view that the Sun is at the center of the solar system.

Hertzsprung–Russell diagram (H-R diagram): A plot of stars according to their absolute magnitudes and spectral types.

High clouds: Clouds that normally have their base above 6000 meters; the base may be lower in winter and at high-latitude locations.

Highs: A high-pressure center characterized by a clockwise flow of air in the Northern Hemisphere.

Hornblende: A dark green to black mineral of the amphibole group, often found in igneous rocks.

Horns: A pyramid-like peak formed by glacial action in three or more cirques surrounding a mountain summit.

Horsts: An elongated, uplifted block of crust bounded by faults.

Host rock: Pre-existing crustal rocks intruded by magma. Host rock may be displaced or assimilated by magmas.

Hot spot: A concentration of heat in the mantle that is capable of producing magma, which in turn extrudes onto Earth’s surface. The intraplate volcanism that produced the Hawaiian islands is one example.

Hot-spot track: A chain of volcanic structures produced as a lithospheric plate moves over a mantle plume.

Hubble’s law: A law that relates the distance to a galaxy and its velocity.

Humidity: A general term referring to water vapor in the air but not to liquid droplets of fog, cloud, or rain.

Hurricanes: A tropical cyclonic storm that has winds in excess of 119 kilometers (74 miles) per hour.

Hydrogen fusion: A nuclear reaction in which hydrogen nuclei are fused into helium nuclei.

Hydrologic cycle: The unending circulation of Earth’s water supply. The cycle is powered by energy from the Sun and is characterized by continuous exchanges of water among the oceans, the atmosphere, the geosphere, and the biosphere.

Hydrosphere: The water portion of our planet; one of the traditional subdivisions of Earth’s physical environment.

Hygrometers: An instrument designed to measure relative humidity.

Hygroscopic nuclei: Condensation nuclei that have a high affinity for water, such as salt particles.

Hypocenter: The zone within Earth where rock displacement produces an earthquake.

Hypothesis: A tentative explanation that is tested to deter-mine whether it is valid.

Ice caps: A mass of glacial ice covering a high upland or plateau and spreading out radially.

Ice sheets: A very large, thick mass of glacial ice flowing outward in all directions from one or more accumulation centers.

Icebergs: A mostly submerged mass of floating ice created by the calving of a glacier.

Igneous rocks: A rock formed by the crystallization of molten magma.

Impact craters: A depression resulting from collisions with bodies such as asteroids and comets.

Incised meanders: A meandering channel that flows in a steep, narrow valley. It forms either when an area is uplifted or when the base level drops.

Inclination of the axis: The tilt of Earth’s axis from the perpendicular to the plane of Earth’s orbit.

Inclusions: A piece of one rock unit contained within another. Inclusions are useful in relative dating.

Index fossils: A fossil that is associated with a particular span of geologic time.

Inertia: A property by which objects at rest tend to remain at rest, and objects in motion tend to stay in motion unless either is acted upon by an outside force.

Infiltration: The movement of surface water into rock or soil through cracks and pore spaces.

Infrared radiation: Radiation with a wavelength from 0.7 to 200 micrometers.

Inner core: The solid innermost layer of Earth, about 1300 kilometers (800 miles) in radius.

Intensity: A measure of the degree of earthquake shaking at a given locale, based on the amount of damage.

Interface: A common boundary where different parts of a system interact.

Interior drainage: A discontinuous pattern of intermittent streams that do not flow to the ocean.

Intermediate composition: A compositional group of igneous rocks, in which the rock contains at least 25 percent dark silicate minerals. The other dominant mineral is plagioclase feldspar.

Intertropical convergence zone (ITCZ): The zone of general convergence between the Northern and Southern Hemisphere trade winds.

Intraplate: Igneous activity that occurs within a tectonic plate away from plate boundaries.

Intrusions: A structure that results from the emplacement and crystallization of magma beneath the surface of Earth. See Pluton.

Intrusive rocks: Igneous rock that formed below Earth’s surface.

Ionic bond: A chemical bond between two oppositely charged ions formed by the transfer of valence electrons from one atom to the other.

Ions: An atom or molecule that possesses an electrical charge.

Irregular galaxies: A galaxy that lacks symmetry.

Island arc: A chain of volcanic islands generally located a few hundred kilometers from a trench where there is active subduction of one oceanic plate beneath another. Also known simply as an island arc.

Island arcs: See Volcanic island arc.

Isobars: A line drawn on a map that connects points of equal atmospheric pressure, usually corrected to sea level.

Isotherm: A line on a diagram that connect points of equal temperature.

Jet streams: Swift, high-altitude winds (120–240 kilometers per hour).

Joints: A fracture in rock along which there has been no movement.

Jovian (Jupiter-like) planets: The Jupiter-like planets Jupiter, Saturn, Uranus, and Neptune. These planets have relatively low densities. Also known as the outer planets.

Kames: A steep-sided hill composed of sand and gravel that originates when sediment collects in openings in stagnant glacial ice.

Karst topography: A topography that consists of numerous depressions called sinkholes.

Kettles: A depression created by a block or blocks of ice becoming lodged in a glacial deposit and subsequently melting.

Kuiper belt: A region outside the orbit of Neptune where most short-period comets are thought to originate.

Laccoliths: A massive igneous body intruded between preexisting strata.

Lahar: Mudflows on the slopes of volcanoes that result when unstable layers of ash and debris become saturated and flow downslope, usually following stream channels.

Lake-effect snows: Snow showers associated with a cP air mass to which moisture and heat are added from below as it traverses a large and relatively warm lake (such as one of the Great Lakes), rendering the air mass humid and unstable.

Laminar flow: The movement of water particles in straight-line paths that are parallel to the channel. The water particles move downstream without mixing.

Land breeze: A local wind that blows from land toward the water during the night in coastal areas.

Latent heat: The energy absorbed or released during a change in state.

Lateral moraines: A ridge of till along the sides of an alpine glacier composed primarily of debris that fell to the glacier from the valley walls.

Lava: Magma that reaches Earth’s surface.

Lava tubes: A tunnel in hardened lava that acts as a horizontal conduit for lava flowing from a volcanic vent. Lava tubes allow fluid lavas to advance great distances.

Law of universal gravitation: Newton’s law stating that any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Lifting condensation level (LCT): The height at which rising air that is cooling at the dry adiabatic rate becomes saturated and condensation begins.

Light silicate minerals: A silicate mineral that lacks iron and/or magnesium. It is generally lighter in color and has a lower specific gravity than a dark silicate mineral.

Light-year: The distance light travels in a year; about 6 trillion miles.

Limestone: A chemical sedimentary rock composed chiefly of calcite. Limestone can form by inorganic means or from biochemical processes.

Liquefaction: A phenomenon, sometimes associated with earthquakes, in which soils and other unconsolidated materials containing abundant water are turned into a fluidlike mass that is not capable of supporting buildings.

Lithification: The process, generally cementation and/or compaction, of converting sediments to solid rock.

Lithosphere: The rigid outer layer of Earth, including the crust and upper mantle.

Lithospheric plates: A coherent unit of Earth’s rigid outer layer that includes the crust and uppermost mantle. Also known as a tectonic plate.

Local Group: The cluster of 20 or so galaxies to which our galaxy belongs.

Local winds: A small-scale wind produced by a locally generated pressure gradient.

Localized convective lifting: Unequal surface heating that causes localized pockets of air (thermals) to rise because of their buoyancy.

Loess: Deposits of windblown silt that are lacking visible layers, generally buff-colored, and capable of maintaining a nearly vertical cliff.

Longitudinal profile: A cross section of a stream channel along its descending course from the head to the mouth.

Longshore currents: A near-shore current that flows parallel to the shore.

Low clouds: Clouds that form below a height of about 2000 meters.

Lows: A low-pressure center characterized by a counterclockwise flow of air in the Northern Hemisphere.

Lunar highlands: The extensively cratered highland areas of the Moon. Also known as terrae.

Lunar regolith: A thin, gray layer on the surface of the Moon, consisting of loosely compacted, fragmented material believed to have been formed by repeated meteoric impacts.

Luster: The appearance or quality of light reflected from the surface of a mineral.

Mafic: A compositional group of igneous rocks in which the rock contains substantial dark silicate minerals and calcium-rich plagioclase feldspar.

Magma: A body of molten rock found at depth, including any dissolved gases and crystals.

Magmatic differentiation: The process of generating more than one rock type from a single magma.

Magnetic reversal: A change in the polarity of Earth’s magnetic field that occurs over time intervals of roughly 200,000 years.

Magnetic time scale: The history of magnetic reversals through geologic time.

Magnetometers: A sensitive instrument used to measure the intensity of Earth’s magnetic field.

Magnitude: In seismology, the total amount of energy released during an earthquake.

Magnitude scales: In seismology, the total amount of energy released during an earthquake.

Main-sequence stars: A sequence of stars on the Hertzsprung-Russell diagram, containing the majority of stars, that runs diagonally from the upper left to the lower right.

Mantle: The 2900-kilometer- (1800-mile-) thick layer of Earth located below the crust.

Mantle plume: Structures that originate at great depth and, upon reaching the crust, spread laterally, creating a localized volcanic zone called a hot spot. A mantle plume is a source of some intraplate basaltic magma.

Marble: A soft metamorphic rock formed from limestone or dolostone. Marble of various colors is used for building stones and monuments.

Maria: The Latin name for the smooth areas of the Moon formerly thought to be seas.

Marine terrace: A wave-cut platform that has been exposed above sea level.

Maritime (m) air masses: An air mass that originates over the ocean. Maritime air masses are relatively humid.

Massive: Descriptive term for an igneous pluton that is not tabular in shape.

Meanders: A looplike bend in the course of a stream.

Mechanical weathering: The physical disintegration of rock, resulting in smaller fragments.

Medial moraines: A ridge of till formed when lateral moraines from two coalescing alpine glaciers join.

Megathrust fault: The plate boundary separating a subducting slab of oceanic lithosphere and the overlying plate.

Mercury barometer: A mercury-filled glass tube in which the height of the mercury column is a measure of air pressure.

Mesosphere: The layer of the atmosphere immediately above the stratosphere and characterized by decreasing temperatures with height.

Mesozoic: A time span on the geologic calendar between the Paleozoic and Cenozoic eras—from about 245 to 66.4 million years ago.

Metallic bonds: A chemical bond that is present in all metals that may be characterized as an extreme type of electron sharing in which the electrons move freely from atom to atom.

Metamorphic rocks: Rock formed by the alteration of preexisting rock deep within Earth (but still in the solid state) by heat, pressure, and/or chemically active fluids.

Metamorphism: The changes in mineral composition and texture of a rock subjected to high temperatures and pressures within Earth.

Meteor showers: Many meteors appearing in the sky that occurs when Earth intercepts a swarm of meteoritic particles.

Meteorites: Any portion of a meteoroid that survives its traverse through Earth’s atmosphere and strikes Earth’s surface.

Meteoroid: Small, solid particles that have orbits in the solar system.

Meteorology: The scientific study of the atmosphere and atmospheric phenomena; the study of weather and climate.

Meteors: The luminous phenomenon observed when a meteoroid enters Earth’s atmosphere and burns up; popularly called a “shooting star.”

Microcontinents: A relatively small fragment of continental crust that may lie above sea level, such as the island of Madagascar, or that may be submerged, as exemplified by the Campbell Plateau located near New Zealand.

Mid-ocean ridge: A continuous elevated zone on the floor of all the major ocean basins representing divergent plate boundaries. See Oceanic ridge (rise).

Middle clouds: Clouds that occupy the height range from 2000 to 6000 meters.

Middle-latitude: A large low-pressure center with a diameter often exceeding 1000 kilometers (600 miles) that moves from west to east and may last from a few days to more than a week and usually has a cold front and a warm front extending from the central area of low pressure.

Midlatitude: A large low-pressure center with a diameter often exceeding 1000 kilometers (600 miles) that moves from west to east and may last from a few days to more than a week and usually has a cold front and a warm front extending from the central area of low pressure.

Mineral: A naturally occurring, inorganic crystalline material that has a unique chemical composition.

Mineralogy: The study of minerals.

Mist: A cloud of water droplets suspended in the atmosphere at or near Earth’s surface.

Mixing ratio: The mass of water vapor in a unit mass of dry air; commonly expressed as grams of water vapor per kilogram of dry air.

Modified Mercalli Intensity scale: A 12-point scale developed to evaluate earthquake intensity based on the amount of damage to various structures.

Mohs scale: A series of 10 minerals used as a standard in determining mineral hardness.

Moment magnitude: A more precise measure of earthquake magnitude than the Richter scale that is derived from the amount of displacement that occurs along a fault zone.

Monoclines: A one-limbed flexure in strata. The strata are usually flat-lying or very gently dipping on both sides of the monocline.

Monsoons: A seasonal reversal of wind direction associated with large continents, especially Asia. In winter, the wind blows from land to sea; in summer, from sea to land.

Mountain breeze: The nightly downslope winds commonly encountered in mountain valleys.

Muscovite: A common member of the mica family of minerals, with excellent cleavage.

Natural levees: An elevated landform that parallels some streams and acts to confine their waters, except during flood stage.

Neap tides: The lowest tidal range, occurring near the times of the first and third quarters of the Moon.

Nebulae: A cloud of interstellar gas and/or dust. See also Solar nebula.

Nebular theory: A model for the origin of the solar system that supposes a rotating nebula of dust and gases that contracted to form the Sun and planets.

Neutron stars: A star of extremely high density that is composed entirely of neutrons.

Neutrons: A subatomic particle found in the nucleus of an atom. A neutron is electronically neutral and has a mass approximately that of a proton.

Nimbostratus: Amorphous layer of dark gray clouds. One of the primary precipitation-producing clouds.

Nimbus: A cloud that is a major producer of precipitation.

Nonconformity: An unconformity in which older metamorphic or intrusive igneous rocks are overlain by younger sedimentary strata.

Nonfoliated: Descriptive term for metamorphic rocks that do not exhibit foliation.

Nonsilicates: A mineral group that lacks silicas in their mineral structures; accounts for less than 10 percent of Earth’s crust.

Nor'easter: The weather associated with an incursion of mP air into the Northeast from the North Atlantic; strong northeast winds, freezing or near-freezing temperatures, and possible precipitation make this an unwelcome weather event.

Normal faults: A fault in which the rock above the fault plane has moved down relative to the rock below.

Normal polarity: A magnetic field that is the same as that which exists at present.

Nucleus: The small, heavy core of an atom that contains all of its positive charge and most of its mass.

Nuée ardentes: A highly heated mixture, largely of ash and pumice fragments, traveling down the flanks of a volcano or along the surface of the ground.

Numerical date: A date that specifies the actual number of years that have passed since an event occurred.

Obsidian: A volcanic glass of felsic composition.

Occluded front: A front formed when a cold front overtakes a warm front.

Oceanic plateaus: An extensive region on the ocean floor that is composed of thick accumulations of pillow basalts and other mafic rocks that, in some cases, exceed 30 kilometers (20 miles) in thickness.

Oceanic ridge: A continuous elevated zone on the floor of all the major ocean basins and varying in width from 500 to 5000 kilometers (300 to 3000 miles). The rifts at the crests of these ridges represent divergent plate boundaries. Also known as a mid-ocean ridge.

Oceanography: The scientific study of the oceans and oceanic phenomena.

Octet rule: A rule which states that atoms combine in order that each may have the electron arrangement of a noble gas—that is, so the outer energy level contains eight neutrons.

Olivine: A high temperature, dark silicate mineral typically found in basalt.

Oort cloud: A spherical shell composed of comets that orbit the Sun at distances generally greater than 10,000 times the Earth–Sun distance.

Orogenesis: The processes that collectively result in the formation of mountains.

Orographic lifting: Mountains acting as barriers to the flow of air and forcing the air to ascend. The air cools adiabatically, and clouds and precipitation may result.

Outer core: A layer beneath the mantle that is about 2200 kilometers (1364 miles) thick and has the properties of a liquid.

Outlet glaciers: A tongue of ice normally flowing rapidly outward from an ice cap or ice sheet, usually through mountainous terrain to the sea.

Outwash plain: A relatively flat, gently sloping plain consisting of materials deposited by meltwater streams in front of the margin of an ice sheet.

Overrunning: Warm air gliding up a retreating cold air mass.

Oxbow lake: A curved lake produced when a stream cuts off a meander.

Ozone: A molecule of oxygen that contains three oxygen atoms.

P waves: A type of seismic wave that involves alternating compression and expansion of the material through which it passes. Also called primary waves.

Pahoehoe flows: A lava flow with a smooth-to-ropey surface.

Paleomagnetism: The natural remnant magnetism in rock bodies. The permanent magnetization acquired by rock that can be used to determine the location of the magnetic poles and the latitude of the rock at the time it became magnetized.

Paleontology: The systematic study of fossils and the history of life on Earth.

Paleozoic: A time span on the geologic calendar between the Precambrian and Mesozoic eras—from about 570 million to 245 million years ago.

Pangaea: A proposed supercontinent that 200 million years ago began to break apart and form the present landmasses.

Parasitic cones: A volcanic cone that forms on the flank of a larger volcano.

Parcel: An imaginary volume of air enclosed in a thin elastic cover. Typically it is considered to be a few hundred cubic meters in volume and is assumed to act independently of the surrounding air.

Partial melting: The process by which most igneous rocks melt. Since individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results.

Passive continental margins: A margin that consists of a continental shelf, continental slope, and continental rise. These margins are not associated with plate boundaries and therefore experience little volcanism and few earthquakes.

Perched water table: A localized zone of saturation above the main water table that is created by an impermeable layer (aquitard).

Peridotite: An igneous rock of ultramafic composition thought to be abundant in the upper mantle.

Periodic table: An arrangement of the elements in which atomic number increases from the left to right and elements with similar properties appear in columns called families or groups.

Periods: A basic unit of the geologic calendar that is a subdivision of an era. Periods may be divided into smaller units called epochs.

Permeability: A measure of a material’s ability to transmit water.

Phanerozoic: The part of geologic time represented by rocks containing abundant fossil evidence. The eon extending from the end of the Proterozoic eon (570 million years ago) to the present.

Phenocrysts: A conspicuously large crystal embedded in a matrix of finer-grained crystals.

Phyllite: A metamorphic rock composed mainly of fine crystals of muscovite, chlorite, or both.

Piedmont glaciers: A glacier that forms when one or more alpine glaciers emerge from the confining walls of mountain valleys and spread out to create a broad sheet in the lowlands at the base of the mountains.

Pillow lavas: Basaltic lava that solidifies in an underwater environment and develops a structure that resembles a pile of pillows.

Plagioclase feldspar: A relatively hard light silicate mineral containing both sodium and calcium ions that freely substitute for one another depending on the crystallization environment.

Planetary nebulae: A shell of incandescent gas expanding from a star.

Planetesimals: A solid celestial body that accumulated during the first stages of planetary formation. Planetesimals aggregated into increasingly larger bodies, ultimately forming the planets.

Plate tectonics: Tested theory proposing that Earth’s outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself.

Plates: A coherent unit of Earth’s rigid outer layer that includes the crust and uppermost mantle. Also known as a tectonic plate.

Playa lake: A temporary lake in a playa.

Plucking: The process by which pieces of bedrock are lifted out of place by a glacier. Sometimes called quarrying.

Plugs: An isolated, steep-sided, erosional remnant consisting of lava that once occupied the vent of a volcano.

Plutonic rocks: Igneous rock that formed below Earth’s surface.

Plutons: A structure that results from the emplacement and crystallization of magma beneath the surface of Earth. Also known as an intrusion.

Pluvial lakes: A lake formed during a period of increased rainfall. During the Pleistocene epoch, this occurred in some nonglaciated regions during periods of ice advance elsewhere.

Point bars: A crescent-shaped accumulation of sand and gravel deposited on the inside of a meander.

Polar (P): A cold air mass that forms in a high-latitude source region.

Polar easterlies: In the global pattern of prevailing winds, winds that blow from the polar high toward the subpolar low. Unlike the trade winds, however, these winds should not be thought of as persistent winds.

Polar front: A stormy frontal zone that separates air masses of polar origin from air masses of tropical origin.

Polar high: An anticyclone that is assumed to occupy the inner polar regions and is believed to be thermally induced, at least in part.

Porosity: The volume of open spaces in rock or soil.

Porphyritic texture: An igneous texture consisting of large crystals embedded in a matrix of much smaller crystals.

Potassium feldspar: An abundant, relatively hard light silicate mineral containing potassium ions in its structure.

Potholes: A depression formed in a stream channel by the abrasive action of the water’s sediment load.

Precambrian: All geologic time prior to the Paleozoic era.

Pressure: The nature of the change in atmospheric pressure over a period of several hours. It can be a useful aid in short-range weather prediction.

Pressure gradient: The force that results from a difference in atmospheric pressure between two locations. Horizontal pressure gradient forces are what cause winds to blow.

Pressure gradient force (PGF): The force that results from a difference in atmospheric pressure between two locations. Horizontal pressure gradient forces are what cause winds to blow.

Prevailing wind: A wind that consistently blows from one direction more than from any other.

Primary waves: A type of seismic wave that involves alternating compression and expansion of the material through which it passes. Also called P waves.

Principle of cross-cutting relationships: A principle of relative dating which states that a rock or fault is younger than any rock or fault through which it cuts.

Principle of fossil succession: A principle by which fossil organisms succeed one another in a definite and determinable order, and any time period can be recognized by its fossil content.

Principle of lateral continuity: A principle by which sedimentary beds originate as continuous layers until they grade into another rock type or thin out. Used to correlate outcrops that are now isolated from each other.

Principle of original horizontality: A principle by which layers of sediment are generally deposited in a horizontal or nearly horizontal position.

Principle of superposition: A principle which states that in any undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below.

Proglacial lakes: A lake created when a glacier acts as a dam, blocking the flow of a river or trapping glacial meltwater. The term refers to the position of such lakes just beyond the outer limits of a glacier.

Protons: A positively charged subatomic particle found in the nucleus of an atom.

Protoplanets: A developing planetary body that grows by the accumulation of planetesimals.

Protostar: A collapsing cloud of gas and dust that is destined to become a star.

Psychrometer: A device consisting of two thermometers (wet-bulb and dry-bulb) that is rapidly whirled and, with the use of tables, yields the relative humidity and dew point.

Ptolemaic system: An Earth-centered system of the universe.

Pulsar: A variable radio source of small size that emits radio pulses in very regular periods.

Pumice: A light-colored, glassy vesicular rock that commonly has a granitic composition.

Pycnocline: A layer of water in which there is a rapid change of density with depth.

Pyroclastic flow: A highly heated mixture, largely of ash and pumice fragments, traveling down the flanks of a volcano or along the surface of the ground.

Pyroclastic materials: The volcanic rock ejected during an eruption, including ash, bombs, and blocks; also called tephra.

Pyroclastic texture: An igneous rock texture resulting from the consolidation of individual rock fragments that are ejected during a violent eruption.

Quartz: A common silicate mineral consisting entirely of silicon and oxygen that resists weathering.

Quartzite: A hard metamorphic rock formed from quartz sandstone.

Quaternary period: The most recent period on the geologic time scale. It began about 2.6 million years ago and extends to the present.

Radiation: The transfer of energy (heat) through space by electromagnetic waves.

Radiation fog: Fog resulting from radiation heat loss by Earth.

Radioactive decay: The spontaneous decay of certain unstable atomic nuclei.

Radiocarbon dating: Dating of events from the very recent geologic past (the past few tens of thousands of years) based on the fact that the radioactive isotope of carbon is produced continuously in the atmosphere.

Radiometric dating: The procedure of calculating the absolute ages of rocks and minerals that contain radio-active isotopes.

Radiosonde: A lightweight package of weather instruments fitted with a radio transmitter and carried aloft by a balloon.

Rain: Drops of water that fall from a cloud and have a diameter of at least 0.5 millimeter.

Red giants: A large, cool star of high luminosity; a star occupying the upper-right portion of the Hertzsprung-Russell diagram.

Reflection: The process by which light, sound, or energy is returned without being absorbed or scattered.

Regional metamorphism: Metamorphism associated with large-scale mountain-building processes.

Relative dates: Rocks placed in their proper sequence or order of formation based on geologic principles.

Relative humidity: The ratio of the air’s water-vapor content to its water-vapor capacity.

Reverse faults: A fault in which the material above the fault plane moves up in relation to the material below.

Reverse polarity: A magnetic field opposite that which exists at present.

Rhyolite: The fine-grained equivalent of the igneous rock granite, composed primarily of the light-colored silicates.

Ridge push: A mechanism that may contribute to plate motion. It involves the oceanic lithosphere sliding down the oceanic ridge under the pull of gravity.

Rift valley: A region of Earth’s crust along which divergence is taking place.

Rift valleys: A region of Earth’s crust along which divergence is taking place.

Rime: A thin coating of ice on objects that is produced when supercooled fog or cloud droplets freeze on contact.

Ring of Fire: The zone of active volcanoes surrounding the Pacific Ocean.

Rock cycle: A model that illustrates the origin of the three basic rock types and the interrelatedness of Earth materials and processes.

Rock flour: Ground-up rock produced by the grinding effect of a glacier.

Rock-forming minerals: One of the few dozen most abundant minerals that make up most common rocks. Minerals rich in oxygen and silicon (silicate minerals) dominate this group.

Rocks: A consolidated mixture of minerals.

Rotation: The spinning of a body, such as Earth, about its axis.

Runoff: Water that flows over land rather than infiltrating into the ground.

S waves: A seismic wave that involves oscillation perpendicular to the direction of propagation. Also called Secondary waves.

Saffir–Simpson hurricane wind scale: A scale from 1 to 5 that is used to rank relative intensities of hurricanes.

Salinity: The proportion of dissolved salts to pure water, usually expressed in parts per thousand ‰.

Sandstone: An abundant, durable sedimentary rock primarily composed of sand-size grains.

Santa Ana: The local name given to a chinook wind in southern California.

Saturation: The maximum possible quantity of water vapor that the air can hold at any given temperature and pressure.

Scattering: The process in which electromagnetic radiation or particles are deflected or diffused.

Schists: Medium- to coarse-grained metamorphic rocks having a foliated texture, in which platy minerals dominate.

Scientific method: The process by which researchers raise questions, gather data, and formulate and test scientific hypotheses.

Scoria: Vesicular ejecta that is the product of basaltic magma.

Scoria cones: A rather small volcano built primarily of pyroclastics ejected from a single vent.

Sea arch: An arch formed by wave erosion when caves on opposite sides of a headland unite.

Sea breeze: A local wind blowing from the sea during the afternoon in coastal areas.

Sea ice: Frozen seawater that is associated with polar regions. The area covered by sea ice expands in winter and shrinks in summer.

Sea stack: An isolated mass of rock standing just offshore, produced by wave erosion of a headland.

Seafloor spreading: The process of producing new sea-floor between two diverging plates.

Seamounts: An isolated volcanic peak that rises at least 1000 meters (3000 feet) above the deep-ocean floor.

Seawall: A barrier constructed to prevent waves from reaching the area behind the wall. Its purpose is to defend property from the force of breaking waves.

Secondary waves: A seismic wave that involves oscillation perpendicular to the direction of propagation. Also called S waves.

Sediment: Unconsolidated particles created by the weathering and erosion of rock, by chemical precipitation from solution in water, or from the secretions of organisms and transported by water, wind, or glaciers.

Sedimentary rocks: Rock formed from the weathered products of preexisting rocks that have been transported, deposited, and lithified.

Seismic waves: A form of elastic energy released during an earthquake that causes vibrations in the materials that transmit them.

Seismograms: A record made by a seismograph.

Seismographs: An instrument that records earthquake waves. Also known as a seismometer.

Seismology: The study of earthquakes and seismic waves.

Seismometers: An instrument that records earthquake waves. Also known as a seismometer.

Sensible heat: The heat we can feel and measure with a thermometer.

Shale: The most common sedimentary rock, consisting of silt- and clay-size particles.

Sheeting: A mechanical weathering process that is characterized by the splitting off of slablike sheets of rock.

Shield volcanoes: A broad, gently sloping volcano built from fluid basaltic lavas.

Silicon–oxygen tetrahedron: A structure composed of four oxygen atoms surrounding a silicon atom that constitutes the basic building block of silicate minerals.

Sills: A tabular igneous body that was intruded parallel to the layering of preexisting rock.

Siltstone: A fine-grained sedimentary rock composed of clay-sized sediment mixed with silt-sized grains.

Sinkholes: A depression produced in a region where soluble rock has been removed by groundwater.

Sinks: A depression produced in a region where soluble rock has been removed by groundwater.

Slab pull: A mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and “pulls” the trailing lithosphere along.

Slate: A very fine-grained metamorphic rock containing platy minerals and having excellent rock cleavage.

Sleet: Frozen or semifrozen rain that forms when raindrops freeze as they pass through a layer of cold air.

Slip face: The steep, leeward slope of a sand dune; it maintains an angle of about 34 degrees.

Small solar system bodies: An object in the solar system that is not a planet or dwarf planet. Examples include comets and asteroids.

Snow: A solid form of precipitation produced by sublimation of water vapor.

Solar nebula: A cloud of interstellar gas and/or dust from which the bodies of our solar system formed.

Sonar: An instrument that uses acoustic signals (sound energy) to measure water depths. Sonar is an acronym for sound navigation and ranging.

Sorting: The process by which solid particles of various sizes are separated by moving water or wind. Also the degree of similarity in particle size in sediment or sedimentary rock.

Source region: The area where an air mass acquires its characteristic properties of temperature and moisture.

Specific gravity: The ratio of a substance’s weight to the weight of an equal volume of water.

Specific heat: The amount of heat needed to raise 1 gram of a substance 1°C at sea level atmospheric pressure.

Spiral galaxy: A flattened, rotating galaxy with pinwheel-like arms of interstellar material and young stars winding out from its nucleus.

Spit: An elongated ridge of sand that projects from the land into the mouth of an adjacent bay.

Spreading centers: A region where the rigid plates are moving apart, typified by the mid-ocean ridges.

Spring: A flow of groundwater that emerges naturally at the ground surface.

Spring tides: The highest tidal range. Occurs near the times of the new and full moons.

Stalactites: An icicle-like structure that hangs from the ceiling of a cavern.

Stalagmites: A columnlike rock formation that grows upward from the floor of a cavern.

Standard rain gauge: A gauge that has a diameter of about 20 centimeters and funnels rain into a cylinder that magnifies precipitation amounts by a factor of 10, allowing for accurate measurement of small amounts.

Stationary front: A situation in which the surface position of a front does not move; the flow on either side of such a boundary is nearly parallel to the position of the front.

Steam fog: Fog that has the appearance of steam; produced by evaporation from a warm-water surface into the cool air above.

Steppe: One of the two types of dry climate. A marginal and more humid variant of the desert that separates it from bordering humid climates.

Stocks: A pluton similar to, but smaller than, a batholith.

Storm surge: The abnormal rise of the sea along a shore as a result of strong winds.

Strata: Parallel layers of sedimentary rock.

Stratified drift: Sediments deposited by glacial meltwater.

Stratocumulus: Soft, gray clouds in globular patches or rolls. Rolls may join together to make a continuous cloud.

Stratosphere: The layer of the atmosphere immediately above the troposphere, characterized by increasing temperatures with height due to the concentration of ozone.

Stratovolcanoes: A volcano composed of both lava flows and pyroclastic material.

Stratus: As one of the three basic cloud forms, refers to sheets or layers (strata) of cloud that cover much or all of the sky. As one of the three low cloud types, refers to a low, uniform cloud layer that resembles fog but does not rest on the ground and that may produce drizzle.

Streak: The color of a mineral in powdered form.

Stream: A flat, benchlike structure produced by a stream, which was left elevated as the stream cut downward.

Stream terraces: A flat, benchlike structure produced by a stream, which was left elevated as the stream cut downward.

Stream valley: The channel, valley floor, and sloping valley walls of a stream.

Strike-slip fault: A fault along which the movement is horizontal.

Subduction erosion: A process at subduction zones in which sediment and rock are scraped off the bottom of the overriding plate and transported into the mantle.

Subduction zones: A long, narrow zone where one lithospheric plate descends beneath another. See also Convergent plate boundary.

Sublimation: The conversion of a solid directly to a gas, without passing through the liquid state.

Submarine canyons: A canyon carved into the outer continental shelf, slope, and rise by turbidity currents.

Submergent coasts: A coast with a form that is largely a result of the partial drowning of a former land surface either because of a rise of sea level or subsidence of the crust or both.

Subpolar low: Low pressure located at about the latitudes of the Arctic and Antarctic Circles. In the Northern Hemisphere, the low takes the form of individual oceanic cells; in the Southern Hemisphere, there is a deep and continuous trough of low pressure.

Subtropical high: Not a continuous belt of high pressure but rather several semipermanent, anticyclonic centers characterized by subsidence and divergence located roughly between latitudes 25° and 35°.

Summer solstice: See Solstice.

Supercontinent: A large landmass that contains all, or nearly all, of the existing continents.

Supercooled: The condition of water droplets that remain in the liquid state at temperatures well below 0°C.

Supergiants: A very large star with high luminosity.

Supernovas: An exploding star that increases in brightness many thousands of times.

Superplumes: A large mantle plume thought to be responsible for bursts of volcanic activity that emit the vast outpourings of lava forming large basaltic provinces.

Surf: A collective term for breakers; also the wave activity in the area between the shoreline and the outer limit of breakers.

Surface waves: Seismic waves that travel along the outer layer of Earth.

Suspended load: The fine sediment carried within the body of flowing water

Synclines: A linear downfold in sedimentary strata; the opposite of anticline.

System: A group of interacting or interdependent parts that form a complex whole.

Tabular: A term used to describe a feature such as an igneous pluton having two dimensions that are much longer than the third; a pluton that is thin in one dimension.

Temperature: A measure of the degree of hotness or coldness of a substance.

Temperature control: A factor that causes temperature to vary from place to place or from time to time.

Temperature gradient: The amount of temperature change per unit of distance.

Tenacity: Describes a mineral’s toughness or resistance to breaking or deforming.

Tendency: See Pressure tendency.

Tephra: The volcanic rock ejected during an eruption, also called pyroclastic materials.

Terrane: A crustal block bounded by faults, whose geologic history is distinct from the histories of adjoining crustal blocks.

Terrestrial (Earth-like) planets: Any of the Earthlike planets, including Mercury, Venus, Mars, and Earth. Also known as the inner planets.

Terrigenous: Seafloor sediment derived from weathering and erosion on land.

Texture: The size, shape, and distribution of the particles that collectively constitute a rock.

Theory: A well-tested and widely accepted view that explains certain observable facts.

Theory of plate tectonics: Tested theory proposing that Earth’s outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself.

Thermocline: A layer of water in which there is a rapid change in temperature in the vertical dimension.

Thermohaline: Movements of ocean water caused by density differences brought about by variations in temperature and salinity.

Thermosphere: The region of the atmosphere immediately above the mesosphere, which is characterized by increasing temperatures due to absorption of very shortwave solar energy by oxygen.

Thrust fault: A low-angle reverse fault.

Thunderstorm: A storm produced by a cumulonimbus cloud and always accompanied by lightning and thunder. It is of relatively short duration and usually accompanied by strong wind gusts, heavy rain, and sometimes hail.

Tidal current: The alternating horizontal movement of water associated with the rise and fall of the tide.

Tidal flats: A marshy or muddy area that is covered and uncovered by the rise and fall of the tide.

Tides: Periodic change in the elevation of the ocean surface.

Till: Unsorted sediment deposited by a glacier.

Tipping-bucket gauge: A recording rain gauge consisting of two compartments (“buckets”), each capable of holding 0.025 centimeter of water. When one compartment fills, it tips, and the other compartment takes its place.

Tombolo: A ridge of sand that connects an island to the mainland or to another island.

Tornado warning: A warning issued when a tornado has actually been sighted in an area or is indicated by radar.

Tornado watches: A forecast issued for areas of about 65,000 square kilometers (25,000 square miles), indicating that conditions are such that tornadoes may develop; it is intended to alert people to the possibility of tornadoes.

Tornadoes: A small, very intense cyclonic storm with exceedingly high winds, most often produced along cold fronts in conjunction with severe thunderstorms.

Trade winds: Two belts of winds that blow almost constantly from easterly directions and are located on the equatorward sides of the subtropical highs.

Transform fault: A major strike-slip fault that cuts through the lithosphere and accommodates motion between two plates. Also called a transform plate boundary.

Transform faults: A major strike-slip fault that cuts through the lithosphere and accommodates motion between two plates. Also called a transform plate boundary.

Transform plate boundary: A boundary in which two plates slide past one another without creating or destroying lithosphere. Also called a transform fault or transform boundary.

Transpiration: The release of water vapor to the atmosphere by plants.

Travertine: A form of limestone (CaCO3) that is deposited by hot springs or as a cave deposit

Trigger: An event, such as an earthquake or heavy rainfall, that initiates a mass wasting process.

Tropic of Cancer: The parallel of latitude, 23½° north latitude, marking the northern limit of the Sun’s vertical rays.

Tropic of Capricorn: The parallel of latitude, 23½° south latitude, marking the southern limit of the Sun’s vertical rays.

Tropical (T) air masses: A warm-to-hot air mass that forms in the subtropics.

Troposphere: The lowermost layer of the atmosphere. It is generally characterized by a decrease in temperature with height.

Tsunami: A rapidly moving ocean wave generated by earthquake activity that is capable of inflicting heavy damage in coastal regions.

Turbidity currents: A downslope movement of dense, sediment-laden water created when sand and mud on the continental shelf and slope are dislodged and thrown into suspension.

Turbulent flow: Erratic movement of water often characterized by swirling, whirlpool-like eddies. Most streamflow is of this type.

Ultramafic: Term for igneous rocks that consist mostly of olivine and pyroxene.

Ultraviolet (UV) radiation: Radiation with a wavelength from 0.2 to 0.4 micrometer.

Unconformity: A surface that represents a break in the rock record, caused by erosion or nondeposition.

Uniformitarianism: The concept that the processes that have shaped Earth in the geologic past are essentially the same as those operating today.

Unsaturated zone: The area above the water table where openings in soil, sediment, and rock are not saturated with water but filled mainly with air.

Upslope fog: Fog created when air moves up a slope and cools adiabatically.

Upwelling: See Coastal upwelling.

Valence electrons: The electrons involved in the bonding process; the electrons occupying the highest principal energy level of an atom.

Valley breeze: The daily upslope winds commonly encountered in a mountain valley.

Valley glaciers: A glacier confined to a mountain valley, which in most instances was previously a stream valley.

Valley train: A relatively narrow body of stratified drift deposited on a valley floor by meltwater streams that issue from a valley glacier.

Vapor pressure: The part of the total atmospheric pressure that is attributable to water-vapor content.

Vent: A conduit that connects a magma chamber to a volcanic crater.

Vesicular texture: A term applied to fine-grained igneous rocks that contain many small cavities called vesicles, which are openings on the outer portion of a lava flow that were created by escaping gases.

Viscosity: A measure of a fluid’s resistance to flow.

Visible light: Radiation with a wavelength from 0.4 to 0.7 micrometer.

Volatiles: Gaseous components of magma dissolved in the melt. Volatiles readily vaporize (form a gas) at surface pressures.

Volcanic cone: A cone-shaped structure built by successive eruptions of lava and/or pyroclastic materials.

Volcanic island arc: A chain of volcanic islands generally located a few hundred kilometers from a trench where there is active subduction of one oceanic plate beneath another. Also known simply as an island arc.

Volcanic island arcs: A chain of volcanic islands generally located a few hundred kilometers from a trench where there is active subduction of one oceanic plate beneath another. Also known simply as an island arc.

Volcanic necks: An isolated, steep-sided, erosional remnant consisting of lava that once occupied the vent of a volcano.

Volcanic rocks: Igneous rock formed when magma solidifies at Earth’s surface.

Warm front: A front along which a warm air mass overrides a retreating mass of cooler air.

Water table: The upper level of the saturated zone of groundwater.

Watershed: The land area that contributes water to a stream. Also called a watershed.

Wave height: The vertical distance between the trough and crest of a wave.

Wave period: The time interval between the passage of successive crests at a stationary point.

Wave refraction: See Refraction.

Wave-cut cliffs: A seawater-facing cliff along a steep shoreline formed by wave erosion at its base and mass wasting.

Wave-cut platform: A bench or shelf in the bedrock at sea level, cut by wave erosion.

Wavelength: The horizontal distance separating successive crests or troughs.

Weather: The state of the atmosphere at any given time.

Weather radar: Instruments that utilize transmitters to send out radio waves at wavelengths that can penetrate clouds, to produce a reflected signal called an echo that can be displayed on a monitor to show the location and intensity of precipitation.

Well: An opening bored into the zone of saturation.

Westerlies: The dominant west-to-east motion of the atmosphere that characterizes the regions on the poleward side of the subtropical highs.

Wet adiabatic rate: The rate of adiabatic temperature change in saturated air. The rate of temperature change is variable, but it is always less than the dry adiabatic rate.

White dwarfs: A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size; believed to be near its final stage of evolution.

Wind vane: An instrument used to determine wind direction.

Wind: Air flowing horizontally with respect to Earth’s surface.

Winter solstice: See Solstice.

Yazoo tributaries: A tributary that flows parallel to the main stream because a natural levee is present.

Zone of accumulation: The part of a glacier characterized by snow accumulation and ice formation. Its outer limit is the snowline.

Zone of saturation: The zone where all open spaces in sediment and rock are completely filled with water.

Zone of wastage: The part of a glacier beyond the zone of accumulation, where all the snow from the previous winter melts, as does some of the glacial ice.