OGRe.net                   Glossary

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.

Arκte - 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 8th 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.

Genus – The category name for a group of closely related species. Genera is the plural.

Geocentric – The concept of an Earth-centered universe.

Geologic record – The record of past events preserved in rocks.

Geologic time scale – The division of Earth history into blocks of time- eons, eras, periods, and epochs.  The time scale was created using relative dating principles.

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

Geostrophic wind – A wind, usually above a height of 600 metres that blows parallel to the isobars.

Geosyncline – Relatively thick sequence of sedimentary and/or volcanic rocks that was deposited within a subsiding linear zone of the crust (generally an orogenic belt) and that ultimately was upheaved to form mountains.

Geothermal energy – Natural steam used for power generation.

Geyser – A fountain of hot water ejected periodically.

Giant (star) – A luminous star of large radius.

Glacial erratic – An ice-transported boulder that was not derived form bedrock near its present site.

Glacial stage – A time of extensive glaciation.  At least four glacial stages are recognized in North America, and six or seven are recognized in Europe.

Glacial striations – Scratches and grooves 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 originating on land form the compaction and recrystallization of snow that shows evidence of past or present flow.

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

Glauconite – A green, mica-like silicate mineral that forms within a marine sedimentary environment wherein sedimentation is very slow.  This allows slow chemical reactions between seawater and clay or mica minerals on the sea floor.  Because it contains some potassium, glauconite can provide direct K-Ar isotopic dates of sedimentation.

Glaze – A coating of ice on objects formed when super-cooled rain freezes on contact.

Globular cluster – A nearly spherically shaped group of densely packed stars.

Globule – A dense, dark nebula through to be the birthplace of stars.

Glossopteris flora – Refers to the dominant gymnosperm tree found in the Southern temperate forests in Gondwanaland during the late Paleozoic and early Mesozoic.

Gondwana orogeny – A major Permo-Triassic mountain-building event recognized in South America, South Africa.  Antarctica, and Australia.

Gondwana rock sequence – A late Paleozoic-early Mesozoic nonmarine stratigraphic sequence unique to the five southern or Gondwana continents, but very different from continents.  Tillites occur in the lower part in close association with coal and the Glossopteris flora; nonmarine vertebrates occur above these, and basaltic lavas and dikes complete the sequence.

Gondwanaland – The southern portion of Pangaea consisting of South America, Africa, Australia, India, and Antarctica.

Graben – A valley formed by the downward displacement of a fault-bounded block.

Graded bed – A sediment layer that is characterized by a decrease in sediment size form bottom to top.

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

Gradualism – A view championed by Lyell and Darwin that changes of species and of the Earth in general have been slow and steady through geologic time.

Grain size – A term relating to the size of the particles making up sediment or sedimentary rock.

Granite (granitic) – Coarse-grained, light-coloured (silicic) igneous rock with quartz and potassium feldspar predominating over plagioclase feldspar, therefore relatively rich in K, Al, and SiO2.

Granite-gneiss complex – One of the two main types of rock bodies characteristic of areas underlain by Archean rocks.

Granules – The fine structure visible on the solar surface caused by convective cells below.

Graptolite – A small, planktonic animal belonging to the phylum Hemichordata.  Graptolites make good guide fossils.  Geologic range is from Cambrian through Mississippian.

Gravity – The attractive force that acts between all objects, e.g., between the Earth and Moon.

Graywacke – A heterogeneous poorly sorted (i.e., texturally immature) dark sandstone.

Grazer – Any animal that crops low-growing  vegetation, especially grasses.

Great American Interchange – The exchange of mammals between South America and North America when the Panamanian land bridge rose in the mid-Pliocene.

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 vapour and carbon dioxide.

Greenstone belts – Zones of the Earth’s crust (chiefly Archean) characterized by mildly metamorphosed volcanic rocks (greenstones) and associated immature clastic sedimentary rocks.

Grenville orogeny – An area in the eastern United States and Canada that was accreted to Laurentia during the late Proterozoic.

Groin – A short wall built at a right angle to the shore

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

Groundwater – Water in the zone of saturation.

Guide fossil – Any easily identifiable fossil that has a wide geographic distribution and short geological ages of strata and to correlate strata of the same age.

Guyot – A submerged flat-topped seamount.

Gymnosperm – A class of plants that originated in the middle Paleozoic, which has seeds borne on leaves or in special cones.

Gyre – The large circular surface current pattern found in each ocean.

Hadean eon – The first eon on the geologic time scale; this eon ended 3.8 billion years ago and preceded the Archean eon.

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.

Halocline – A layer of water in which there is a high rate of change in salinity in the vertical dimension.

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

Hardness – The resistance a mineral offers to scratching.

Haversian canals – Abundant canals in the bone of endotherms that are filled with blood vessels.

Heat – The kinetic energy of random molecular motion.

Heliocentric – The view that the Sun is at the centre of the solar system.

Herbivore – Animals that eat plants for their primary source of food.

Hercynian orogeny – A major late Paleozoic mountain-building event that affected most of Europe and southern Asia.  It is approximately equivalent to the Appalachian orogeny of North America as well as to the upheaval of the Ural Mountain belt between Europe and Siberia in western Russia.

Hertzsprung-Russell diagram – See H-R diagram.

Heterotrophs – Organisms that must obtain their food by consuming nutrients from their environment (as in animals).

Hiatus – The interval of geologic time not represented by strata in a sequence represented by strata containing an unconformity.

High – A centre of high pressure characterized by anticyclonic winds.

High cloud – A cloud that normally has its base above 6000 metres; the base may be lower in winter and at high-latitude locations.

Highland climate – Complex pattern of climate conditions associated with mountains.  Highland climates are characterized by large differences that occur over short distances.

Hogback – A narrow, sharp-crested ridge formed by the upturned edge of a steeply dipping bed of resistant rock.

Holocene Epoch – The last of two epochs comprising the Quaternary Period.  Began 10,000 years ago.

Homeothermy – The ability of an animal to maintain a constant body temperature.

Hominid – Members of the zoological family Hominidae, including modern humans and their ancestors.

Homogeneous accretion – A model for the differentiation of the Earth into a core, mantle, and crust; holds that the Earth was originally compositionally homogeneous, but heated up, allowing heavier elements to sink to the core.

Homology – Organs in two or more different organisms that have different functions but the same evolutionary origin.

Horizon – A layer in a soil profile.

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

Horst – An elongate, uplift block of crust bounded by faults.

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

Hot spring – A spring in which the water is 6-9 degree Celsius (10-15 degree F) warmer than the mean annual air temperature of its locality.

H-R diagram – A plot of stars according to their absolute magnitudes and spectral types.

Hubble law – Relates the distance to a galaxy and its velocity.

Humid continental climate – A relatively serve climate characteristic of broad continents in the middle latitudes between approximately 40 and 50 degrees north latitude.  This climate is not found in the Southern Hemisphere, where the middle latitudes are dominated by the oceans.

Humid subtropical climate – A climate generally located on the eastern side of a continent and characterized by hot, sultry summers and cool winters.

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

Hummocky stratification – A distinctive undulatory, lamination in fine sandstones formed in marine shelf environments (e.g., epeiric seas) by deposition influenced by large storm waves.

Humus – Organic matter in soil produced  by the decomposition of plants and animals.

Hurricane – A tropical cyclone storm having winds in excess of 119 kilometres per hour.

Hydrogenous sediment – Seafloor sediments consisting of minerals that crystallize from seawater.  An important example is manganese nodules.

Hydrothermal solution – The hot, watery solution that escapes from a mass of magma during the later stages of crystallization.  Such solutions may alter the surrounding country rock and are frequently the source of significant ore deposits.

Hygrometer – An instrument designed to measure relative humidity.

Hygroscopic nuclei – Condensation nuclei having a high affinity for water, such as salt particles.

Hypothesis – A tentative explanation that is tested to determine if it is valid.

Hypothetico-deductive method – A method of scientific inquiry wherein hypotheses are proposed to explain some phenomenon and are then eliminated by testing (deduction.)

Hyracotherium – A small Early Eocene mammal; the oldest genus assigned to the horse family (family Equidae).

Iapetus Ocean – A Paleozoic ocean basin that separated North America from Europe began moving toward one another, and it was eliminated when the continents collided during the Late Paleozoic.

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

Ice cap climate – A climate that has no monthly means above freezing and supports no vegetative cover except in a few scattered high mountain areas.  This climate, with its perpetual ice and snow, is confined largely to the ice sheets of Greenland and Antarctica.

Ice sheets – A very large, thick mass of glacial ice flowing outward in all directions from on or more accumulation centres.

Ichthyosaur – Any of the porpoise-like, Mesozoic marine reptiles.

Igneous rock – A rock formed by the crystallization of molten magma.

Immature soil – A soil lacking horizons.

Incised meander – Meandering channel that flows in a steep, narrow valley.  They form either when an area is up lifted or when base level drops.

Inclination angle – Vertical angle of inclination of rock remanent magnetization (“fossil magnets”) relative to the surface to the Earth.  Indicative of paleolatitude of a specimen because steep inclination of the Earth’s magnetic field is characteristic of high latitudes and low inclination is characteristic of low latitudes.

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

Included-fragment age relationship – Any fragment included within a clastic sediment or an igneous rock must be older than the enclosing rock; an extension of the superposition principle (q.v.) for relative age.

Inclusion – A piece of rock unit contained within another.  Inclusions are used in relative dating.  The rock mass adjacent to the one containing the inclusion must have been there first in order to provide the fragment.

Index fossil – A fossil that is associated with a particular span of geology time.

Index fossils – fossils unique to a limited thickness of strata, widespread geographically, and easily recognized.  As a result, they are important for correlation of strata.

Inertia – A property of matter that resists a change in its motion.

Infauna – Organisms living within the sediment of the bottoms of aqueous bodies, such as oceans, lakes, or streams.

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

Infrared – Radiation with a wavelength from .7 to 200 micrometres.

Inheritance of acquired characters – The origin of new species by passing on to offspring new habits (hence, ultimately new characteristics) acquired during the lifetime of an adult because of changing (new) needs.  The theory usually associated with Lamarck, although held by many eighteenth and nineteenth century naturalists, including Darwin.

Inhomogeneous accretion – A model for the differentiation of the Earth into a core, mantle, and crust by the sequential condensation of core, mantle, and crust from hot nebular gases.

Inner core – The solid innermost layer of Earth, about 1300 kilometres in radius.

Inorganic evolution – Irreversible evolutionary change in the nonliving realm, such as change form an oxygen-free early atmosphere to an oxygen-rich later one, and from no initial continental crust to the present configuration.

Inselberg – An isolated mountain remnant characteristic of the late stage of erosion of arid region.

Intensity (earthquake) – An indication of the destructive effects of an earthquake at a particular place.  Intensity is affected by such factors as distance to the epicenter and the nature of the surface materials.

Interactive evolution – Repeated evolution of a certain body form after the first species of that shape became extinct.

Interglacial stage – A time between glacial stages when glaciers cover much less area and global temperatures are warmer than during a glacial stage.

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

Intermediate composition – The composition of igneous rocks lying  between felsic and mafic.

Intermediate magma – A magma having a silica content between 53% and 65% and an overall composition intermediate between felsic and mafic magmas.

Interstellar matter – Dust and gases found between stars.

Intertidal zone – The area where land and sea meet and overlap; the zone between high and low tides.

Intertonguing facies – Two sedimentary facies, such as sandstone and shale, have a complex lateral gradation from one to the other so that long “tongues” (or fingers) of one penetrate into the other.  Such tongues reflect repeated lateral shifting of adjacent sedimentary environments during deposition of both facies.

Intracontinental rift – A linear zone of deformation within a continent produced by tensional forces; characterized by normal faults and volcanism, e.g., the East African Rift System.

Intrusive rock – Igneous rock that formed below Earth’s surface.

Ion – An atom or molecule that possesses an electrical charge.

Ionic bond – A bond that results from the attraction of positively and negatively charged ions.

Ionosphere – a complex zone or ionized gases that coincides with the lower portion of the thermosphere.

Iridium – A rear-Earth element in the platinum group of metals, depleted in the Earth’s crust  but more abundant in the mantle and in meteorites.  Its abundance at the Cretaceous/Tertiary boundary has been used as evidence for an asteroid impact.

Iron formation – Evenly laminated or “banded” iron-rich layers alternating with either chert or carbonate layers; unique to Cryptozoic sequences.

Iron meteorite – One of the three main categories of meteorites.  This group is composed largely of iron with varying amounts of nickel (5-20 percent).  Most meteorite finds are irons.

Irregular galaxy – A galaxy that lacks symmetry.

Isobar – A line drawn on a map connecting points of equal atmospheric pressure, usually corrected to sea level.

Isolation – The separation of one unit of population of a species form other population so that there no longer is gene flow between them.  Isolated subpopulations tend to evolve and become more distinct with time.

Isostasy – Gravitational equilibrium such that large parts of the brittle crust “float” on the plastic mantle; relative elevations are functions of thickness and density of the crustal blocks.

Isostasy – The concept that Earth’s crust is “floating” in gravitational balance upon the material of the mantle.

Isostatic rebound – Rise of the crust in response to removal of a load that had previously depressed the crust , as in Scandinavia and around Hudson Bay after the melting of huge glaciers.

Isotherms – Lines connecting pints of equal temperature.

Isotopes – Varieties of the same element that have different mass numbers; their nuclei contain the same number of protons but different numbers of neutrons.  

Isotopic dating – Mineral or rock dating using radioactive isotopes.  In the usual case, the ratio of radiogenic daughter isotope to its unstable parent isotope contained in a mineral is multiplied by the rate of decay of the parent to determine how long the parent isotope has been incorporated in that mineral.

Jet Steam – Swift (120 240-kilometre per hour), high-altitude winds.

Jetties – A pair of structures extending into the ocean at the entrance to a harbor or river that are built for the purpose of protecting against storm waves and sediment deposition.

Joint – A fracture in rock along which there has been no movement.

Jovian planet – The Jupiter-like planets Jupiter, Saturn, Uranus, and Neptune.  These planets have relatively low densities.

Kame – A steep-sided hill composed of sand and gravel originating when  sediment collected in openings in stagnant glacial ice.

Karst – A topography consisting of numerous depressions called sinkholes.

Kaskaskia sequence – A widespread sequence of Devonian and Mississippian sedimentary rocks bounded above and below by unconformities; deposited during a transgressive –regressive cycle of the Kaskaskia Sea.

Kazakhstania – One of six major Paleozoic continents; a triangular-shaped continent centered on Kazakhstan (part of Asia).

Kettle holes – Depressions created when blocks of ice became lodged in glacial deposits and subsequently melted.

Kimberlites – A species of ultramafic peridotite with high-pressure minerals such as diamond.  Kimberlite is derived by rapid explosive intrusions into the crust from the mantle and occurs in vertical cylindrical, pipe-like bodies.

Komatiite – A fine-grained ultramafic volcanic rock with the same composition as coarser-grained, deep-seated peridotite.  Found only among early Cryptozoic (Archean) volcanic rocks.

Koppen classification – A system for classifying climate devised by Wladimir Koppen that is based on mean monthly and annual values of temperature and precipitation.

Labyrinthodont – Any of the amphibians, from the Devonian to the Triassic, characterized by the labyrinthine wrinkling and folding of their teeth.

Laccolith – A massive igneous body intruded between pre-existing 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-snow – Snow showers associated with a cP air mass to which moisture and heat are added from below as the air mass traverse a large and relatively warm lake (such as one of the Great Lakes), rendering the air mass humid and unstable.

Land breeze – A local wind blowing from land toward the water during night in coastal areas.

Land bridge – A narrow strip of land connecting two continents, and permitting land organisms previously restricted to one continent to migrate to the other.

Lapse rate (normal) – The average drop in temperature (6.5 degrees C per kilometre) with increased altitude in the troposphere.

Laramide orogeny -  Latest Cretaceous-middle Eocene episode of mountain building in the Cordillera, that uplifted deep basement rocks in the Rocky Mountain region, forming large mountains with deep basins n between. 

Latent heat – the energy absorbed or released during a change in state.

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

Laterite – A red, highly leached soil type found in the tropics that is rich in oxides of iron and aluminum.

Laurasia – The northern portion of Pangea consisting of North America and Eurasia.

Laurentia – The name given to a Proterozoic continent that was composed mostly of North America and Greenland, parts of northwestern Scotland, and perhaps parts of the Baltic shield of Scandinavia.

Lava – Magma that reaches Earth’s surface.

Law of conservation of angular momentum – The product of the velocity of an object around a centre of rotation (axis) and the distance squared of the object from the axis is constant.

Leaching – The depletion of soluble materials from the upper soil by downward-percolating water.

Lightning – A sudden flash of light generated by the flow of electrons between oppositely charged parts of a cumulonimbus cloud or between the cloud and the ground.

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

Limestone – Sedimentary rock composed of calcium carbonate (CaCo3)2 made up largely of invertebrate fossils skeletal debris.

Lipid – The major biochemical component of all fasts and oils; made up of many fatty acids.

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

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

Lithology – The total characteristics of a rock, including texture, composition, fossil constituents, colour, etc.

Lithosphere – Rigid outer portion of the earth (100 to 250 km) above a low-seismic-velocity zone; includes continental and oceanic crust, and upper mantle.

Lithospheric plates – Large, irregular plate-like units of lithosphere whose margins are delineated today by the major zones of active earthquakes.

Lithostratigraphic unit – A unit of sedimentary rock, such as a formation, defined by its lithologic characteristics rather than it biological content or time of origin.

Little Ice Age – An interval of nearly four centuries (1300 A.D. to the mid-to late 1800s) during which glaciers expanded to greatest historic extents.

Living fossil – A living organism that has descended from ancient ancestors with little apparent change.

Lobe-finned fish – A type of fish in which the fin contains a series of articulating bones and is attached to the body by a fleshy shaft; one of two major subgroups of bony fishes.

Locale group – The cluster of 20 or so galaxies to which our galaxy belongs.

Loess – Deposits of wind-blown silt, lacking visible layers, generally buff coloured, and capable of maintaining a nearly vertical cliff.

Longitudinal (self dunes) – Long ridges of and oriented parallel to the prevailing wind; these dunes form where sand supplies are limited.

Longshore current – A near-shore current flows parallel to the shore.

Low – A centre of low pressure characterized by cyclonic winds.

Low cloud – A cloud that forms below a height of 2000 metres.

Low-seismic-velocity zone – Zone in upper mantle (below 100 to 250 km) that defines base of lithosphere.  This zone has a seismic velocity lower than those above or below, indicating less rigidity.  Seems to be the zone in which most lithosphere plate motion is concentrated.

Low-velocity zone – See Asthenosphere.

Luminosity – The brightness of a star.  The amount of energy radiated by a star.

Lunar breccia – A lunar rock formed when angular fragments and dust are welded together by the heat generated by the impact of a meteoroid.

Lunar eclipse – An eclipse of the Moon.

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 meteoritic impacts.

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

Lycopsid flora – Refers to the dominant tree found in the Northern tropical forests in Laurasia during the late Paleozoic and early Mesozoic.

Macroevolution – The evolution of large-scale differences between organisms, such as the origin of jaws of flight.

Mafic – Igneous rocks with a low silica content and a high iron-magnesium content.

Mafic magma – A type of magma containing between 45 and 52% silica and proportionately more calcium, iron, and magnesium than a felsic magma.

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 form a single magma.

Magnetic anomalies – Any local deviations from normal (either greater or lesser) intensity of the Earth’s magnetic field, such as the narrow positive and negative sea-floor-anomaly stripes parallel to ocean ridges.

Magnetic reversal – The phenomenon of the complete reversal of the north and south magnetic poles.

Magnetic transgression – The invasion of coastal areas or much of a continent by the sea level or subsidence of the land.

Magnitude (stellar) – A number given to a celestial object to express its relative brightness.

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

Manganese nodules – Rounded lumps of hydrogenous sediment scattered on the ocean floor consisting mainly of manganese and iron, and usually containing small amounts of copper, nickel, and cobalt.

Mantle – The 2900 kilometre thick layer of Earth located below the crust.

Mantle plume – A hypothetical plume of partially molten material rising up through the mantle and doming the crust above; may cause initiation of rifts, such as the Red Sea, and probably causes midplate volcanism in the Hawaiian Islands.

Marine regression – The withdrawal of the sea from a continent or coastal area resulting in the emergence of land as sea level falls or the land rises with respect to sea level.

Marine west coast climate – A climate found on windward coast from latitudes 40 to 65 degrees and dominated by maritime air masses.  Winters are mild and summers are cool.

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

Marsupials – A group of pouched mammals that give birth to their young prematurely (unlike placental mammals).  The embryo than climbs up to the mother’s pouch and attaches to a nipple inside, where it continues to mature.  Familiar marsupials include opossum, kangaroos, koalas, and wombats.

Mass extinction – The relatively sudden loss of large numbers of organisms in relation to the number of new species being added to the fauna; usually occurs near the end of a geologic system.

Mass number – The number of neutrons and protons in the nucleus of an atom.

Mass wasting – The downslope movement of rock, regolith, and soil under the direct influence of gravity.

Mean solar day – The average time between two passages of the Sun across the local celestial meridian.

Meander – A loop-like bend in the course of a stream.

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

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

Meiosis – A type of cell division during which the number of chromosomes is reduced by one-half.  The cell division process that yields sex cells, sperm and eggs in animals, and pollen and ovules in plants.

Melange – Chaotically mixed-up and sheared mass of rocks formed by the shearing due to subduction.  Usually found in the accretionary wedge.

Melt – The liquid portion of magma excluding the solid crystals.

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

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

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

Mesocyclone – An intense, rotating wind system in the lower part of a thunderstorm that proceeds tornado development.

Mesopause – The boundary between the mesosphere and the thermosphere.

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

Mesozoic era – A time span on he geologic calendar between the Paleozoic and Cenozoic eras-from about 245 million to 66.4 million years ago.

Mesozoic marine revolution – The Triassic-Jurassic decline of vulnerable invertebrates (especially brachiopods) that lived on the sea bottom, and their replacement by armored, mobile, or burrowing invertebrates (especially molluscs).  It is presumed to be the result of the appearance of a number of shell-crushing predators.

Metamorphic rock – Rocks formed by the alternation 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 temperature and pressure within Earth.

Metazoan – A multicellular organism with differentiated tissues, which are used for reproduction, respiration, food processing, etc. 

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

Meteor shower – Many meteors appearing in the sky caused when Earth intercepts a swarm of meteoritic particles.

Meteorite – 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.

Miacid – Small, short-limbed, Early Tertiary carnivorous mammals; ancestors of all later members of the order Carnivora.

Micrite – Microcrystalline calcium carbonate mud; a limestone composed of micrite.

Microbiotic crust (or cryptogamic soils) – A community of fungi, bacteria, and algae that form a soft spongy mat on otherwise barren clays and silts.

Microcontinent – Any crustal block smaller than a continent with a continental-type crust.

Microevolution – Small-scale evolutionary changes, usually within the normal range of variation of a species.

Microplate – Small lithospheric plate.

Microsphere – Minute globular masses of complex organic molecules found in the Precambrian and believed to be precursors of cells.

Midcontinent rift – A Late Proterozoic intracontinental rift within Laurentia; contains thick accumulations of extrusive igneous rocks and detrital sedimentary rocks.

Middle cloud – A cloud occupying the height range from 2000 to 6000 metres.

Middle-latitude cyclone – Large centre of low pressure with an associated cold front and often a warm front.  Frequently accompanied by abundant precipitation.

Mid-ocean ridge – A continuous mountainous ridge on the floor of all the major ocean basins and varying in width from 500 to 5000 kilometres.  The rifts at the crests of these ridges represent divergent plate boundaries.

Milankovitch effect – Cyclic change of temperature of the
Earth’s surface due to combined effects of variations of tilt of axis, wobble of axis (precession),and ellipticity of orbit.

Mineral – A naturally occurring, inorganic crystalline material with a unique chemical composition.

Mineral resource – All discovered and undiscovered deposits of a useful mineral that can be extracted now or at some time in the future.

Mitosis – A type of cell division during which the two cells resulting from division receive the same number of chromosomes as the parent cell possessed.

Mixed tide – A tide characterized by a large inequality in successive high water heights, low water heights, or both.

Mixing depth – The height to which convectional movements extend above Earth’s surface.  The greater the mixing depth, the better the air quality.

Mobile belt – Elongated area of deformation as indicated by folds and faults; generally located adjacent to a craton, e.g., the Appalachian mobile belt.

Modern synthesis – A synthesis of the ideas of geneticists, paleontologists, population biologists, and others to yield a neo-Darwinian view of evolution; includes chromosome theory of inheritance, mutation as a source of variation, and gradualism.

Mohorovicic discontinuity (Moho) – The boundary separating the crust from the mantle, discernible by an increase in seismic velocity.

Mohs' scale – A series of 10 minerals used as a standard in determining hardness.

Molarization – An evolutionary trend in the hoofed mammals (ungulates) during which the premolars become more molar-like.

Molars – The teeth of mammals used for grinding and chewing.

Molasse – Collective term for coarse, clastic sediments with large-scale cross stratification, channel structures, coal, and in some cases red beds; generally non-marine deposits accompanying the uplift of a mountain belt.

Mold – A cavity or impression of an organism or part thereof in sediment or sedimentary rock, e.g., a mold of a clam shell.

Monomer – A comparatively simple organic molecule, such as an amino acid, that is capable of linking with other monomers to form polymers.

Monotreme – The egg-laying mammals; only two types of monotremes now exist, the platypus and spiny anteater of the Australian region.

Monsoon – 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.

Monthly mean temperature – The mean temperature of a month that is calculated by averaging the daily means.

Morphology – The form or structure of an organism or any of its parts.

Mosaic evolution – The concept that all features of an organisms do not evolve  at the same rate; all organisms are mosaic of characteristics some of which are retained form the ancestral condition, and some of which are more recently evolved.

Mountain breeze – The mean temperature for a month that is calculated by averaging the daily means.

Multicellular organism – Any organism consisting of many cells as opposed to a single cell; possesses cells specialized to perform specific functions such as reproduction and respiration.

Mutation – Any change in the genetic determinants or genes of organisms.

Nappe structure – Very large, recumbent (flat-lying) folds associated with the thrust-fault zones where orogenic belts impinge upon craton margins.  In young belts like the Alps and Himalaya, nappes are obvious, but in older belts, erosion has removed all but their roots.

Natural levees – The elevated landforms that parallel some streams and act to confine their waters, except during floodstage.

Natural selection – The process postulated by Darwin that results in the differential reproductive success of those individuals best adapted for particular changes in the environment.

Neanderthal – A type of human that inhabited Europe and the Near East form 150,000 to 32,000 years ago; considered by some to be a variety or subspecies (Homo sapiens neanderthalensis) of Homo sapiens and by some as a separate species (Homo neanderthalensis).

Neap tide – Lowest tidal range, occurring rear the times of the first-and third-quarter phases of the Moon.

Nebula – A cloud of interstellar gas and/or dust.

Nebular hypothesis – The basic idea that the sun and planets formed from the same cloud of gas and dust in interstellar space.

Negative-feedback mechanism – As used in climatic change, any effect that is opposite of the initial change and tends to offset it.

Nektonic – Organisms that swim.

Neptunism – Archaic theory championed by Werner, which held that all rocks (including granite and basalt) were deposited from an early, universal ocean.

Neritic zone – The marine life zone that extends from the low tide line out to the shelf break.

Neutralism – The idea that some genetic changes are unaffected by natural selection, thus are selectively neutral.

Neutron – A subatomic particle found in the nucleus of an atom.  The neutron is electrically neutral and has a mass approximately that of a proton.

Neutron star – A star of extremely high density composed entirely of neutrons.

Nevadan orogeny – Late Jurassic to Cretaceous phase of the Cordilleran orogeny; most strongly affected the western part of the Cordilleran mobile belt.

New World monkey – Any of the monkeys native of Central and South America.  A superclass of the anthropoids.

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

Nonfoliated – Metamorphic rocks that do not exhibit foliation.

Nonmetallic mineral resource – Mineral resource that is not a fuel or processed for the metals it contains.

Nonrenewable resource – Resource that forms or accumulates over such long time spans that it must be considered as fixed in total quantity.

Nonvascular – Plants lacking specialized tissues for conducting fluids.

Normal fault – 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.

North America Cordillera – A complex mountainous region in western North America extending from Alaska into central Mexico.

Nova - A star that explosively increases in brightness.

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

Nuee ardente – Incandescent volcanic debris buoyed up by hot gases that moves downslope in an avalanche fashion.

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

Obduction – The overthrusting of a thick slab of one plate over another at a convergent lithosphere plate margin.  Commonly it has involved sea-floor (ophiolite) material obducted upon continental or arc crust.

Objective lens – In a refracting telescope, the long-focal-length lens that forms an image of the object viewed.

Oblique fault – A fault having both vertical and horizontal movement.

Obliquity – The angle between the planes of Earth’s equator and orbit.

Obsidian – A volcanic glass of felsic composition.

Occultation – An eclipse of a star or planet by the Moon or a planet.

Oceanic zone – The marine life zone beyond the continental shelf.

Oceanic-continental plate boundary – A type of convergent plate boundary along which oceanic lithosphere and continental lithosphere collide; characterized by subduction of the oceanic plate and by volcanism and seismicity.

Oceanic-oceanic plate boundary – A type of convergent plate boundary along which two oceanic lithospheric plates collide.

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

Old World monkey – Any of the monkeys native to Africa, Asia, and southern Europe.  A superfamily of the anthropoids.

Ontogeny – The historical changes in an individual’s development from fertilized egg to death.

Oolite – Sedimentary rock commonly of limestone composition consisting of spheres approximately 1mm in diameter called oolites with microscopic concentric layers,  which were precipitated form shallow, supersaturated waters as the grains were agitated vigorously by waves or currents.

Open cluster – A loosely formed group of stars of similar origin.

Ophiolite – A sequence of igneous rocks thought to represent a fragment of oceanic lithosphere; composed of peridotite overlain successively by gabbro, sheeted basalt, and pillow basalts.

Ophiolite suite – European term for sequences found in many mountain belts that consist of peridotite overlain successively by basalt, chert, and black shale.  Ophiolite rocks represent uplift deep sea floor.

Orbit – The path of a body in revolution around a centre of mass.

Ordinary chondrite – The most abundant type of stony meteorite; composed of high-temperature ferromagnesian minerals such olivine and some pyroxenes.

Ore – Usually a useful metallic mineral that can be mined at a profit.  The term is also applied to certain nonmetallic minerals such as fluorite and sulfur.

Organelles – Structures within a cell that process food, eliminate waste, trigger reproduction, and perform many other functions.

Organic reef – A wave-resistant limestone structure with a structural framework of animal skeletons, e.g., stromatoporoid reef or coral reef.

Original horizontality – Layers of sediments are generally deposited in a horizontal or nearly position.

Original lateral continuity – Inference of original extent of strata now partially eroded.   Such extent must be estimated in order to infer past expanse of land and sea or to reconstruct deeply eroded structures, such as cratonic arches.

Ornithischian – The “bird-hipped” dinosaurs belonging to the order Ornithischia, including the duckbills, horned dinosaurs, armored dinosaurs, armored dinosaurs, stegosaurs and their relatives.

Orogen – A linear part of the Earth’s crust that was deformed during an orogeny.

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

Orogenic belt – Linear or arcuate zones of the crust that experienced unusually severe compressional tectonics for long spans of time.  They are sites of convergence and subduction of lithosphere plates, of andesitic volcanism, granitic batholiths, and mountain ranges.

Orogeny – Mountain building, especially a mountain-building event in history, such as the Taconic orogeny.

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

Ostracoderm – The “bony-skinned” fish; first appeared during the Late Cambrian and thus are the oldest known vertebrates; characterized by a lack of jaws and teeth and presence of bony armor.

Ouachita mobile belt – A mobile belt located along the southern margin of the North American craton; probably continuous with the Appalachian mobile belt.

Ouachita orogeny – An orogeny that deformed the Ouachita mobile belt during the Pennsylvanian Period.

Outer core – A layer beneath the mantle about 2200 kilometres thick that has the properties of a liquid.

Outgassing – The escape of gases that had been dissolved in magma.

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

Overprinting – Superimposing of later deformation and/or metamorphism upon earlier ones; commonly results in discordant isotope dates by the resetting (re-equilibration) of “isotopic clocks.”

Oxbow lake – A Curved lake produced when a stream cuts off a meander.

Oxygen-isotope analysis – Ratio of 18O to 16O in shells composed of CaCO3 provides indication of paleotemperature of seawater when shell was formed.

Ozone – A molecule of oxygen containing three oxygen atoms.

Ozone layer – A zone in the lower atmosphere between 15 and 30 km above the Earth’s surface that is enriched in ozone (O3).  It is formed by splitting of O2 molecules by ultraviolet radiation followed by the combining of the stray oxygen atoms with other O2 molecules.  Because O3 is unstable, however, it soon breaks up again.  The process is a steady-state one because ozone is forming at the same rate as it breaks down.  There is also an important feedback effect, for the ozone layer filters most of the lethal high-energy a ultraviolet radiation so that it does not reach the Earth’s surface; this makes land areas habitable for life.

Pahoehoe – A lava flow with a smooth-to-ropy surface.

Paleoclimate – A climate that existed during the past.

Paleocurrent – A term referring to an ancient current direction; determined by measuring the orientations of various sedimentary structures such as cross-bedding.

Paleoecology – The study of fossil organisms in relation to pat environments.

Paleogeography – The restoration of geography of any past time, including land and sea distributions, and such things as mountains and lowlands, paleocurrents and paleowinds, hot tropics and cold areas, etc.

Paleolatitude – Latitude of a continent or any other part of the crust at some past time.  Due to continental drift, a continent may have changed latitude significantly over time.

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 era – A time span on the geological calendar between the Precambrian and Mesozoic eras-from about 570 million to 245 million years ago.

Pan-African orogeny – Widespread metamorphism and mountain building between 400 and 600 m.y. ago in all southern continents except India.

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

Panselectionism – The belief that natural selection operates on every feature of the organism and its genes.

Panthalassa Ocean – The Late Paleozoic worldwide ocean that surrounded the supercontinent Pangaea.

Parabolic dunes – The shape of these dunes resembles barchans except their tips point the wind; they often form along coasts that have strong onshore winds, abundant sand, and vegetation that partly covers the sand.

Parallax – The apparent shift of an object when viewed from two different locations.

Parallel evolution – The development of similarities in two or more closely related but separate lines of descent as a consequence of similar adaptations.

Paratethys Sea – Relict of northern part of Tethys seaway now represented by Black Sea, Caspian Sea, and Aral Sea.

Parent element – An unstable element that by radioactive decay is changed into a stable daughter element.

Parent material – The material upon which a soil develops.

Parsec – The distance at which an object would have a parallax angle of 1 second of arc (3.26 light-years).

Partial melting – Only a portion of the mantle or crust is melted to form magma; e.g., andesitic magma seems to represent selective, partial melting of sodium, aluminum, and silica from an iron-magnesium-rich source.

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 margin – Margins that consist of a continental shelf, continental slope, and continental rise.  They are not associated with plate boundaries and therefore experience little volcanism and few earthquakes.

Pedalfer – Soil of humid regions characterized by the accumulation of iron oxides and aluminum-rich clays in the B horizon.

Pedocal – Soil associated with drier regions and characterized by an accumulation of calcium carbonate in the upper horizons.

Pelagic – Organisms that live in open seawater; also may refer to very fine sediment particles that settle slowly from seawater.

Pelagic zone – Open ocean of any depth.  Animals in this zone swim or float freely.

Pelycosaur – Pennsylvanian to Permian “finback retiles”; possessed some mammalian characteristics.

Penumbra – The portion of a shadow from which only part of the light source is blocked by an opaque body.

Perched water table – A localized zone of saturation above the main water table created by an impermeable layer (aquiclude).

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

Perihelion – The point in the orbit of a planet where it is closest to the sun.

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

Perissodactyl – Any member of the order Perissodactyla, the odd-toed hoofed mammals; living perissodactyls include horses, rhinoceroses, and tapirs.

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

Perturbation – The gravitational disturbance of the orbit of one celestial body by another.

pH scale – A common measure of the degree of acidity or alkalinity of a solution, it is a logarithmic scale ranging from 0 to 14.  A value of 7 denotes a neutral solution, values below 7 indicate greater acidity, and numbers above 7 indicate greater alkalinity.

Phanerozoic eon – 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.

Phases of the Moon – The progression of changes in the Moon’s appearance during the month.

Phenocryst – Conspicuously large crystals imbedded in a matrix of finer grained crystals.

Phenotype – The anatomical results of growth and development form a given genotype.

Photic zone – The upper 100 metres or so of water in which sufficient sunlight can penetrate to allow photosynthesis.

Photochemical dissociation – a process whereby water molecules in the upper  atmosphere are disrupted by ultraviolet radiation, yielding oxygen (O2) and hydrogen (H).

Photochemical reactions – Chemical reactions that are triggered by solar radiation, e.g., the formation of some of the gases in smog by the impingement of sunlight on the atmosphere hydrocarbon pollutants.  Postulated also as one mechanism for the generation of CO2 and N2 in the early atmosphere.

Photon – A discrete amount (quantum) of electromagnetic energy.

Photosphere – The region of the sun that radiates energy to space.  The visible surface to the sun.

Photosynthesis – The method by which plants chemically transform carbon dioxide and water into simple carbohydrates (basic food) utilizing the sun’s radiation.  Free oxygen (O2) is a by-product.

Phyletic gradualism – Gradual transformation of fossils species through time.

Phylogeny – The evolution history of a group of related organisms.

Piedmont glacier – A glacier that forms when one or more valley 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 – Lava flows with distinctive internal ellipsoidal structures resembling a pile of pillows; forms when lava is erupted in or flows into water.

Pipe – A vertical conduit through which magmatic materials have passed.

Placental – Any of the mammals that have a placenta to nourish the embryo then give birth when they are nearly fully developed (unlike marsupials); fusion of the amnion of the amniote egg with the walls of the uterus forms the placenta; most mammals, living and fossil, are placentals.

Placer – Deposit formed when heavy minerals are mechanically concentrated by currents, most commonly streams and waves.  Placers are sources of gold, tin, platinum, diamonds, and other valuable minerals.

Placoderm – The “plate-skinned” fishes; Late Silurian through Permian; characterized by jaws and bony armor especially in the head-shoulder region.

Plane of the ecliptic – The imaginary plane that connects Earth’s orbit with the celestial sphere.

Planetary nebula – A shell of incandescent gas expanding from a star.

Planetesimals – Meteorite-like objects thought to have condensed form the solar nebula and to have aggregated together with gases and ices to form protoplanets between 4.6 and 5.0 b.y. ago.

Planets – Terrestrial or inner planets are relatively small, dense, stony bodies orbiting around the sun and thought to have lost most of their original light gases early in solar system history (Mercury, Venus, Earth, and Mars).  Gaseous or outer planets are larger, low-density bodies that have retained most of the original gases derived from the solar nebula (Jupiter, Saturn, Uranus, Neptune, and Pluto).

Plankton – Microscopic organisms that float in water.

Plate – One of numerous rigid sections of the lithosphere that move as a unit over the material of the asthenosphere.

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

Plateau basalts – Basalts formed by eruptions of lava flows form many fissures, resulting in great thickness of flow-upon-flow to produce large plateaus.

Playa – A flat area on the floor of an undrained desert basin.  Following heavy rain, the playa becomes a lake.

Playa lake – A temporary lake in a playa.

Pleistocene epoch – An epoch of the Quaternary period beginning about 1.6 million years ago and ending about 10,000 years ago.  Best known as a time of extensive continental glaciation.

Plesiosaurs – A group of Mesozoic, marine reptiles.

Plucking (quarrying) – The process by which pieces of bedrock are lifted out of place by a glacier.

Pluton – An intrusive igneous body that forms when magma cools and crystallizes within the Earth’s crust.

Plutonic (intrusive igneous) rock – Igneous rock that crystallizes from magma intruded into or formed in place within the Earth’s crust.

Plutonism – Hutton’s theory of the origin of all granitic rocks (and mountains) from molten magmas forcibly intruded upward into the crust due to subterranean heat.

Pluvial lake – 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.

Poikilothermy – The ability to let body temperature fluctuate to conserve energy.

Point bar – The sediment body deposition on the gently sloping side of a meander loop.

Polar (P) air mass – 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.  These winds, however, should not be thought of as persistent winds, such as the trade winds.

Polar front – The stormy frontal zone separating air masses of polar origin from air masses of tropical origin.

Polar high – Anticyclones that are assumed to occupy the inner polar regions and are believed to be thermally induced, at least in part.

Polar wandering – As the result of paleomagnetic studies in the 1950s, researchers proposed that either the magnetic poles migrated greatly through time or the continents had gradually shifted their positions.

Polarity reversals – Nonperiodic reversals of polarity of the Earth’s magnetic field (last of which occurred about 780,000 years ago).  Provide basis for a magnetic reversal time scale.

Pollen analysis – Identification and statistical analysis of pollen from sedimentary rocks; such analyses provide information about ancient floras and climates.

Polymeres – Molecules made up of simpler components linked into complex chains and other three-dimensional structures.

Population density – The number of individuals occupying a given area or stratum.

Population I – Stars rich in atoms heavier than helium.  Nearly always relatively young stars found in the disk of the galaxy.

Population II – Stars poor in atoms heavier than helium.  Nearly always relatively old stars found in the halo, globular clusters, or nuclear bulge.

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

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

Positive-feedback mechanism – As used in climatic change, any effect that acts to reinforce the initial change.

Preadaptation – Evolutionary  features that apparently arose for one purpose, and then were taken over for another purpose.

Precambrian – All geologic time prior to the Paleozoic era.

Precambrian borderland – Hypothetical high land composed of Precambrian metamorphic and igneous rocks postulated by Americans before 1940 to be the source of all Paleozoic and younger clastic sediments deposited sediments deposited in geosynclines.

Precambrian shield – A vast area of exposed Precambrian rocks on a continent; areas of relative stability for long periods of time, e.g., the Canadian shield.

Precession – A slow motion of Earth’s axis which traces out a cone over a period of 26,000 years.

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

Pressure gradient – The amount of pressure change occurring over a given distance.

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

Prevailing wind – A wind that consistently blows from one direction more than from another.

Primary (P) wave – A type of seismic wave that involves alternating compression and expansion of he material through which it passes.

Primary pollutants – Those pollutants emitted directly from identifiable sources.

Primary producer – Those organisms in a food chain, such as green plants and bacteria, upon which all other members of the food chain depend directly or indirectly; those organisms not dependent on an external source of nutrients.

Primates – The order of mammals that includes prosimians (lemurs and tarsiers), monkeys, apes, and humans; characteristics include large brain, stereoscopic vision, and grasping hand.

Primordial lead – The ratios among four lead isotopes found in meteorites.  Because neither parent Th or U are present in the meteorites, these ratios are assumed to represent the primary or primordial ratios in the solar system 4.5 to 4.7 b.y. ago.

Principle of cross-cutting relationships – A principle used to determine the relative ages of events; holds that an igneous intrusion or fault must be younger than the rocks that it intrudes or cuts.

Principle of fossil succession – The principle based on the work of William  Smith that holds that fossils, and especially assemblages of fossils, succeed one another through time in a regular and determinable order.

Principle of inclusions – A principal that holds that inclusions, or fragments, in a rock unit are older than the rock unit itself, e.g., granite fragments in a sandstone are older than the sandstone rock unit.

Principle of lateral continuity – A principle that holds that sediment layers extend outward in all directions until they terminate.

Principle of superposition – A principle that holds that younger strata are deposited on top of older layers.

Principle of uniformitarianism – A principle that holds that we can interpret past events by understanding present-day processes; based on the assumption that natural laws have not changed through time.

Proboscidian – The elephants and their extinct relatives (order Proboscidea); characterized by an elongate or snout-like feeding organ (a trunk).

Proglacial lake – A lake formed by melt-water accumulating along the margins of a glacier.

Progradation – Lateral expansion of any sedimentary environment as deposition occurs, for example, the regression of the sea along a shoreline (e.g. a delta front) where sedimentation is rapid enough to push the sea back; the shore progrades or “migrates forward”.

Progradation – The seaward (or lake-ward) advance of the shoreline as a result of nearshore sedimentation.

Prokaryotes – Primitive single-celled organisms lacking a cell nucleus.

Prominence – A concentration of material above the solar surface that appears as a bright archlike structure.

Prosimian – Any member of the primate suborder Prosimii; includes tree shrews, lemurs, tarsiers, and lorises; commonly called lower primates.

Protein – Large organic molecules constructed of amino acids; the basis molecule in all cells.

Proteinoids – Droplets of proteins surrounded by a lipid membrane that have many of the properties of simple cells.

Proterozoic eon – The eon following the Archean and preceding the Phanerozoic.  It extends between 2500 million and 570 million years ago.

Proton – A positively charged subatomic particle found in the nucleus of an atom.

Proton-proton chain – A chain of thermonuclear reactions by which nuclei of hydrogen are build up into nuclei of helium.

Protostar – A collapsing cloud of gas and dust 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.

Pterosaur – Any of the Mesozoic flying reptiles.

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

Pulsar – A variable radio pulses in very regular periods.

Pulsating variable – A variable star that pulsates in size and luminosity.

Punctuated equilibrium – The geological abrupt appearance of new species followed by very long periods of no visible change.

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

Pyroclastic material – 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 material –The volcanic rock ejected during an eruption, including ash, bombs, and blocks.

Quadrupedal – Referring to locomotion on all four legs; as opposed to bipedal.

Quartzite-carbonate-shale assemblage – A suite of sedimentary rocks characteristic of passive continental margins, but also known from intracratonic basins and back-arc basins.

Quaternary – A term for a geologic period or system comprising all geological time or rocks from the end of the Tertiary to the present; consist of two epochs or series, the Pleistocene and the Recent (Holocene).

Queenston Delta – The clastic wedge resulting from the erosion of the high-lands formed during the Taconic orogeny; deposited on the west side of the Taconic Highlands.

Radial pattern – A system of streams running in all directions away from a central elevated structure, such as volcano.

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

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

Radiation pressure – The force exerted by electromagnetic radiation from an object such as the Sun.

Radio interferometer – Two or more radio telescopes that combine their signals to achieve the resolving power of a larger telescope.

Radio telescope – A telescope designed to make observations in radio wavelengths.

Radioactive decay – Spontaneous change of an unstable atomic nucleus to a stable one.  By emission of particles or radiation, an unstable parent isotope changes to a stable daughter isotope.

Radioactivity – The spontaneous decay of certain unstable atomic nuclei.

Radiocarbon (carbon-14) – The radioactive isotope of carbon, which is produced continuously in the atmosphere and is used in dating events as far back as 40,000 years.

Radiometric dating – The procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes.

Rainshadow desert – A dry area on the lee side of a mountain range.  Many middle-latitude deserts are of this type.

Ramapithecine – A family of Miocene apelike primates; possible ancestors of orangutans.

Range zone – Biostratigraphic unit defined by the occurrence of a single type of organism such as a species or genus.

Rapids – A part of a stream channel in which the water suddenly begins flowing more swiftly and turbulently because of an abrupt steepening of the gradient.

Ray (lunar) – Any of a system of bright elongated streaks, sometimes associated with a crater on the Moon.

Ray-finned fish – A subclass (Arctinopterygii) of the bony fish (class Osteichthyes).  A term describing the way the fins are supported by thin bones that project from the body.

Recessional moraine – An end moraine formed as the ice front stagnated during glacial retreat.

Recessive genes – Genes that are not expressed unless no dominant genes are present in the genotype.

Recombination – The mixing of genetic material from both parents when egg and sperm meet.  This mixing results in new genetic variability among the offspring.

Rectangular pattern – A drainage pattern characterized by numerous right angle bends that develops on jointed or fractured bedrock.

Red beds – Any sedimentary rocks that are red-coloured due to disseminated, fully oxidized iron coatings on grains.

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

Reef – Most densely populated environment of the sea floor where calcareous skeletons of many animals and algae build mounds.  Optimal growth conditions occur in the warm, shallow, lighted (photic) zone with oxygenation of the water and abundant nutrients.  Fringing reefs grow directly adjacent to shore; barrier reefs are separated from shore by a shallow shelf; atolls are rings of reefs with a central shallow lagoon.  Today most reefs grow  only in shallow seas between 30 degrees N and 30 degrees S latitude.

Reflecting telescope – A telescope that concentrates light form distant objects by using a concave mirror.

Reflection nebula – A relatively dense dust cloud in interstellar space that is illuminated by starlight.

Refracting telescope – A telescope that employs a lens to bend and concentrate the light form distant objects.

Refraction – The process by which the portion of a wave in shallow water slows, causing the wave to bend and ten to align itself with the underwater contours.

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

Regolith – The layer of rock and mineral fragments that nearly everywhere covers Earth’s surface.

Regressive facies – Retreat of the sea away from a land area results in deposition of facies that characteristically become coarser upward at any one locality and generally have an unconformity at the top.

Relative dating – Rocks are placed in their proper sequence or order.  Only the chronological order of events is determined.

Relative humidity – The ratio of the air’s water vapour content to its water vapour capacity.

Remanent magnetism – Component of a rock’s magnetism induced by the Earth’s magnetic field when that rock formed.  Measurement of its orientation provides a clue to paleolatitude of the sample (thus of continents) at time of formation.

Renewable resource – A resource that is virtually inexhaustible or that can be replenished over relatively short time spans.

Repetitive – Changing conditions, such as fluctuations of sea level or climatic changes, implied by a distinct succession of sediment types; this succession is repeated many times in a vertical sequence.  Especially conspicuous in upper Paleozoic strata.  May or may not be regularly (periodically) recurring.

Reserve – Already identified deposits from which minerals can be extracted profitably.

Resetting of isotopic clocks – Loss of daughter isotopes from minerals generally due to heating by metamorphism (e.g., loss of 40Ar gas), which results in apparent ages younger than true age.  Valuable in dating metamorphic events.

Residual soil – Soil developed directly from the weathering of the bedrock below.

Resolving power – The ability of a telescope to separate objects that would otherwise appear as one.

Respiration – The release of energy by the controlled oxidation of food molecules.

Restored cross section – Cross section that shows thickness and facies of strata with any later structural deformation and erosion ignored (i.e., restored); valuable for portraying inferred conditions at some past time.

Retrograde motion – The apparent westward motion of the planets with respect to the stars.

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

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

Revolution – The motion of one body about another, as Earth about the Sun.

Ribosomes – Knots of RNA and protein in the cell that help decode the DNA.

Ribozymes – RNAs from the ribosomes that contain not only a genetic code, but also have the ability to catalyze the reactions to reproduce themselves.

Richter scale – A scale of earthquake magnitude based on the motion of a seismograph.

Rift – Long, narrow zone of normal faulting due to tension of the crust, as in aulacogens, ocean-ridge crests, etc.  

Rift zone – A region of Earth’s crust along which divergence is taking place.

Right ascension – An angular distance measured eastward along the celestial equator from the vernal equinox.  Used with declination in a coordinate system to describe in a coordination system to describe the position of celestial bodies.

Rime – A thin coating of ice on objects produced when supercooled fog droplets freeze on contact.

Ripple mark – Wavelike (undulating) structure produced in granular sediment such as sand; formed by wind, unidirectional water currents, or wave currents.

RNA (ribonucleic acid) – Single-stranded, helically-coiled molecule with sugars and phosphates making up the strand, and a variety of bases lined up along it.  In the double-stranded form, it is DNA.  Some organisms (especially viruses) have no DNA and use RNA for their genetic code, and all organisms use a variety of RNAs for transferring and copying genetic information.

Rock – A consolidated mixture of minerals.

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 sequence – Very widespread, relatively thick, formally designated intervals of strata definable chiefly on cratons.  They are bounded by profound, widespread unconformities and include many formations.

Rock-forming mineral – A common mineral that makes up a significant portion of a rock.

Rockslide – The rapid slide of a mass of rock downslope along planes of weakness.

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

Rounding – The process by which the sharp corners and edges of sedimentary particles are abraded during transport.

Rudistid clams – Large, irregular, and conical organisms that formed coral-like reefs in the Cretaceous.  

Ruminant – Any of the cud-chewing placental mammals such as deer, cattle, camels, bison, sheep, and goats.

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

Sabkha – An arid supratidal salt flat, typically found along the margins of a shallow tropical sea, such as the Persian Gulf.

Salinity – The proportion of dissolved salts to pure water, usually expressed in parts per thousand (%).

Salt dome – A structure resulting from the upward movement of a mass of salt through overlying layers of sedimentary rocks.  Oil and gas fields are commonly associated with salt domes.

Saltation – Transportation of sediment through a series of leaps or bounces.

Samfrau belt – Permo-Triassic geosyncline or orogenic belt extending across Gondwanaland from South America along southern Africa and Antarctica to eastern Australia.

San Andrea transform fault – A major transform fault extending through part of California; connects with spreading centers in the Gulf of Mexico and in the Pacific Ocean off the northwest coast of the United States.

Sandstone maturity – Relative purity of a sandstone in terms of compositional maturity or of relative uniformity of texture in terms of textural maturity.

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

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

Sauk Sequence – The first Phanerozoic unconformity-bounded stratigraphic sequences of the North American craton.  The base varies in age from Vendian beyond the craton margin to Late Cambrian in the centre.  The top is a cratonwide unconformity eroded on Early Ordovician (and older) strata.

Saurischian – The “lizard-hipped” dinosaurs, including the giant sauropods and the carnivorous theropods.

Scientific method – A logical, orderly approach that involves data gathering, formulating and testing of hypotheses, and proposing theories.

Scoria – Hardened lava that has retained the vesicles produced by escaping gases.

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 during the afternoon in coastal areas.

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

Seafloor spreading – The process of producing new seafloor between two diverging plates.

Sea-level-fluctuation (or “Vail”) curve – Graph of apparent worldwide changes of sea level through Phanerozoic time of 200 to 300 metres above or below present level, which resulted in half a dozen profound unconformities that seem to be present on several continents.

Seamount – An isolated volcanic peak that rises at least 1000 metres 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 (S) wave – A seismic wave that involves oscillation perpendicular to the direction of propagation.

Secondary enrichment – The concentration of minor amounts of metals that are scattered through unweathered rock into economically valuable concentrations by weathering processes.

Secondary pollutants – Pollutants that are produced in the atmosphere by chemical reactions that occur among primary pollutants.

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

Sedimentary facies – Overall lithology of strata reflecting environment of deposition; facies characteristic of one environment, such as beach sand, grade laterally into facies of another environment, such as offshore mud.

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

Sedimentary structure – Any structure in sedimentary rock such as cross-bedding, desiccation cracks, and animal burrows.

Sediment-deposit feeder – Any animal that ingests sediment and extracts the nutrients from it.

Seedless vascular plant – A type of land plant with vascular tissues for transport of fluids and nutrients throughout the plant; reproduces by spores rather than seeds, e.g., ferns and horsetail rushes.

Seismic sea wave – A rapidly moving ocean wave generated by earthquake activity capable of inflicting heavy damage in coastal regions.

Seismogram – The record made by a seismograph.

Seismograph – An instrument that records earthquake waves.

Seismology – The study of earthquakes and seismic waves.

Semiarid – See Steppe.

Semidiurnal tide – The predominant type of tide throughout the world, with two high waters and two low waters each tidal day.

Sequence stratigraphy – The study of rock relationships within a time-stratigraphic framework of related facies bounded by widespread unconformities.

Sevier orogeny – Late Cretaceous phase of mountain building in the Cordillera due to subduction that produced the Sierran arc.  It is responsible for extensive backarc thrusting from southern Nevada to Alberta.

Shadow zone – The zone between 104 and 143 degrees distance form an earthquake epicenter in which direct waves do not arrive because of refraction by Earth’s core.

Sheeting – A mechanical weathering process characterized by the splitting off of slab-like sheets of rock.

Shelf break – The point where a rapid steepening of the gradient occurs, marking the outer edge of the continental shelf and the beginning of the continental slope.

Shield – A large, relatively flat expanse of ancient metamorphic rock within the stable continental interior.

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

Siberia – One of six major Paleozoic continents; composed of Russia east of the Ural Mountains, and Asia north of Kazakhstan and south of Mongolia.

Sidereal day – The period of rotation of Earth with respect to the stars.

Sidereal month – A time period based on the revolution of the Moon around Earth with respect to the stars.

Silicate – Any one of numerous minerals that have the oxygen and silicon tetrahedron as their basic structure.

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

Sill – A tabular igneous body that was intruded parallel to the layering of pre-existing rock.

Sink hole – A depression produced in a region where soluble rock has been removed by groundwater.

Slab gap (or slab-free window) – Hypothesis that the tectonics of the western Cordillera is due to a gap between two remnants of the Farallon plate, where no Pacific plate was subducted to fill the gap.  Consequently, in this region the North American plate lies directly over hot mantle.

Sleet – Frozen or semi frozen rain formed when raindrops freeze as they pass through a layer of cold air.

Slide – A movement common to mass wasting processes in which the material moving downslope remains fairly coherent and moves along a well-defined surface.

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

Slump – The downward slipping of a mass of rock or unconsolidated material moving as a unit along a curved surface.

Snow – A solid form of precipitation produced by sublimation of water vapour.

Snowfield – An area where snow persists year-round.

Snowline – Lower limit of perennial snow.

Soil – A combination of mineral and organic matter, water, and air; that portion of the regolith that supports plant life.

Soil horizon – A layer of soil that has identifiable characteristics produced by chemical weathering and other soil-forming processes.

Soil profile – A vertical section through a soil showing its succession of horizons and the underlying parent material.

Soil texture – The relative proportions of clay, silt, and sand in a soil.  Texture strongly influences the soil’s ability to retain and transmit water and air.

Solar constant – The rate at which solar radiation is received outside Earth’s atmosphere on a surface perpendicular to the Sun’s rays when Earth is at an average distance form the Sun.

Solar eclipse – An eclipse of the Sun.

Solar flare – A sudden and tremendous eruption in the solar chromosphere.

Solar nebula – The spinning, disc-shaped cloud of gas, dust, and ices from which the entire solar system condensed 4.6 to 5.0 b.y. ago.

Solar nebular theory – A theory for the evolution of the solar system from a rotating cloud of gas.

Solar winds – Subatomic particles ejected at high speed form the solar corona.

Solifluction – Slow, downslope flow of water-saturated materials common to permafrost areas.

Solstice – The time when the vertical rays of the Sun are striking either the Tropic of Cancer or the Tropic of Capricorn.  Solstice represents the longest or shortest day (length of daylight) of the year.

Solum – The O.A, and B horizons in a soil profile.  Living roots and other plant and animals life are largely confined to this zone.

Sonoman orogeny – A Permo-Triassic mountain-building event in western United States through to have resulted from the collision of a volcanic arc with the continental margin.

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.

Speciation – The formation of a new species, thought to be the result of mutation and reproductive isolation.

Species – A population of similar individuals that in nature can reproduce and produce fertile offspring.

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

Specific humidity – The weight of water vapour compared with the total weight of the air, including the water vapour.

Spectral class – A classification of a star according to the characteristics of its spectrum.

Spectroscope – An instrument for directly viewing the spectrum of a light source.

Spectroscopy – The study of spectra.

Spheroidal weathering – Any weathering process that tends to produce a spherical shape from an initially blocky shape.

Spicule – A narrow jet of rising material in the solar chromosphere, or a small needle-like or multispined skeletal element of sponges, corals and echinoderms.

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.

Spontaneous generation – A discredited belief that new life originates from dead material in dark, damp places.

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

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

Spring tide – Highest tidal range that occurs near the times of the new and full moons.

Stalactite – The icicle-like structure that hangs from the ceiling of a cavern.

Stalagmite – The column-like form that grows upward from the floor of a cavern.

Star dune – Isolated hill of sand that exhibits a complex form and develops where wind directions are variable.

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.

Steady state – A dynamic system (such as a beach) is said to be in a steady state when energy is being dissipated, but the system appears the same from time to time; also called dynamic equilibrium

Stellar parallax – A measure of stellar distance.

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

Stony meteorite – One of the three main categories of meteorites.  Such meteorites are composed largely of silicate minerals with inclusions of other minerals.

Stony-iron meteorite – One of the three main categories of meteorites.  This group, as the name implies, is a mixture of iron and silicate minerals.

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

Strata – Parallel layers of sedimentary rock.

Stratification – Layering characteristic of sedimentary deposits, which reflects differences of colour, texture, or composition.

Stratified drift – Sediments deposited by glacial meltwater.

Stratigraphy – A branch of geology; concerned with the composition, origin, and areal and age relationships of stratified rocks.

Stratopause – The boundary between the stratosphere and the mesosphere.

Stratosphere – The layer of the atmosphere immediately above the troposphere characterized by increasing temperatures with height owing to the concentration of ozone.

Stratovolcano – See Composite cone.

Stratus – One of three basic cloud forms; also the name given one of the flow clouds.  They are sheets or layers that cover much or all of the sky.

Streak – The colour of a mineral in powdered form.

Stream fog – Fog having the appearance of steam; produced by evaporation from a warm water surface into the cool air above.

Striations (glacial) – Scratches or grooves in a bedrock surface caused by the grinding action of a glacier and its load of sediment.

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

Stromatolite – Structures that are deposited by algae and consist of layered mounds of calcium carbonate.

Subarctic climate – A climate found north of the humid continental climate and south of the polar climate and characterized by bitterly cold winters and short cool summers.  Places within this climatic realm experience the highest annual temperature ranges on Earth.

Subduction – The process of thrusting oceanic lithosphere into the mantle along a convergent boundary.

Subduction complex – Intensely sheared and fractured assemblages of rocks with many overthrust faults; characteristic of lithosphere plate margins above subduction zones.

Subduction zone – A long, narrow zone where one lithospheric plate descends beneath another.

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

Submarine – A cone-shaped sedimentary deposit that accumulates on the continental slope and rise.

Submarine canyon – A seaward extension of a valley that was cut on the continental shelf during a time when sea level was lower, or a canyon carved into the outer continental shelf, slope, and rise by turbidity currents.

Submergent coast – A coast with a form this is largely the result of the partial drowning of a former land surface either because of low rise of sea level or substance 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.

Subsidence – Sinking of a large part of he Earth’s crust due to such causes as loading by ice or thick sediments, gradual increase of density as by cooling of young oceanic crust,  downward convective flow in the mantle beneath, or loading of the crust by a thick pile of thrust faults.

Subsoil – A term applied to the B horizon of a soil profile.

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

Summer solstice – The solstice that occurs on June 21-22 in the Northern Hemisphere and on December 21-22 in the Southern Hemisphere.

Sundance Sea – A wide seaway that existed in western North America during the Middle Jurassic Period.

Sunspot – A dark spot on the Sun, which is cool by contrast to the surrounding photosphere.

Supercontinent – The result of joining two or more continents by collision.  Gondwanaland was an example before it broke up to spawn modern India, Africa, South America, Antarctica, and Australia.

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

Supergiant – A very large star of high luminosity.

Supernova – An exploding star that increase in brightness many thousands of times.

Superposition – Relationship among strata such that the lowest layer in a sequence is the oldest; basis of relative age of strata.  For deformed strata, original superposition must be determined with fossils, isotopic dating, or features like graded bedding that indicate the original up direction.

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

Surface soil – The uppermost layer in a soil profile: the A horizon.

Surface waves – Seismic waves that travel along the outer layered of the Earth.

Suspect terrane – A tectonic region (terrain) whose original location is uncertain (suspect); some examples have motivated thousands of kilometres and collided with a continent or another suspect terrane.

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

Suspension feeder – An animal that consumes microscopic plants, animals, or dissolved nutrients from the water.

Suture zones – The zones of collision between continental plates, which are generally characterized by intense deformation, metamorphism, and (especially) strips of mafic and ultramafic oceanic rocks.

SWEAT hypothesis – Idea that southwestern North America and East Antarctica were once connected in the late Proterozoic, than rifted apart.

Sweepstakes routes – Dispersal routes across hostile regions so that migration is rare and difficult.  Chance plays a major role in determining which organisms will make it across.

Swells – Wind-generated waves that have moved into an area of weaker winds or calm.

Sympatric speciation – Species formed within populations that are not physically isolated from one another.

Synapsid – The lineage of land vertebrates that eventually led to mammals (formerly called “mammal-like reptiles”).

Syncline – A linear downfold in sedimentary strata; the opposite of anticline.

Synodic month – The period of revolution of the Moon with respect to the Sun, or its cycle of phases.

Synthetic theory of evolution – The modernized version of
Darwin’s theory incorporating genetics, systematic zoology, ecology, and paleontology.

System – The fundamental unit in the time-stratigraphic hierarchy of units; the Devonian System refers to rocks deposited during a specific interval of geologic time, the Devonian Period.

Taconic orogeny – Major Late Ordovician mountain-building event on the southeastern margin of North America.  The previously passive continental margin was converted to an active convergent margin in Middle Ordovician time, and the orogeny then resulted from collision of a volcanic arc.

Talus – An accumulation of rock debris at the base of a cliff.

Tarn – A small lake in a cirque.

Taxonomy – The grouping together of related organisms into categories arranged in a classification.

Tectonic land – Large, mountainous land raised within an orogeny belt by an orogeny and composed of only slightly older sedimentary, volcanic, and plutonic rocks; principal source of so-called geosynclinal sediments.

Tectonics – The study of the large-scale processes that collectively deform Earth’s crust.

Tejas epeiric sea – A Cenozoic epeiric sea that was largely restricted to the Atlantic and Gulf Coastal plains and parts of coastal California, but did extend into the continental interior in the Mississippi Valley.

Temperature – A measure of the degree of hotness or coldness of a substance; a measure of the average kinetic energy of individual atoms or molecules in a substance.

Temperature inversion – A layer in the atmosphere of limited depth where the temperature increases rather than decreases with height.

Template – A framework or scaffold that helps support and organize the assembly of larger, more complex structures, as in clays, pyrite, or zeolites aligning organic molecules to form complex polymers.

Temporary (local) base level – The level of a lake, resistant rock layer, or any other base level that stand above sea level.

Terminal moraine – The end moraine marking the farthest advance of a glacier.

Terrace – A flat, bench-like structure produced by a stream, which was left elevated as the stream cut downward.

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

Terrestrial planet – Any of the Earth-like planets including Mercury, Venus, Mars, and Earth.

Terrigenous sediment – Seafloor sediments derived from terrestrial weathering and erosion.

Tertiary – A term for a geologic period or system comprising all geological time or rocks form the end of the Cretaceous to the beginning of the Quaternary.  The Tertiary consists of five epochs or series: Paleocene, Eocene, Oligocene, Miocene, and Pliocene.

Tethys sea – Permian through Eocene seaway between Gondwanaland and Laurasia with a distinctive, very diverse tropical fauna.

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

Thecodontian – An informal term for a variety of Permian and Triassic reptiles that had teeth set in individual sockets.  Small, bipedal thecodonts are the probable ancestors of dinosaurs.

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

Theory of Evolution - The theory that all living things are related and that they descended with modification from organisms that lived during the past.

Therapsid - Permian to Triassic reptiles that possessed mammalian characteristics and thus are called mammal-like reptiles; one group of therapsids, the cynodonts, gave rise to mammals.

Thermal convection – Slow circulation or turnover of mantle material by plastic flow due to differential heating as air rises from a radiator, cools, and then sinks on the opposite side of a room.

Thermal convection cell - A type of circulation of material in the asthenosphere during which hot material rises, moves laterally, cools and sinks, and its reheated and reenters they cycle.

Thermal gradient – The increase in temperature with depth.  It averages 1 degree C per 30 metres (1-2 degrees F per 100 feet) in the crust.

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

Thermohaline circulation – 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 and characterized by increasing temperatures due to absorption of very short-wave solar energy by oxygen.

Thrust fault – A low-angle reverse fault.

Thunder – The sound emitted by rapidly expanding gases along the channel of lightning discharge.

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 delta – A delta-like feature created when a rapidly moving tidal current emerges from a narrow inlet and slows, depositing its load of sediment.

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

Tide – Periodic change in the elevation of the ocean surface.

Tiering – Subdivision of the regions above and below the sea floor into different feeding levels, or tiers, which allows more different kinds of organisms to live in the same area of sea floor.

Till – Unsorted sediment deposited directly by a glacier.

Tillite – Lithified glacial till characterized by heterogeneous composition, unsorted texture, and a lack of stratification.

Time transgressive - Refers to a rock unit that was deposited in an environment that shifted with time, as during a marine transgression.  Thus, the age of the rock unit varies over its geographic extent.

Time unit - Any of the units such as eon, era, period, epoch, and age used to refer to specific intervals of geologic time.

Time-stratigraphic unit - A unit of strata that was deposited during a specific interval of geologic time, e.g., the Devonian System, a time-stratigraphic unit, was deposited during the part of the geological time designated as the Devonian Period.

Tippecanoe Sequence – The second Phanerozoic unconformity-bounded stratigraphic sequence of the North American craton.  The basal unconformity is overlain in the central craton by the Middle Ordovician St. Peter Sandstone; the top is a cratonwide unconformity developed on Early Devonian and older rocks.

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

Tommotian – One of the earliest stages of the Cambrian Period; marked by abundant small shelly fossils, but no trilobites yet.

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

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

Tornado watch – A warning issued for areas of about 65,000 square kilometres indicating that conditions are such that tornadoes may develop; it is intended to alert people to the possibility of tornadoes.

Total-rock dating – Isotopic dating using an entire rock specimen rather than a single mineral.  Thus, parent-daughter isotope ratios are determined for a specimen containing several different minerals.

Trace fossils – Indirect evidence of life, such as footprints, trails, and burrows.

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

Transcontinental Arch - An area consisting of several large islands extending from New Mexico to Minnesota that was above sea level during the Cambrian transgression of the Sauk Sea.

Transcurrent fault – A lateral or strike-slip fault with horizontal displacement, such that one side moves laterally past the other (e.g., San Andreas fault of California).

Transform boundary – A boundary in which two plates slide past one another without creating or destroying lithosphere.

Transform fault – A type of transcurrent or strike-slip fault characteristic of oceanic spreading ridges, which offsets the ridge axes as spreading progresses.

Transgressive facies – Advance of the sea over a land area results in deposition of transgressive facies,  which overlie an unconformity and are characterized by deposits that become finer upward at any one locality.

Transpiration – The release of water vapour to the atmosphere by plants.

Transported soil – Soils that form on unconsolidated deposits.

Transverse dunes – A series of long ridges oriented at right angles to the prevailing wind; these dunes form where vegetation is sparse and sand is very plentiful.

Travertine – A form of limestone (CaCo3) that is deposited by hot springs or as a cave deposit.

Tree-ring dating - The process of determining the age of a tree or wood in structures by counting the number of annual growth rings.

Trellis pattern – A system of streams in which nearly parallel tributaries occupy valleys cut in folded strata.

Trench – An elongated depression in the seafloor produced by bending of oceanic crust during subduction.

Trilobite - A group of benthonic, detritus-feeding, extinct marine invertebrate animals (phylum Arthropoda), having skeletons of an organic compound called chitin.

Triple junction – Junction of three linear tectonic elements, such as three spreading ridges or three subduction zones.

Trophic level - The complex interrelationships among producers, consumers, and decomposers in a community of organisms.  There are several trophic levels of production and consumption through which energy flows in a feeding hierarchy.

Tropic of Cancer – The parallel of latitude, 23-1/2 degrees north latitude, marking the northern limit of the Sun’s vertical rays.

Tropic of Capricorn – The parallel of latitude, 23-1/2 degrees south latitude, marking the southern limit of the Sun’s vertical rays.

Tropical depression – By international agreement, a tropical cyclone with maximum winds that so not exceed 61 kilometres per hour.

Tropical rain forest – A luxuriant broadleaf evergreen forest; also the name given the climate associated with this vegetation.

Tropical storm – By international agreement, a tropical cyclone with maximum winds between 61 and 119 kilometres per hour.

Tropical wet and dry – A climate that is transitional between the wet tropics  and the subtropical steppes.

Tropopause – The boundary between the troposphere and the stratosphere.

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

Tsunami – The Japanese word for a seismic sea wave.

Tundra climate – Found almost exclusively in the Northern Hemisphere or at high altitudes in many mountainous regions.  A treeless climatic realm of sedges, grasses, mosses, and lichens that is dominated by along, bitterly cold winter.

Turbidite –Turbidity current deposit characterized by graded bedding.

Turbidity current – 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.

Ultimate base level – Sea level; the lowest level to which stream erosion could lower the land.

Ultramafic – Igneous rocks composed mainly of iron and magnesium-rich minerals.

Ultraviolet – Radiation with a wavelength from .2 to .4 micrometre.

Umbra – The central, completely dark part of a shadow produced during an eclipse.

Unaltered remains - Fossil remains that retain their original composition and structure.

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

Ungulate - An informal term referring to the hoofed mammals, especially the orders Artiodactyla and Perissodactyla.

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

Uplift – Raising of large regions by structural forces, such as upward convective flow in the mantle, rise of masses of magma through the crust, or compression of the crust by subduction or continental collision.

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

Upwelling – The rising of cold water from deeper layers to replace warmer surface water that has been moved away.

Urban heat island – Refers to the fact that temperatures within a city are generally higher than in surrounding rural areas.

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

Valley glacier – See Alpine glacier.

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

Vapour pressure – That part of the total atmospheric pressure attributable to water vapour content.

Varve - A couplet of sedimentary laminae representing deposition that occurred in one year, e.g., the dark (winter) and light (summer) layers in varved sediments deposited in glacial lakes.

Vascular - A term referring so some land plants possessing specialized tissues for transporting fluids.

Vein deposit – A mineral filling a fracture or fault in a host rock.  Such deposits have a sheet-like, or tabular form.

Vendian – Period just before the Cambrian Period when the Ediacaran soft-bodied fauna was dominant.

Ventifact – A cobble or pebble polished and shaped by the sandblasting effect of wind.

Vertebrate - Any animal having a segmented vertebral column; members of the subphylum Vertebrata;  includes fishes, amphibians, reptiles, mammals, and birds.

Vesicular – A term applied to igneous rocks that contain small cavities called vesicles, which are formed when gases escape from lava.

Vestigial organs – Organs which are no longer of any use to an organism, but their presence indicates that organisms once had these organs in their evolutionary past.

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

Visible light – Radiation with a wavelength from .4 to .7 micrometre.

Volcanic (extrusive igneous) rock - An igneous rock that forms when magma is extruded onto the Earth's surface is extruded onto the Earth's surface and cools and crystallizes or when pyroclastic materials become consolidated.

Volcanic arc – An arcuate chain of volcanic islands or volcanoes along a continental margin, which erupt mainly andesitic lavas and ash (but including also basalts and rhyolites); generally associated with topographic trenches and much seismicity.  All these phenomena are the results of subduction of lithosphere beneath the arc.

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

Volcano – A mountain formed of lava and/or pyroclastics.

Walther’s Law – Observation that, when environments change position, adjacent sedimentary facies will succeed each other in vertical depositional sequences.  Vertical changes of lithology due to transgression or regression are mirrored by similar lateral changes (e.g., coarsening laterally as well as upward).

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

Wash – A common term for a desert stream course that is typically dry except for brief periods immediately following a rain.

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

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

Wave of oscillation – A water wave in which the wave form advances as the water particles move in circular orbits.

Wave of translation – The turbulent advance of water created by breaking waves.

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

Wave refraction – See Refraction.

Wave-cut cliff – A seaward-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.

Weak nuclear force - A force responsible for the breakdown of atomic nuclei, thus producing radioactive decay.

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

Weathering – The disintegration and decomposition of rock at or near Earth’s surface.  

Welded tuff – A pyroclastic rock composed of particles that have been fused together by the combination of heat still contained in the deposit after it has come to rest and by the weight of overlying material.

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 dwarf – 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.

White frost – Ice crystals instead of dew that form on surfaces when the dew point is below freezing.

Wilson cycle - The relationship between mountain building (orogeny) and the opening and closing of ocean basins.

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

Wind vane – An instrument used to determine wine direction.

Winter solstice – The solstice that occurs on December 21-22 in the Northern hemisphere and on June 21-22 in the Southern Hemisphere.

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

Zodiac – A band along the ecliptic containing the 12 constellations of the zodiac.

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

Zone of aeration – Area above the water table where opening in soil, sediment, and rock are not saturated but filled mainly with air.

Zone of fracture – The upper portion of a glacier consisting of brittle ice.

Zone of saturation – 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 of the snow from the previous winter melts, as does some of the glacial ice.

Zuni sequence - An Early Jurassic to Early Paleocene sequence of sedimentary rocks bounded above and below by unconformities; deposited during a transgressive-regressive cycle of the Zuni Sea.