As with any field, the Earth Sciences has a vocabulary that is unique to its field. This page is intended to take the guesswork out of
understanding the material on this site and contained in the Ontario Curriculum.

A comprehensive online dictionary of Earth Sciences terms is available at the United States Geological Survey's website.
 
 
 

Aa - 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.

Absaroka sequence – A widespread sequence of Pennsylvanian and Permian sedimentary rocks bounded above and below by unconformities; deposited during a transgressive-regressive cycle of the Absaroka Sea.

Absolute dating – The process of assigning an actual age to geologic events.  Various radioactive decay dating techniques yield absolute ages.

Absolute humidity - The weight of water vapour in a given volume of air (usually expressed in grams/m3).

Absolute instability - Air that has a lapse rater greater than the adiabatic rate.

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 - Air with a lapse rate less than the wet adiabatic rate.

Absorption spectrum - A continuous spectrum with dark lines superimposed.

Abyssal plain - Very level area of the deep-ocean floor, usually lying at the foot of the continental rise.

Abyssal zone - A subdivision of the benthic zone characterized by extremely high pressures, low temperatures, low oxygen, few nutrients, and no sunlight.

Acadian orogeny – A major mountain-building event affecting eastern North America in Late Devonian and Early Mississippian time.  Apparently it was caused by collision with the Avalonia microcontinent.

Acanthodian – Any of the fish first having a jaw or jaw-like mechanism; a class of fishes (class Acanthodii) appearing during the Early Silurian and becoming extinct during the Permian.

Accretionary wedge (or accretionary prism} - A large wedge-shaped mass of sediment that accumulates in subduction zones.  Here sediment is scraped from the subducted oceanic plate and accreted to the overriding crustal block.

Achondrite – A type of stony meteorite lacking chondrules; composition similar to that of terrestrial basalt.

Acid precipitation - Rain or snow with a pH value that is less than the pH of unpolluted precipitation.

Acritarchs – Organic walled microfossils that probably represent the cysts of planktonic algae; appeared in the fossil record about 1.4 billion year ago and become abundant during the Late Proterozoic through Devonian.

Acritarchs – Organic-walled microfossils thought to be the cysts of early eukaryotes.  They were common in the late Proterozoic and early Paleozoic, but become extinct shortly thereafter.

Active continental margin - Usually narrow and consisting of highly deformed sediments.  They occur where oceanic lithosphere is being subducted oceanic lithosphere is being subducted beneath the margin of a continent.

Activity – The process of emission of particles and/or radiation from the nuclei of unstable atoms during radioactive decay (see Emissions).  Rate of activity or decay provides a basis for calculating isotopic dates.

Actualism – The assumption that present laws of science would apply at all times; thus, presently known processes are presumed to have always acted in the same way but with greatly varying intensities.

Adaptation – A feature of an organism that makes it better fit for its environment.

Adaptive radiation – Diversification by means of speciation into a new environment and subsequent specialization by new species of related ancestry.

Adiabatic temperature change - Cooling or warming of air caused when air is allowed to expand or is compressed, not because heat is added or subtracted.

Advection - Horizontal convective motion, such as wind.

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

Aerobic – An environment rich in oxygen; also an organism that requires free oxygen in its environment.

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

Aftershocks - Smaller earthquakes that follow the main earthquake.

Air - A mixture of many discrete gases, of which nitrogen and oxygen are most abundant, in which varying quantities of tiny solid and liquid particles are suspended.

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

Air pollutants - Airborne particles and gases that occur in concentrations that endanger the health and well-being of organisms or disrupt the orderly functioning of the environment.

Air pressure - The force exerted by the weight of a column of air above a given point.

Air-mass weather - The conditions experienced in an area as an air mass passes over it.  Because air masses are largely and fairly homogenous, 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.

Alleghenian orogeny – Pennsylvanian to Permian orogenic event during which the Appalachian mobile belt from New York to Alabama was deformed; occurred in the present-day Appalachian Mountains.

Allele – One of the two or more different states of a given gene.

Allopatric speciation – Small populations isolated from the main population trend to be come genetically different from their ancestors, and eventually can become new species. 

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

Alluvium - Unconsolidated sediment deposited by a stream.

Alpha decay – Alternative form of a gene controlling the same trait.

Alpine glacier - A glacier confined to a mountain valley, which in most instances had previously been a stream valley.

Alpine-Himalayan belt – A major linear belt of deformation extending from the Atlantic Ocean eastward across southern Europe and northern Africa, through the Middle East, and into southeast Asia; one of two major Mesozoic-Cenozoic orogenic belts.

Alpine-Himalayan orogeny – A major mountain building event extending from Spain through the Alps, southeastern Europe, southwestern Asia, and Himalayan Ranges to Indonesia during Oligocene and Miocene time.  It resulted from several continent-continent collisions.

Altered remains – Fossils remains that have been changed from their original composition or structure or both.

Altitude (of the Sun) - The angle of the Sun above the horizon.

Amino acids – Relatively simple organic compounds that are the building blocks of proteins.

Amniote egg – An egg in which the embryo develops in a liquid filled cavity called the amnion.  The embryo is also supplied with a yolk sac and a waste sac.  The amniote egg is shelled in reptiles, birds, and egg-laying mammals and is retained but modified in all other mammals.

Anaerobic – An environment poor in oxygen; also organisms that live without oxygen.

Analogous organs - Body parts, such as wings of insects and birds, that serve the same function, but differ in structure and development.

Ancestral Rockies – Late Paleozoic uplift in the southwestern part of the North American craton.

Andean-type plate margin – A margin like that of western South America with oceanic crust being subducted beneath continental crust to produce a volcanic arc (Andes volcanoes) on the edge of a continent.

Andesite – Volcanic rock of intermediate composition like that of coarser diorite; characteristics of volcanic arcs above subduction zones.

Anemometer - An instrument used to determine wind speed.

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

Angiosperm – Vascular plants having flowers and encapsulated seeds; the flowering plants.

Angle of repose - The steepest angle at which loose material remains stationary without sliding down slope.

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

Annual mean temperature - An average of the 12 monthly temperature means.

Anthracite - A hard, metamorphic form of coal that burns clean and hot.

Anthropoid – Any member of he primate suborder Anthropoidea; includes New World and Old World monkeys, apes, and humans.

Anticline - A fold in sedimentary strata resembling an arch.

Anticyclone - A high-pressure centre characterized by a clockwise flow of air in the Northern Hemisphere.

Antler orogeny –A mountain-building event in western North America during Late Devonian to Early Mississippian time.  It may have resulted from a collision of a volcanic arc or microcontinent.  It is unusual in that neither significant granitic plutons nor metamorphism occurred.

Aphelion - The place in the orbit of a planet where the planet is farthest from the Sun.

Aphotic zone - That portion of the ocean where there is no sunlight.

Appalachian mobile belt – A mobile belt located along the eastern margin of the North American craton; extends from Newfoundland to Georgia; probably continuous to the southwest with the Ouachita mobile belt.

Appalachian orogeny – The last major mountain building event that affected eastern North America (Permian and possibly Early Triassic time).  It produced most of the folding and thrust faulting so prominent in the present Appalachian Mountains, however, much of the present topography there resulted from Cenozoic upwarping, which rejuvenated rivers to cause incising of their valleys.

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

Aquifer - Rock or soil through which groundwater moves easily.

Aquitard - Impermeable beds that hinder or prevent groundwater movement. 

Arc orogen – An  area of deformation, such as island arc, that results from subduction of an oceanic plate; characterized by deformation and igneous activity.

Arch – Large-scale cratonic structure that has a broad anticlinal form; total deposition was less and was interrupted  by more unconformities than in its adjacent basins.

Archaebacteria – The most primitive group of single-celled organisms; typically found in extreme environments, such as hot springs.

Archaeocyathid – A benthonic sessile suspension feeder that lived during the Cambrian and constructed reef-like structures.

Archean eon - The second eon of Precambrian time, following the Hadean and preceding the Proterozoic.  It extends between 3.8 billion and 2.5 billion years before the present.

Archosaurs – The group of retiles that include birds, dinosaurs, crocodiles, and their extinct relatives.

Arête - A narrow knifelike ridge separating tow adjacent glaciated valleys.

Arid - see Desert.

Arkose - A feldspar-rich sandstone.

Artesian well - A well in which the water rises above the level where it was initially encountered.

Artificial selection – The practice of selective breeding of plants and animals for desirable traits.

Artiodactyl – Any member of the order Artiodactyla, the even-toed hoofed mammals.  Living artiodactyls include swine, sheep, goats, camels, deer, bison, and musk oxen.

Asteroids - Thousands of small planet-like bodies, ranging in size from a few hundred kilometres to less than a kilometre, whose orbits lie manly 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 kilometres and in some regions extends as deep as 700 kilometres.  The rock within this zone is easily deformed.

Astronomical theory - A theory of climatic change first developed by the Yugoslavian astronomer Milankovitch.  It is based upon changes in the shape of Earth's axis, and the wobbling of Earth's axis.

Astronomical Unit (AU) - Average distance form Earth to the Sun; 1.5 x 10 to the 8^th km.

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 subdivision of Earth's physical environment.

Atoll - A continuous or broken ring of coral reef surrounding a central lagoon.

Atom - the smallest particle that exists as an element.

Atomic mass  – The total number of protons and neutrons in the nucleus of an atom.

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

Atomic weight - The average of the atomic masses of isotopes for a given element.

Aulacogen – Fault-bounded trough (graben) generally extending into a craton margin and filled with thick strata; thought to form when a supercontinent begins to break apart.

Aurora - A bright display of ever-changing light caused by solar radiation interacting with the upper atmosphere in the region of the poles.

Australopithecine – A term referring to several extinct species of the genus Australopithecus that existed in South and East Africa during the Pliocene and Pleistocene epochs.

Autotroph – An organism that manufactures its own food.

Autumnal equinox - The equinox that occurs on September 21-23 in the Northern Hemisphere and on March 21-22 in the Southern Hemisphere.

Axis of spreading – Hypothetical axis that defines the rotation of the Earth.

Back-arc marginal basin – A basin formed on the continentward side of a volcanic island arc; thought to form by back-arc spreading; the site of a marginal sea, e.g., the Sea of Japan.

Backswamp - A poorly drained area on a floodplain that results when natural levees are present.

Baltica – One of six major Paleozoic continents; composed of Russia west of the Ural Mountains, Scandinavia, Poland, and northern Germany.

Banded iron formation (BIF) – Sedimentary rocks consisting of alternating thin layers of silica (chert) and iron minerals (mostly the iron oxides hematite and magnetite).

Barchan dune - Solitary sand dune shaped like a crescent with its tips pointing downward.

Barchanoid dune - Dunes forming scalloped rows of sand oriented at right angles to the wind.  This form is intermediate between isolated barchans and extensive waves of transverse dunes.

Barograph - A recording barometer.

Barometer - An instrument that measures atmospheric pressure.

Barometric tendency - see Pressure tendency.

Barred spiral - A galaxy having straight arms extending from its nucleus.

Barrier island - A low, elongate ridge of sand that parallels the coast.

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

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

Basin - A circular downfolded structure.

Basin and Range Province - Area of Nevada and western Utah (and parts of adjacent states) characterized by north-south-trending fault-block ranges and grabens, and by extreme crustal thinning.

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

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

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 - Large quantities of sand are added to the beach system to offset losses caused by wave erosion.

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

Bedding (stratification) – The layering in sedimentary rocks.  Layers less than 1 cm thick are laminae, whereas beds are thicker.

Bedding plane – The boundary surface that separates one layer of strata from another.

Benioff zone - The marine life zone that includes any seabottom surface regardless of its distance from shore.

Benthonic - Organisms that live on the bottom of lakes, streams, or oceans.  They are either sessile (stationary) or vagrant (can move around), and may live on the bottom (epifaunal) or beneath the surface (infaunal).

Bentonite - Volcanic ash that settled on the sea floor and became altered to clay.

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

Beta decay – A type of radioactive decay during which a fast-moving electrons is emitted from a neutron and thus is concerted to a proton; results in an increase of one atomic number, but no change in atomic mass number.

Big bang theory - The theory that proposes that the universe originated as a single mass, which subsequently exploded.

Binary stars - Two starts revolving around a common centre of mass under their mutual gravitational attraction.

Biogenic sedimentary structure – Any structure in sedimentary rocks produced by the activities of organisms, e.g., tracks, trails, burrows.

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

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

Biostratigraphic unit – A unit of sedimentary rock defined by a fossil content.

Biotic - Pertaining to plants and animals.

Bioturbation – The process of churning or stirring of sediments by organisms.

Bipedal – Walking on two legs as a means of locomotion.

Bituminous - The most common form of coal, often called soft black coal.

Black dwarf - A final state of evolution for a star, in which all of its energy sources are exhausted and it no longer emits radiation.

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

Block faulting - Fragmentation or rifting of the crust due to tension, which results in parallel fault blocks dropped down relative to one another.

Blocking temperature - As cooling occurs, it is the temperature below at which a given material becomes a closed chemical system for a particular radioactive decay series.  The parent-daughter isotope ratio dates the time of this closure.

Blowout (deflation hollow) - A depression excavated by the wind in easily eroded deposits.

Blueschists - Unusual metamorphic  rocks composed of blue-coloured minerals that are produced in regions of high pressure but relatively low temperature, primarily in subduction zones.

Bode's law - A sequence of numbers that approximates the mean distances of the planets from the Sun.

Body fossils – The actual remains of any prehistoric organism; includes shells, teeth, bones, and rarely, the soft parts of organisms.

Body waves - Seismic waves that travel through Earth's interior.

Bonding – The process whereby atoms are joined to other atoms.

Bony fish – A class of fishes (class Osteichthyes) that evolved during the Devonian; he most common fishes; characterized by an internal skeleton of bone; divided into two subgroups, the ray-finned fishes and lobe-finned fishes.

Brachiopod – Any member of a group of bivalved, suspension-feeding marine, invertebrate animals.

Braided stream - A stream consisting of numerous intertwining channels.

Breakwater - a structure protecting a near-shore area from breaking waves.

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

Bright nebula - A cloud of glowing gas excited by ultraviolet radiation from hot stars.

Bright-line spectrum - The bright lines produced by an incandescent gas under low pressure.

Browser – An animal that eats tender shoots, twigs, and leaves.  Compare with grazer.

Cactolith - A quasi-horizontal chonolith composed of anastomosing ductoliths, whose distal ends curl like a harpolith, thin like a sphenolith, or bulge discordantly like an akmolith or ethmolith.

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

Caledonian orogeny - A middle Paleozoic mountain-building event (culminated in Siluro-Devonian time) that affected northwestern Europe and East Greenland.  It resulted from a collision between these two continental masses.

Calorie - The amount of heat required to raise the temperature of one gram of water 1degree Celsius.

Calving - Wastage of a glacier that occurs when large pieces of ice break off into water.

Canadian shield – The Precambrian shield of North America; exposed mostly in Canada, but outcrops occur in Minnesota, Wisconsin, Michigan, and New York.

Capacity - The total amount of sediment a steam is able to transport.

Captorhinomorph – The oldest known reptiles; evolved during the Early Pennsylvanian; ancestors of all other reptiles, thus commonly called the stem reptiles.

Carbon 14 dating – An absolute dating method that relies upon determining the ratio of C14 to C12 in a sample; useful back to about 70,000 years ago; can be applied only to organic substances.

Carbonaceous chondrite – A type of stony meteorite; same as ordinary chondrites except they contain about 5% organic compounds including inorganically produced amino acids.

Carbonate mineral – A mineral that contains the negatively charged carbonate ion (CO3)-2 (e.g., calcite [CaCO3] and dolomite [CaMg(CO3)2].

Carbonate rock – A rock containing predominately carbonate minerals (e.g., limestone and dolostone).

Carnassials – A pair of specialized upper and lower shearing teeth in members of the order Carnivora.

Carnivore - An animal (rarely a plant) that eats animals for its basic food.

Cartilaginous fish – Fishes such as living sharks, rays, and skates, and their extinct relatives that have a skeleton composed of cartilage.

Cassini division - A wide gap in the ring system of Saturn between the A ring and B ring.

Cast – A replica of an object such as a shell or bone formed when a mold of that object is filled by sediment or mineral matter.

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

Catskill Delta – The Devonian clastic wedge that was deposited adjacent to the highlands that formed during the Acadian orogeny.

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

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

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

Cepheid variable - A star whose brightness varies periodically because it expands and contracts.  A type of pulsating star.

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

Chemical sediments - Accumulated by precipitation of grains from chemically saturated water (e.g., the evaporite deposits, salt, and gypsum).

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

Chemostat - The chemical regulatory system between seawater, atmosphere, solid Earth, and life, which helps to maintain a chemical balance or equilibrium among all four.

China – One of six major Paleozoic continents; composed of all of southeast Asia, including China, Indochina, part of Thailand, and the Malay Peninsula.

Chinook - A wind blowing down the leeward side of a mountain and warming by compression.

Chondrite – A stony meteorite that contains chondrules.

Chondrule – A small, round mineral body formed by rapid cooling; found in chondritic meteorites.

Chordate – All members of the phylum Chordata; characterized by a notochord, a dorsal, hollow nerve cord, and gill slits at some time during the animal’s life cycle.

Chromosome - A structure in a cell nucleus carrying genes.  During cell division, the chromosome splits into two sections so that identical genetic information is passed on to the daughter cells.

Chromosphere - The first layer of the solar atmosphere found directly above the photosphere.

Cinder cone - A rather small volcano build primarily of pyroclastics ejected from a single vent.

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

Circum-Pacific belt – One of two major Mesozoic-Cenozoic orogenic belts; located around the margins of the Pacific Ocean basin; includes the orogens of South and Central America, the Cordillera of western North America, and the Aleutian, Japan, and Philippine arcs.

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

Cirrus - One of three basic cloud forms; also one of the three high cloud types.  They are thin, delicate ice crystal clouds often appearing as veil-like patches or thin, wispy fibers.

Cladistics - The study of the relationship between organisms by analyzing derived and primitive features.

Classification - Any system of grouping species into a more inclusive category-the genera into families and families into orders, classes, and phyla, each more inclusive than the preceding category.

Clastic rock - A sedimentary rock made of broken fragments of preexisting rock.

Clastic sediments - Fragmental sediments formed of broken grains of older minerals, rocks, shells, or other skeletal materials.  Terrigenous clastic sediments were derived by erosion of older rocks exposed in land areas; nonterrigenous clastic materials were derived from skeletons formed in the sea than washed about on the sea floor.

Clastic texture – Sedimentary rocks consisting of the broken particles of preexisting rocks or organic structures such as shells are said to have a clastic texture.

Clastic wedge - Prism of clastic sediments with wedge-shaped cross section, which was deposited to rising mountains.  As the mountains rose, their debris was deposited on either adjacent lowlands or in the sea, depending upon relative subsidence and sedimentation. 

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.

Climate System - The exchanges of energy and moisture that occur among the atmosphere, hydrosphere, solid earth, biosphere, and cryosphere.

Climatic feedback mechanism - One of the several different outcomes that may result if one of the many elements in the atmosphere's extremely complex interactive system is altered.

Climatology - The scientific study of climate.

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

Clouds of vertical development - A cloud that has its base in the low height range but extends upward into the middle or high altitudes.

Cluster (star) - A large group of starts.

Col - A pass between mountain valleys where the headwalls of two cirques intersect.

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

Collision - The crashing together of various large divisions of the Earth's crust or lithosphere to form orogenic belts.  They may involve continent-continent, microcontinent, arc-continent, ridge-continent, arc-arc,etc., collisions.

Collision-coalescence process - A theory of raindrop formation in warm clouds (above 0 degrees C) in which large cloud droplets ("giants") collide and join together with smaller droplets to form a raindrop.  Opposite electrical charges may bind the cloud droplets together.

Column - A feature found in caves that is formed when a stalactite and stalagmite join.

Columnar joints - A pattern of cracks that form during cooling of molten rock to generate columns that are generally six-sided.

Coma - The fuzzy, gaseous component of a comet's head.

Comet - A small body that generally revolves about the Sun in an elongated orbit.

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

Composite cone - A volcano composed of both lava flows and pyroclastic material.

Compound - 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.

Concurrent range zone – A type of biozone established by plotting the overlapping ranges of fossils that have different geologic ranges; the first and last occurrences of fossils are used to establish concurrent range zone boundaries.

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

Condensation nuclei - Tiny bits of particulate matter that serve as surfaces on which water vapour condenses.

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

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

Conformable - Strata lacking any discontinuities (unconformities), thus representing an apparently complete record of continuous deposition.

Conglomerate - A sedimentary rock composed of rounded gravel-sized particles.

Conodonts - Tooth-like phosphate microfossils, now thought to be part of the feeding apparatus of a primitive eel-like relative of the vertebrates.  Common throughout the Paleozoic, then vanished in the Triassic.

Constellation - An apparent group of stars originally named for mythical characters.  The sky is presently divided into 88 constellations.

Contact metamorphism - Changes in rock caused the heat form a nearby magma body.

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

Continental accretion - An increase through time of the volume and area of continental crust by the formation of new granitic and andesitic rocks within orogenic belts as well as by the collision of volcanic arcs and microcontinents with continents.

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

Continental margin - That portion of the sea floor adjacent to the continents.  It may include the continental shelf, continental slope, and continental rise.

Continental rise - The gentle sloping surface at the base of the continental slope.

Continental shelf - The gently sloping submerged portion of the continental margin extending form 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.

Continuous spectrum - An uninterrupted band of light emitted by an incandescent solid, liquid, or gas under pressure.

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.  Because convergence at lower levels is associated with an upward movement of air, areas of convergent winds are regions favourable to cloud formation and precipitation.

Convergent boundary - A boundary in which tow plates move together, causing one of the slabs of lithosphere to be consumed into the mantle as it descends beneath on overriding plate.

Convergent evolution – The development of similarities in two or more distantly organisms as a consequence of adapting to a similar lift-style, e.g., ichthyosaurs and porpoises.

Convergent plate margin - Margin along which two lithosphere plates moves toward each other.  In some cases an oceanic plate is subducted (or consumed) beneath the other, causing andesitic volcanism and granite intrusion, whereas in other cases, two continents or arcs may converge and collide.

Coral reef - Structure formed n a warm, shallow, sunlit ocean environment consists primarily of the calcite-rich remains of corals as well as the limy secretions of algae and the hard parts of many other small organisms.

Cordillera - The mountainous regions of western North America, including the Rocky Mountains, the Cascades, and the Sierra Nevadas.

Cordilleran mobile belt – A mobile belt in western North America bounded on the west by the Pacific Ocean and on the east by the Great Plains; extends north-south from Alaska into central Mexico.

Cordilleran orogeny – A protracted episode of deformation affecting the western margin of North America form Jurassic to Early Cenozoic time; typically divided into three separate phases called the Nevadan, Sevier, and Laramide orogenies.

Core - Located beneath the mantle, it is the innermost layer of Earth.  The core is divided into an outer core and an inner core.

Coriolis force (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.

Corona - The outer, tenuous layer of the solar atmosphere.

Correlation - Comparison of strata at two or more localities to establish a similarity of age.  Most commonly correlation is established by comparing index fossils, but it may also be based upon similar isotopic dates, magnetic polarity reversals, isotope variations, etc.

Cosmic particles - High-energy particles from extraterrestrial space, which may shatter nuclei of oxygen and neutrons that then produce 14C by collision with other nitrogen atoms.

Cosmogenists - Those who developed all-encompassing theories for the origin of the Earth and solar system; e.g. Buffon, Descartes.

Cosmopolitan species - Species found over vast stretches of the world (such as the Norwegian rat).

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

Covalent bond – A bond formed by the sharing of electrons between atoms.

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

Craton - Large, tectonically stable nucleus of a continent that has relatively subdued topography.  Most existing continental cratons have been relatively stable throughout most of Phanerozoic time (past 700 m.y.).

Cratonic sequence – A widespread sequence of sedimentary rocks bounded above and below by unconformities; deposited during a transgressive-regressive cycle of an epeiric sea, e.g., the Sauk sequence.

Creep - The slow downhill movement of soil and regolith.

Cretaceous Interior Seaway – An interior seaway that existed during the Late Cretaceous; formed as northward-transgressing waters form the Gulf of Mexico joined with southward-transgressing water from the Arctic; effectively divided North America into two large landmasses.

Crevasse - A deep crack in the brittle surface of a glacier.

Cro-Magnon – A race of Homo sapiens that lived mostly in Europe from 35,000 to 10,000 years ago.

Cross stratification - Smaller-scale lamination at an angle (typically 20 to 30 degrees) to that of the major stratification, and produced by the migration of ripples or dunes.

Cross-bedding - Structure in which relatively thin layers are inclined at an angle to the main bedding.  Formed by currents of wind or water.

Cross-cutting age relationships - Any igneous intrusions or fault must be younger than all rocks penetrated by it but older than any that cut it.  Inference of relative age form such relationships is an extension of the superposition principle.

Crossopterygian – A specific type of lobe-finned fish; possessed lungs; ancestral to amphibians.

Crust - The very thin outermost layer of Earth.

Crystal - An orderly arrangement of atoms.

Crystal form - The external appearance of a mineral as determined by its internal arrangement of atoms.

Crystalline solid – A solid in which the constituent atoms are arranged in a regular, three-dimensional framework.

Crystalline texture – A texture of rocks consisting of an interlocking mosaic of mineral crystals.

Crystallization - The formation and growth of a crystalline solid from a liquid or gas.

Cumulus - One of three basic cloud forms; also the name given one of the clouds of vertical development.  Cumulus are billowy individual cloud masses that often have flat bases.

Cup anemometer - see Anemometer

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

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

Cyclone - A low-pressure centre characterized by a counterclockwise flow of air in the Northern Hemisphere.

Cyclothem – A vertical sequence of cyclically repeated sedimentary rocks resulting from alternating periods of marine and nonmarine deposition; commonly contain a coal bed.

Cynodont – A type of therapsid (advanced mammal-like reptile); the ancestors of mammals.

Daily mean - The mean temperature for a day that is determined by averaging the 24 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 temperatures for a day.

Dark nebula - A cloud of interstellar dust that obscures the light of more distant stars and appears as an opaque curtain.

Dark-line spectrum - see Absorption spectrum

Daughter element – An element formed by the radioactive decay of another element, e.g., argon 40 is the daughter element of potassium 40.

Daughter product - An isotope resulting from radioactive decay.

Daughter-isotope dating - Comparison of ratios among different radiogenic daughter isotopes are generated at different rates, their rations to one another change progressively through time, providing a basis for dating.

Debris flow - A relatively rapid type of mass wasting that involves a flow of soil and regolith containing a large amount of water.  Also called mudflows.

Decay series - Radiogenic parent isotope and all daughter isotopes derived therefrom by decay; e.g., 40k to 40Ar, 87Rb to 87Sr, and 238U to 206Pb series.

Deccan traps - Voluminous mass of basalts erupted from deep fissures in western India during the latest Cretaceous to Paleocene.

Declination (shelter) - The angular distance north or south of the celestial equator denoting the position of a celestial body.

Deep-ocean trench - 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.

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

Dendritic pattern - A stream system that resembles the pattern of a branching tree.

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

Deposition - The process by which water vapour is changed directly to a solid without passing through the liquid state.

Depositional environment – Any area in which sediment is deposited; a depositional site that differs in physical aspects, chemistry, and biology from adjacent environments.

Derived characters - Specialized characters that evolved subsequent to the ancestral (or primitive) characters.  Used in the cladistic classification method.

Desalination - The removal of salts and other chemicals from seawater.

Desert - one of the two types of dry climate; the driest of the dry climates.

Desert pavement - A layer of coarse pebbles and gravel created when wind removed the finer material.

Desiccation crack – A crack formed in clay-rich sediments in response to drying and shrinkage.

Detrital sedimentary rock - Rock formed from the accumulation of material that originated and was transported in the form of solid particles derived from both mechanical and chemical weathering.

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

Differential weathering - The variation in the rate and degree of weathering cause by such factors as mineral makeup, degree of jointing, and climate.

Differentiation of the Earth - The chemical and physical separation of the Earth's components from a homogeneous protoplanet to form core, mantle, crust, oceans, and atmosphere.  Most of this separation occurred very early, but some differentiation of crust continues today through volcanism and orogenesis.

Diffused light - Solar energy scattered and reflected in the atmosphere that reaches Earth's surface in the form of diffuse blue light from the sky.

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

Diluvialists - People who hold that most strata and fossils were deposited by the biblical Flood.

Diorite – Coarse-grained igneous rock of intermediate composition; contains feldspar and amphibole, but little or no quartz.

Dip-slip fault - 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 dates – Two or more differing isotopic dates obtained by different methods for the same methods for the same rock, generally due to selective resetting of one decay series that is more susceptible to heating (e.g., K-Ar); a valuable clue to complex metamorphic histories.

Disjunct floras and faunas – Portions of the same flora or fauna now isolated but originally spread over all the area between present separated outposts.

Dissolved load - That portion of a stream's load carried in solution.

Distributary - A section of a stream that leaves the main flow.

Diurnal tide - Tides characterized by a single high and low water height each tidal day.

Divergence - The condition that exists when the distribution of winds within a given area results in a net horizontal outflow of air form 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 unfavourable to cloud formation and precipitation.

Divergent boundary - A region where the rigid plates are moving apart, typified by the mid-oceanic ridges.

Divergent evolution – The diversification of a species into two or more descendant species.

Divergent plate margin – Margin of lithospheric plate that is moving away from a spreading zone; characterized initially by tensional faulting and basaltic volcanism followed by stable tectonic behaviour.  New oceanic lithosphere is created here.

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

DNA (deoxyribonucleic acid) – A complex molecule arranged in a double-strained helix.  The backbone strands are made of sugars and phosphates, and they are held together by combinations of four bases.  The sequence of these bases gives the information for the genetic code.

Dolomite – Sedimentary rock composed of calcium-magnesium carbonate [CaMg(CO3)2]; most commonly formed by replacement of limestone (CaCO3) through introduction of magnesium ions carried in solution in pore water.

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

Dominant genes – Genes that are expressed at the expense of recessive genes.

Doppler effect - The apparent change in wavelength of radiation caused by the relative motions of the source and the observer.

Doppler radar - In addition to the tasks performed by conventional radar, this new generation of weather radar can detect motion directly and hence greatly improve tornado and severe storm warnings.

Drainage basin - The land area that contributes water to a stream.

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

Drift - The general term for any glacial deposit.

Drumlin - 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 1degree Celsius per 100 metres.

Dry climate - A climate in which yearly precipitation is not as great the potential loss of water by evaporation.

Dryopithecine – Any of the members of a Miocene family of apelike primates; possible ancestors of apes and humans.

Dry-summer subtropical climate - A climate located on the west sides of continents between latitudes 30 and 45 degrees.  It is the only humid climate with a strong winter precipitation maximum.

Dune - A hill or ridge of wind-deposited sand.

Dynamo theory – An explanation of the Earth’s magnetic field by analogy with an electric dynamo.  An electrical conductor moving in a magnetic field has an electric current generated within it; the Earth’s metallic core is a conductor, and current generated therein as it spins maintains the magnetic field (as in an electric dynamo with some of the current it generates being fed back into its electromagnets to maintain the field required for further generation of current in the moving conductor).

Earthflow - The downslope movement of water-saturated, clay-rich sediment.  Most characteristic of humid regions.

Earthquake – The vibration of Earth produced by the rapid release of energy.

Ebb current – The movement of a tidal current away from the shore.

Eccentricity – The variation of an ellipse from circle.

Echo sounder – 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 form the bottom.

Eclipse – The yearly path of the Sun plotted against the background of stars.

Ecliptic – The yearly path of the Sun plotted against the background of stars.

Ecologic niche – A habitat occupied by a species or group of organisms especially adapted for existing in that habit.

Ecologic replacement – The filling of a vacated ecologic niche by the evolution of a new species or the migration of a different species.

Ecosystem – All living and nonliving things in a given area that have a relationship which is self-renewing.

Ectothermy – The ability to obtain body heat from the surrounding environment.

Ediacaran fauna – The late Proterozoic (about 600 m.y. old) soft-bodied fauna of multicellular organisms found in many parts of the world.

El Nino – The name given to the periodic warming of the ocean that occurs n the central and eastern Pacific.  A major ‘El Nino episode can cause extreme weather in many parts of the world.

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

Electromagnet radiation – See Radiation.

Electromagnetic force – A combination of electricity and magnetism into one force; binds atoms into molecules.

Electromagnetic spectrum – The distribution of electromagnetic radiation by wavelength.

Electron – A negatively charged subatomic particle that has a negligible mass and is found outside an atom’s nucleus.

Electron capture decay – A type of radioactive decay involving the capture of an electron by a proton and its conversion to a neutron; results in a loss of one atomic number, but no change in atomic mass number.

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

Elements of weather and climate - Those quantities or properties of the atmosphere that are measured regularly and that are used to express the nature of weather and climate.

Ellesmerian orogeny – Major mountain-building event in Arctic Canada and northern Greenland (Late Devonian to Early Mississippian age).  Its cause is unclear, but may have involved a microcontinent collision.

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

Eluviation – The washing out of fine soil components from the A horizon by downward-percolating water.

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

Emission nebula – A gaseous nebula that derives its visible light from the fluorescence of ultraviolet light from a star in or near the nebula.

Emissions – Spontaneous expulsions from an atomic nucleus of one or more of the following caused by radioactive decay:  alpha particle (He nucleus), beta particle (electron), or gamma ray (similar to x-ray).  Emission rate reflects decay rate.

Encounter theory – A theory for the evolution of the solar system; involves a star passing close to the Sun and thus pulling away gaseous filaments that later accreted into solid bodies.

End moraine – A ridge of till marking a former position of the front of a glacier.

Endemic species – A species known only from a restricted geographic region.

Endothermy – The ability to produce body heat from the metabolic burning of food.

Environment lapse rate – The rate of temperature decrease with increasing height in the troposphere.

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

Epeiric sea – Shallow seas that flooded continental cratons frequently through geologic time; also called epicontinental seas (epi = above, thus “seas above or over continents”).

Ephemeral stream – 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 focus of an earthquake.

Epifauna – Animals living on the bottom of aqueous bodies, such as oceans, lakes, and streams, but above the bottom sediments.

Episodic events – Phenomena that occur irregularly in time (i.e., nonperiodic) and involve large deviations from the average intensity of processes for a given environment.  They include so-called rare events like the “500-year flood” or “200-year typhoon.”

Epoch – A unit of the geologic calendar that is a subdivision of a period.

Equatorial low – A belt of low pressure lying near the equator and between the subtropical highs.

Equatorial system – A method of locating stellar objects much like the coordinate system used on Earth’s surface.

Equilibrium – A balance between two or more processes; e.g., uplift and erosion, or new snowfall and glacier melting, or rise of sea level and upward growth of reefs.

Equinox – The time when the vertical rays of the Sun are striking the equator.  The length of daylight and darkness is equal at all latitudes at equinox.

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

Erosion – The incorporation and transportation of material by a mobile agent, such as water, wind, or ice.

Eruptive variable – A star that varies in brightness.

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

Esker – Sinuous ridge composed largely of sand and gravel deposited by a stream flowing in a tunnel beneath a glacier near its terminus.

Estuary – A partially enclosed coastal water body that is connected to the ocean.  Salinity here is measurable reduced by the freshwater flow of rivers.

Eubacteria – The more familiar advanced bacteria, including those that are responsible for digestion and many diseases.

Eukaryotes – All cells that have a nucleus and certain organelles.

Eukaryotic cell – A type of cell with a membrane-bounded nucleus containing chromosomes; also contains such organelles as plastids and mitochondria that are absent in prokaryotic cells.

Eupantothere – Any member of a group of mammals that included the ancestors of both marsupial and placental mammals.

Euphotic zone – The portion of the photic zone near the surface where light is bright enough for photosynthesis to occur.

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

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

Evaporite intrusions (salt domes) – Upward intrusions by plastic flow of salt or gypsum from deeply buried evaporite layers, which become isostatically unstable because they are less dense than overlying strata.

Evolution, (Theory of) – A fundamental theory in biology and paleontology that sets forth the process by which members of a population of organisms come to differ from their ancestors.  Organisms evolve by means of mutations, natural selection, and genetic factors.  Modern species are descended form related but different species that lived in earlier times.

Exfoliation dome – Large, dome-shaped structure, usually composed of granite, formed by sheeting.

Exotic stream – A permanent stream that traverses a desert and has its source in well-watered areas outside the desert.

Exponential change – Change at an increasing (or decreasing) rather than a constant rate (e.g., population growth).

External process – Process such as weathering, mass wasting or erosion that is powered by the Sun and transforms solid rock into sediment.

Extrusive – Igneous activity that occurs outside the crust.

Eye – A zone of scattered clouds and calm averaging about 20 kilometres in diameter at the centre of a hurricane.

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

Eyepiece – A short-focal-length lens used to enlarge the image in a telescope.  The lens nearest the eye.

Facies fossils – Types of fossils that tend to be restricted to a single lithology or facies (such as graptolites in black shales) so they are not useful for correlation from that facies into other facies.

Facies map – Shows distribution of different sedimentary facies (i.e., lithologies) over some geographic area for some specified moment or interval of geologic time.  Important for interpreting ancient depositional environments and paleogeography.

Fall – A type of movement common to mass wasting processes that refers to the free falling of detached individual pieces of any size.

Farallon plate – A Late Mesozoic-Cenozoic oceanic plate that was largely subducted beneath North America; remnants of the Farallon plate are the Juan de Fuca and Cocos plates.

Fatty acids – The basic molecular building block of fats and oils; they combine to form lipids.

Fault – A break in a rock mass along which movement has occurred.

Fault creep – Displacement along a fault that is so slow and gradual that little seismic activity occurs.

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

Fauna – All the animals in a given region or time period or all the species of a phylum in a given region or time period.

Feedback – The result of a process may “feed back into” the system and modify the further development of that same process.  There may be either amplification or suppression of the process, and so feedback changes the conditions of equilibrium in the system.  Feedback occurs in both living and nonliving systems as well as between the two; thus the evolution of the one realm has influenced the evolution of the other.

Felsic – The group of igneous rocks composed primarily of feldspar and quartz.

Felsic magma – A type of magma containing more than 65% silica and considerable sodium, potassium, and aluminum, but little calcium, iron, and magnesium.

Fermentation – Bacterial action causing the breakdown of large organic molecules, usually under anaerobic (oxygen-depleted) conditions.

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

Filaments – Dark, thin streaks that appear across the bright solar disk.

Filter feeders – Organisms that can strain water to remove food particles.

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

Fiord – A steep-sided inlet of the se4a formed when a glacial trough was partially submerged.

Fission tracks – Imperfections in minerals and volcanic glass caused by spontaneous fission of an unstable atomic nucleus, which propels energy particles through surrounding material.  Density of tracks is a function of numbers of atoms that have undergone fission, thus also of age.

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

Five-hundred-year flood – Concept of violent natural phenomena such as floods that are rare events on the human time scale, but not on the geologic time scale.

Flare – A sudden brightening of an area on the Sun.

Flood basalts – Flows of basaltic lava that issue from numerous cracks or fissures and commonly cover extensive areas to thickness of hundreds of metres.

Flood current – The tidal current associated with the increase in the height of the tide.

Floodplain – The flat, low-lying portion of a stream valley subject to periodic inundation.

Flora – All the plants in a given region or time period.

Flow – A type of movement common to mass wasting processes in which water-saturated material moves downslope as a viscous fluid.

Fluorescence – The absorption of ultraviolet light, which is reemitted as visible light

Fluvial – A term referring to streams, stream action, and the deposits of streams.

Flysch – Collective term for very evenly layered, alternating thin sandstones and shales.  Characteristic of sequences in which the sandstones were deposited by turbidity currents before an orogenic collision.

Focal length – The focal length of a lens is the distance from the lens to the point where it focuses parallel rays of light.

Focus (earthquake) – The zone within Earth where rock displaces an earthquake.

Focus (light) – The point where a lens or mirror causes light rays to converge.

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

Fold – A bend rock layer or series of layers that were originally horizontal and subsequently deformed.

Foliated – A texture of metamorphic rocks that gives the rock a layered appearance.

Foliated texture – A texture of metamorphic rocks in which platy and elongate minerals are arranged in a parallel fashion.

Forearc basin – A basin formed between the volcanic arc and the accretionary prism in a subduction zone.

Foreland basin – A sedimentary basin formed by subsidence of the margin of a craton in front of (“fore”)- and apparently because of loading by-the overthrusting of an orogenic belt onto the craton.  Generally such basins are filled by clastic wedges  whose sediments have been derived from erosion of the orogenic belt.  Depending upon rate of initial subsidence, the basin may begin with relatively deep or shallow marine water over it; rapid sedimentation generally overtakes substance, causing a progression to nonmarine conditions.

Foreshocks – Small earthquakes that often precede major earthquake.

Formation – The most fundamental local rock division of stratigraphic classification, which has some distinctive homogeneity of colour, texture, fossil content, or the like; generally named formally for some geographic locality (e.g., Morrison Formation for Morrison, Colorado).

Fossil fuel – General term for any hydrocarbon that may be used as a fuel, including coal, oil, and natural gas.

Fossil succession – Fossil organisms succeed one another in a definite and determinable order, and any time period can be recognized by its fossil content.

Fossil zone – Restricted thickness of strata characterized by a distinctive index fossil (q.v.), which constitutes a biologic datum for correlation.

Fossils – The remains or traces of organisms preserved from the geological past.

Founder effect – When a small population becomes genetically isolated form its ancestral population, it can have unusual gene combinations.  These come to dominate the genes of all the descendants of the founder population, so that they could become a new species.

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

Franklin mobile belt – The most northerly mobile belt in North America; extends from northwestern Greenland westward across the Canadian Arctic islands.

Freezing – The change of state from a liquid to a solid.

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

Front – The boundary between two adjoining air masses having contrasting characteristics.

Frontal fog – Fog formed when rain evaporates as it falls through a later of cool air.

Frontal wedging – Lifting of air resulting when cool air acts as a barrier over which warmer, lighter air will rise.

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

Galactic cluster – A system of galaxies containing form several to thousands of member galaxies.

Gene – The fundamental inheritance unit that carries a characteristic from parent to offspring; composed of a linear segment of the DNA molecule.

Gene pool – The actively breeding portion of a population in which genes are exchanged as a result of reproduction.

Genetics – The study of heredity and the causes of variation of organisms.

Genotype – The total genetic information that codes for an individual.