GLOSSARY

Note: If you can't find a term in this glossary, try a standard dictionary. Whenever you are studying, you should always keep a good dictionary at hand. Check Appendix I for additional information.

Abyssal plain

• • A flat, sediment-covered region of the deep sea floor.

Accreted terrane

• • A region that formed apart from the landmass to which it is now attached, and that became attached or accreted to it at a later time.

Accretion

• • The process of growth of planets and planetesimals by the infalling of matter that is swept up from interplanetary space.

Achondrites

• • Stony meteorites containing little or no metal that are igneous in origin and probably resemble Earth's mantle rocks.

Aerobic

• • Pertaining to life that requires free oxygen.

Airglow

• • A faint glow produced in the lower ionosphere and upper mesosphere due to the absorption of solar radiation and particles in that region.

Albedo

• • A measure of the extent to which a surface reflects incident light.

Algorithm

• • A procedure for obtaining a desired result, generally applied to mathematical procedures.

Amino acid

• • Simple organic compounds that are the fundamental building blocks for living organisms.

Anaerobic

• • pertaining to organisms that require or can live in the absence of free oxygen.

Andesite

• • A generally fine-grained igneous rock of intermediate chemical composition.

Anthracite

• • The highest grade of coal, with high heating value and generally low sulfur content.

Anticline

• • A fold in rock strata that is convex upward.

Asteroid

• • Small rocky bodies in the solar system, many of which are found between the orbits of Mars and Jupiter.

Asthenosphere

• • The weak, soft region of the mantle just below the lithosphere.

Aulacogen

• • The failed rift-valley arm of what began as a triple spreading ridge junction.

Aurora

• • The Northern Lights (Aurora Borealis) and the Southern Lights (Aurora Australis).

Auroral oval

• • A circular band roughly centered on the magnetic poles in which aurora are most frequently visible.

Baltica

• • Northeastern Europe, including Scandinavia.

Basalt

• • A fine-grained mafic igneous rock.

Basement

• • The igneous or metamorphic layer beneath sedimentary strata.

Bathymetric map

• • A map showing the depths to the ocean floor.

Bauxite

• • The principal ore of aluminum, occurring as an oxide.

Benthic storm

• • Turbulent transport of sediment along the deep-sea floor by rapidly-moving currents.

Biogenic

• • Of biological origin.

Biogeochemical cycle

• • The process in which elements such as carbon, oxygen, nitrogen, phosphorus, sulfur, and others are cycled through the biosphere and geosphere.

Biomass

• • Pertaining to all organic matter produced by presently (or very recently) living organisms.

Biosphere

• • That portion of the rocky and fluid world inhabited by life.

Bitumen

• • A general name for various solid and semisolid hydrocarbons.

Bituminous coal

• • A dense black coal of high grade that burns with a flame.

Black smokers

• • Rock chimneys on the sea floor venting hot water with dissolved minerals suspended in it, giving the appearance of smoke.

Brachiopods

• • Marine shelled animals with two unequal shells.

Bow shock

• • The boundary between the magnetosheath and the solar wind.

Breeder reactor

• • A reactor in which some of the neutrons produced are used to convert nonfissionable Uranium-238 into fissionable Plutonium-239.

Caldera

• • The crater or bowl-shaped depression formed at the summit of a volcano or by the collapse of a magma chamber.

Callisto

• • A moon of Jupiter.

Carnivore

• • A meat eater.

Catastrophism

• • The invocation of a general catastrophe at some time in the past in order to explain differences between fossils and fossil abundances found in different strata.

Centripetal acceleration

• • The acceleration of an object toward a fixed central point that produces circular motion.

Chemosynthesis

• • The use by organisms of energy from chemical reactions, rather than from light, as in photosynthesis .

Ceres

• • The largest asteroid.

Charon

• • The satellite of Pluto.

Chlorofluorocarbons

• • A class of relatively inert gases used as propellants in spray cans and as refrigerants in air conditioners and refrigerators.

Chondrites

• • Stony meteorites whose compositions match that of the atmosphere of the Sun. They contain small grains of nickel-iron metal.

Chloroplasts

• • Organelles that carry out photosynthesis within cells.

Chromite

• • The principal ore of chromium, an oxide.

Chromosphere

• • The region of the Sun's surface that lies between the photosphere and the transition zone. It appear magenta or pink during total eclipses of the Sun.

Climate proxy

• • An indicator of past climates.

Comets

• • Small bodies in orbit around the Sun that are composed of rock and ice. When they approach the Sun, vapor is produced from the ice that is swept by the solar wind away from the Sun, producing a tail.

Conduction

• • The process by which heat travels through solid bodies from a warm region to a cooler region.

Continental margin

• • The edge of the continent consisting of the continental shelf, slope, and rise.

Continental rise

• • The gradual slope between the continental slope and the deep ocean floor.

Continental shelf

• • The relatively flat portion of continental crust that is covered by shallow sea water.

Continental slope

• • The edge of the continental shelf where it slopes down toward the deep ocean floor at an average angle of four degrees.

Convection

• • The process by which heat is carried by a moving fluid (gas or liquid) from a warm region to a cooler region. Convection is caused by density differences in the fluid caused by heat.

Coriolis Effect

• • The apparent tendency of moving objects, as seen from the rotating surface of a planet, to veer off to one side or the other, depending on whether the object is north or south of the equator.

Corona

• • The outermost portion of the Sun, visible during total solar eclipses.

Coronagraph

• • A telescopic device that blocks out the bright disk of the Sun, allowing the much dimmer corona to be photographed.

Coronal holes

• • Regions of the Sun in which magnetic field lines extend far out into interplanetary space before looping back to the Sun.

Cracking

• • The process of breaking large, complex molecules (especially hydrocarbons) into smaller, simpler ones.

Creationists

• • Those who believe that life originated in one or a series of specific creations, as opposed to evolutionists, who believe life evolved in a slow and relatively gradual process from simple to complex forms. Creationists generally believe that geological history should be consistent with events described in the Book of Genesis in the Bible.

Crust

• • The outermost layer of the Earth, above the Mohorovicic discontinuity.

Cyclone

• • A circulating air mass that rotates counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. Also used in many parts of the eastern hemisphere as synonymous to "hurricane".

Declination

• • The angle between true or geographic north and magnetic north.

Deimos

• • A satellite of Mars.

Demographic transition

• • The transition of a society from a situation with high birth and death rates to one in which birth and death rates are low. During the transition, population usually increases rapidly.

Dendrochronology

• • Dating via tree rings.

Desertification

• • The spread of deserts.

Deuterium

• • Heavy hydrogen, an atom whose nucleus consists of one proton and one neutron. Ordinary hydrogen has only one proton.

Dione

• • A satellite of Saturn.

Diorite

• • A coarse-grained igneous rock of intermediate chemical composition.

Dipole magnetic field

• • A magnetic field that has two antipodal poles of opposite polarity.

Doldrums

• • A region of light winds in the intertropical convergence zone.

Dolomite

• • A calcium-magnesium carbonate, often similar in appearance to limestone.

Early Faint Sun Paradox

• • While the Sun has increased its output of energy by as much as 30% during the course of its evolution, worldwide climate has not changed so much as to prevent the continuance of life.

Echo sounder

• • An acoustic device for measuring the depth of the ocean floor.

Eckman-layer flow

• • The tendency of water in the upper layer of the ocean to flow at right angles to the direction of the wind that blows along the sea surface.

Ecliptic

• • The plane of Earth's orbit as projected on the sky. This is the apparent path of the Sun through the constellations of the Zodiac.

El Niño

• • A change in the normal interaction of prevailing winds and ocean currents in the equatorial Pacific Ocean, characterized by a failure of the normal upwelling currents off the coast of Peru and Chile and other effects.

Eolian

• • Applied to the erosive action of the wind and its ability to transport sediment.

Epicenter

• • The point on the surface of the Earth directly above the principal energy release during an earthquake.

Erosion

• • Any process that loosens or disintegrates rock or soil and allows it to be transported elsewhere.

Escape velocity

• • The velocity that must be imparted to a mass (neglecting air friction) in order for it to permanently leave the vicinity of a planet or star.

    Eukaryote

    An organism whose genetic material is organized into a well-defined nucleus which is surrounded by a nuclear wall membrane.

Europa

• • A satellite of Jupiter.

Eutrophication

• • An oversupply of nutrients in that can cause algal blooms and depletion of free oxygen in bodies of water.

Evaporite

• • Mineral deposits left behind after evaporation of the fluid in which they were dissolved.

Extratropical cyclone

• • The air circulation system around low-pressure cells; not necessarily associated with major storms .

Fault

• • A fracture in a rock mass across which there has been displacement of one side relative to the other.

Felsic

• • Minerals and rocks characterized by high silica content and low iron and magnesium content. Generally low in density and light in color.

Fetch

• • The extent of open ocean across which prevailing winds may blow unhindered.

Firestorm

• • A general conflagration characterized by high temperatures, fierce hot winds, and exhaustion of oxygen in the air.

Flares

• • See Solar flares.

Flood basalt

• • Huge outpouring of basaltic lava from fissures in the ground; not necessarily associated with volcanic mountains.

Fossil fuels

• • Fuels derived from organic matter in geologic deposits coal, oil, natural gas, tar sands, and oil shales.

Fossils

• • The remains or traces of animals or plants that have been preserved in rocks by natural processes.

Fractionation

• • Separation of one or more substances from a mixture.

Fraunhofer lines

• • Dark absorption lines in the solar spectrum that can be used to determine the composition of the outer layers of the Sun.

Fumarole

• • A vent in the ground emitting volcanic gases and sometimes hot water.

Fusilinids

• • Important microfossils in the Pennsylvanian and Permian systems. They became extinct in the great dying at the end of the Paleozoic.

Gabbro

• • A coarse-grained mafic igneous rock.

Galilean satellites

• • The four largest moons of Jupiter Io, Europa, Ganymede, and Callisto, that were observed first by Galileo.

Gamma ray

• • Extremely energetic electromagnetic radiation, with wavelengths less than 1 x lO-10 meter.

Ganymede

• • A satellite of Jupiter.

Geocentric model of the solar system

• • An early model of the solar system in which the Sun revolves about the Earth, which stands in the center.

Geoid

• • The shape of the Earth as defined by sea level.

Geophone

• • A sensitive microphone designed to detect low-frequency vibrations produced in seismic surveying.

Geosphere

• • The solid part of the Earth.

Geostrophic wind

• • A wind that moves parallel to isobars and perpendicular to the pressure gradient. It is a consequence of the Coriolis Force.

Geothermal energy

• • Energy derived from the Earth's crust, especially in volcanic regions.

Geyser

• • A hot spring in which water and steam are periodically expelled with some force.

Globigerina

• • A microscopic single-celled marine organism that secretes a hard calcium carbonate test, or outer shell, which makes it a useful fossil for stratigraphic studies.

Gondwana

• • A supercontinent consisting of South America, Africa, Madagascar, Antarctica, Australia, New Zealand, and other smaller landmasses, that existed throughout the Paleozoic.

Grab sampler

• • A scoop lowered to the ocean floor for sampling sediment and rock.

Granite

• • A coarse-grained felsic igneous rock.

Granulation

• • A mottling of the photosphere of the Sun due to the motion of convection cells below it.

Gravimeter

• • A device designed to measure the strength of the force of gravity.

Greenhouse Effect

• • Warming in the lower atmosphere due to the ability of air to transmit visible light from the Sun but to absorb infrared radiation attempting to escape into space.

Gyre

• • A large-scale rotating mass of fluid, such as those found in the ocean basins.

Hadley cell

• • Vertically-oriented circulating air patterns present in the tropical atmosphere. See Figure VII-9 on page 229.

Half-life

• • The time needed for the atoms of a parent radioisotope to decay to one-half their original number.

Heat islands

• • A region of somewhat higher temperatures produced by man-made activities around cities.

Heavy oil

• • Tar or very viscous oil that cannot be pumped from the ground like crude oil.

Heat capacity

• • The amount of heat that must be put into a gram of a substance in order to produce a one degree Celsius temperature rise in it. Similarly, the amount of heat that leaves a gram of the substance when it cools by one degree Celsius.

Heliocentric model of the solar system

• • The present model of the solar system in which Earth and the other planets revolve in orbits around the Sun, which stands in the center.

Herbivore

• • Plant eaters.

Holocene epoch

• • The relatively warm period since the last Ice Age. Approximately the last 10,000 years.

Horse latitudes

• • A region of light winds in the high pressure zone between the westerlies and the trade winds.

Hot spot

• • A localized source of heat within the mantle that produces volcanism above it. Hot spots often produce lines of volcanoes.

Humidity

• • See Relative humidity.

Hydrocarbons

• • Compounds consisting only of carbon and hydrogen.

Hydrologic cycle

• • The recirculating path that water takes in the natural world, evaporating from the oceans, falling as rain, traveling in rivers back to the oceans.

Hydrothermal

• • Hot or warm water, usually within porous or permeable rocks.

Hyperion

• • A satellite of Saturn.

Hypsometric diagram

• • A diagram showing the proportion of Earth's rocky surface that stands at different elevations .

Iapetus

• • A satellite of Saturn.

Igneous rocks

• • Rock produced by the freezing of molten rock.

Impact craters

• • Depressions blasted out of rock by infalling meteorites.

Inclination

• • The angle that the magnetic field makes with the horizontal.

Inner core

• • The innermost part of the Earth, probably solid iron.

Interglacial

• • A warm period separating Ice Ages.

Intertidal zone

• • The elevation zone between high and low tides.

Intertropical convergence zone

• • A region, generally near the equator, where the trade winds converge to replace air that rises into the Hadley cells.

Io

• • The innermost satellite of Jupiter.

Ion

• • An atom or molecule that has become electrically charged through the loss or gain of one or more electrons.

Ionization

• • The process of producing ions from neutral (uncharged) atoms or molecules.

Ionosphere

• • The lower portion of the thermosphere, characterized by the presence of ions. The aurora are produced within this region.

Isobar

• • A line on a map showing those points that all have the same atmospheric pressure.

Isoseismal map

• • A map depicting zones of equal intensity on the Modified Mercalli scale for a given earthquake .

Isostasy

• • The concept that explains large-scale surface elevations in terms of the buoyancy of the low-density crust as it "floats" on the higher-density mantle.

Isotopes

• • Forms of single element that have differing numbers of neutrons in their nuclei, but all with the same number of protons. Different isotopes of an element have different masses, but nearly the same chemical properties.

Jet stream

• • Fast-flowing rivers of air in the upper atmosphere.

Jovian planets

• • Jupiter, Saturn, Uranus, Neptune -- the large, gaseous, low-density outer planets with many satellites and rings.

Kazakhstania

• • A portion of central Asia east of the Ural Mountains that may have traveled as part of a separate microplate at some time in the Paleozoic.

Kinematics

• • The description of motion, as opposed to dynamics, which is a description of the forces that produce motion.

Land bridges

• • An early explanation for the appearance of similar land fossils on continents now separated by ocean basins, in which a narrow bridge of land connected the continents and has since sunk into the sea. Continental drift is now the favored explanation.

LANDSAT

• • A remote-sensing satellite designed to carry out multispectral studies of Earth's landmasses.

Latent heat of vaporization

• • The energy required to vaporize a unit mass of liquid. The same amount of heat is released when the vapor condenses back into liquid.

Laterite

• • A red soil characteristic of tropical regions, in which silica has been leached out, leaving behind a residual soil rich in aluminum and iron oxides.

Laurentia

• • A continental landmass consisting of present-day North America, Greenland, and a bit of northern Europe, including Scotland, that apparently existed during the Early Paleozoic era.

Laurussia

• • A continental landmass formed by the union of Laurentia and Baltica that existed during the Late Paleozoic.

Lava

• • Molten rock extruded onto the surface of the Earth, as by a volcano.

Light water reactor

• • The most common type of nuclear reactor in use today.

Lignite

• • A brownish black coal that is higher-grade than peat but lower-grade than sub-bituminous coal.

Limestones

• • Sedimentary rocks formed from calcium-rich sediments, often made up largely of the shells and skeletal remains of biota.

Lineaments

• • Straight lines that appear on maps of landscape features, often indicating the presence of faults .

Lithification

• • The process by which sediments are made into rock.

Lithosphere

• • The rigid outermost portion of the Earth, comprising the moving plates. It includes the crust and a portion of the upper mantle.

Little Ice Age

• • A period extending from roughly 1500-1850 AD in which global temperatures were significantly colder than more recent averages.

Love wave

• • A type of surface wave in which the particle motion is horizontal.

Low velocity zone

• • A region of low seismic velocity that is observed most clearly beneath oceanic crust at a depth of roughly 100 to 300 km. It is a portion of the asthenosphere.

Mafic

• • Rocks characterized by high iron and magnesium content and low silica content. Generally dense and dark in color.

Magma

• • Molten rock occurring at depth within the Earth.

Magmatic concentration

• • The process in which magma tends to segregate into different constituents as it cools and solidifies.

Magnetic polarity reversal

• • An event in which the direction of the Earth's magnetic field reverses its direction everywhere on Earth's surface. In the process, the North and South Magnetic Poles are interchanged.

Magnetic storm

• • Short-term changes in Earth's magnetic field excited by events on the Sun.

Magnetograms

• • Applied to the Sun's magnetic field, a photograph that allows the polarity of the Sun's local magnetic fields to be determined. Applied to Earth's magnetic field, a record of short-term magnetic field activity that generally reflects the level of solar activity.

Magnetometer

• • A device designed to measure the strength, and sometimes the direction, of a magnetic field.

Magnetopause

• • The boundary between the Earth's magnetic field and that of the solar wind.

Magnetosheath

• • The region of magnetic turbulence between the magnetopause and the bow shock.

Magnetosphere

• • The regional interaction between Earth's and the solar magnetic fields.

Magnetospheric substorm

• • Disturbances that originate on the night side of the magnetosphere that can produce auroral displays.

Magnetotail

• • A long tail-shaped region of the Earth's magnetic field that points away from the Sun due to the action of the solar wind.

Manganese nodules

• • Concretions of manganese and iron oxides with other metals found in certain places lying on the deep-sea floor.

Mantle

• • The largest part of the Earth's interior, below the crust and above the core, probably composed of ultramafic rocks.

Maria

• • Large basaltic plains on the Moon.

Maunder Minimum

• • A period from 1645 to 1715 AD during which sunspots were hardly ever observed. Solar activity was presumably uncommonly low during this period.

Mechanical concentration

• • A process of concentration for heavy metals that do not react readily with other elements. They are concentrated by stream action or other mechanical action due to their density.

Medieval Optimum

• • An abnormally warm period that extended from about 900 to 1200 AD.

Mercalli Intensity Scale

• • See Modified Mercalli Intensity Scale.

Mesoscale

• • Middle scale; that is, neither the largest nor smallest possible scale of phenomena being described.

Mesoscale eddy

• • Rotating mass of water on a smaller scale than that of a gyre. Same as ring current.

Mesosphere

• • A region of the atmosphere above the stratosphere in which temperature falls with increasing elevation.

Metamorphic rocks

• • Any rocks that have heated and/or squeezed to such an extent that their chemical and physical makeup are altered.

Meteor

• • A piece of extraterrestrial rock, metal, or ice falling to Earth and glowing incandescent in its flight through the atmosphere.

Meteorite

• • A piece of extraterrestrial rock or metal that has survived its fall to Earth.

Meteorology

• • The study of the atmosphere and weather.

Microplate

• • A smaller than continent-sized plate.

Milankovitch hypothesis

• • A hypothesis that relates Pleistocene climate changes to slight changes in Earth's orbit around the Sun.

Mimas

• • A satellite of Saturn.

Mineral

• • A naturally-occurring compound or element of definite composition.

Mitochondria

• • Organelles within cells that act to combine carbohydrates and fatty acids with oxygen in order to release useful energy.

Modified Mercalli Intensity Scale

• • A measure of the destruction caused by an earthquake.

Mohorovicic discontinuity (Moho)

• • The boundary between the crust and mantle.

Monsoons

• • Seasonal shifts in atmospheric circulation that brings alternating wet and dry seasons, especially marked in the eastern hemisphere.

Moraine

• • Piles of gravel and rocks left by retreating glaciers.

Multispectral scanner

• • A device that is capable of scanning (and often, constructing images) at several specific wavelengths.

Neap tide

• • A lower-than-average tidal amplitude due to opposing effects of the Sun and Moon.

Nebula

• • An interstellar dust cloud.

Nemesis

• • The name given to a hypothetical companion star to the Sun, which has been postulated to explain periodic bombardments of the inner solar system by large numbers of comets.

Neutrino

• • A subatomic particle with no electric charge and a vanishingly small rest mass that travels nearly at the speed of light and seldom interacts with other matter.

Nitrogen fixation

• • The production of water-soluble nitrogen compounds from free nitrogen gas.

Noble gas

• • One of the inert gases helium, neon, krypton, radon. They do not normally form chemical compounds and are found in nature as elements.

Normal fault

• • A fault that occurs in response to horizontal tensional forces.

Nova

• • The explosion of a star, often triggered by the exhaustion of the star's nuclear fuels. An enormous amount of energy is released, accompanied by extremely high temperatures and pressures that can cause the nucleosynthesis of heavy elements.

Nuclear Winter

• • Global lowering of temperature expected to take place in the aftermath of a substantial nuclear weapons exchange.

Nutrients

• • Elements essential to life.

Occluded front

• • Occurs when a cold front overtakes a warm front. The warm air mass is lifted above the cold air masses and does not contact the ground.

Ocean basin

• • The deep parts of an ocean.

Oil shale

• • Fine-grained sediments containing organic matter that can be distilled to yield oil.

Olivine

• • An ultramafic class of minerals, magnesium-iron silicates, that are presumed to be abundant in the mantle.

Omnivores

• • Creatures that eat both plants and meat.

Oort cloud

• • A cloud of comets that is presumed to be orbiting the Sun beyond the orbit of Pluto.

Ooze

• • Ocean sediments rich in biogenic material.

Ophiolite

• • A suite of mafic and ultramafic igneous rocks associated with deep-sea sediments found on a landmass; now believed to be portions of uplifted sea floor.

Orbital ellipticity

• • The extent to which the Earth's orbit around the Sun is elliptical rather than circular.

Ore

• • Rocks in which certain minerals are concentrated by natural processes to such an extent that it is profitable to mine them.

Organelles

• • Small specialized units within eukaryotic cells.

Outer core

• • The liquid portion of the Earth between the inner core and the mantle, probably composed largely of molten iron.

Ozone hole

• • Circular regions surrounding the North and South Poles in which stratospheric ozone concentrations drop dramatically during the polar summer season and then rebuild during the winter.

Ozone layer

• • A concentration of ozone (03) in the lower stratosphere that absorbs much of the ultraviolet light from the Sun.

P wave

• • The first-arriving seismic wave characterized by particle motion parallel to the direction of wave travel.

Paleoclimatology

• • The study of ancient climates.

Paleomagnetism

• • The study of the ancient Earth's magnetic field as recorded in rocks.

Paleontology

• • The study of fossils.

Pangea

• • The supercontinent that existed at the end of the Paleozoic Era and that included all the landmasses of the world.

Peat

• • Partially decomposed remains of marsh plants that may be used as a low-grade fuel.

Pegmatite dikes

• • Coarse-grained igneous rocks found in dikes or cracks in the surrounding rock; usually associated with igneous intrusions.

Penumbra

• • The mottled outer portion of sunspots.

Peridotite

• • A dark, dense, ultramafic rock. An important constituent of the mantle.

Permeability

• • The ability of a rock to transmit a fluid through pores and cracks.

Permo-Carboniferous

• • At the boundary between the Carboniferous and Permian Eras, 290 million years ago.

Phobos

• • A satellite of Mars.

Phosphorites

• • Phosphorus-rich deposits that tend to form on the sea floor near continents.

Photosphere

• • The outer portion of the Sun that constitutes its visible surface.

Photovoltaic solar cells

• • Solid-state cells that are capable of using radiant energy from the Sun directly to generate a flow of electricity.

Phytoplankton

• • Microscopic marine plants.

Pillow basalts

• • Rounded pillow-shaped basalt tongues that form underwater as fluid lava breaks through a cooled outer crust. The result looks somewhat like a pile of sandbags.

Pinger

• • An acoustic device lowered on an instrument package that determines how far the package is from the sea floor.

Piston corer

• • A device for sampling sediment on the ocean floor.

Placer deposit

• • Mechanically concentrated deposits of metals such as gold found in the sand and gravel of streams.

Planetesimal

• • A body in the solar system of smaller size than a planet.

Plankton

• • Free-floating marine organisms that are not able to swim rapidly. Many are quite small.

Plasma

• • A gas consisting largely of ionized particles.

Pleistocene epoch

• • The time of the Ice Ages, spanning roughly the past two million years.

Polar wander

• • The apparent motion of the magnetic poles as seen from a particular landmass. In most cases, it turns out that the pole is fixed and it is the landmass that is moving.

Polarity reversals

• • See Magnetic polarity reversals.

Porosity

• • The proportion of a rock that is pore space (the space between grains). Fluids may migrate freely through porous rocks.

Porphyry copper deposits

• • Large and often relatively low-grade copper deposits associated with igneous intrusions.

Positron

• • A sub-atomic particle of the same mass as an electron but with a positive charge.

Precession of the Equinoxes

• • A gradual and periodic change in direction of the Earth's axis in space.

Pressure gradient

• • The change in pressure over a unit horizontal distance, where the distance is measured in the direction of maximum pressure change.

Prevailing winds

• • The average direction of winds in a particular place and season.

Primordial cloud

• • The dust cloud from which the Sun and the rest of the solar system formed.

Principle of Original Horizontality

• • States that sedimentary rocks generally form with each layer flat and approximately horizontal.

Principle of Superposition

• • States that in an undisturbed series of strata, the oldest are on the bottom and the youngest are on the top.

Prokaryote

• • Primitive single-celled organisms, represented today by blue-green algae and bacteria.

Prominences

• • See Solar prominences.

Pyroclastic surge

• • A mixture of molten or nearly molten rock and hot gases that may be explosively erupted from volcanoes.

Pyroxene

• • An ultramafic class of minerals.

Radiant energy

• • Energy that travels in the form of electromagnetic waves.

Radioisotope

• • Radioactive isotopes.

Radiolaria

• • Marine protozoans that possess complex internal siliceous skeletons. They may be used as an indicator of ocean temperatures at the time that they lived.

Radiosonde

• • A weather balloon carrying measuring instruments and a radio for transmitting information back to the ground.

Rain shadow

• • The lee side of a mountain chain with respect to the prevailing winds, where dry conditions often result.

Rayleigh wave

• • A surface seismic wave in which the particle motion is similar to that in a water wave.

Refraction

• • The bending of a wave as it travels across the boundary between two substances with different wave velocities.

Relative humidity

• • A measure of the amount of moisture (as vapor) in the air compared to its carrying capacity .

Replacement

• • The process in which rocks are altered when they come into contact with hot fluids, sometimes forming mineral ores.

Reserves

• • Known deposits from which minerals can be extracted profitably using existing technology under present economic and legal conditions.

Residual concentration

• • Concentration of a mineral because it is less easily weathered than surrounding rock, which is carried away.

Resolution of a model

• • The smallest increment, in space or time, that can be resolved or distinguished in a numerical model.

Resources

• • Known potential sources of extractable minerals that might be used in the future if changes in technology or economic and legal conditions allow.

Rhea

• • A satellite of Saturn.

Rhyolite

• • A fine-grained felsic igneous rock.

Richter magnitude scale

• • A measure of the strength of an earthquake, based on measurement of the amplitude of seismic waves that is corrected for distance to the earthquake.

Rift valley

• • A valley caused by extensional forces acting to produce normal faults.

Rilles

• • Sinuous valleys cut into the lunar maria.

Ring current

• • See mesoscale eddy.

Rock

• • A solid, naturally-occurring mass of mineral matter. Rocks can occur with a wide variety of mineral content.

Rock magnetism

• • The study of magnetic properties and behavior of rock materials.

S wave

• • A seismic wave characterized by particle motion perpendicular to the direction of wave motion. It travels slower than P waves.

Salinity

• • Measure of the quantity of total dissolved solids in water.

Sandstones

• • Sedimentary rocks formed from sandy deposits.

Scientific Method

• • The logical procedure by which scientists attempt to explain natural phenomena. It is based on an interplay between observations and explanations (hypotheses, theories, models).

Sea-floor spreading

• • The process by which plates move apart at oceanic ridges, creating new sea floor.

Seasat

• • The first remote-sensing satellite dedicated solely to study of the oceans.

Sedimentary basin

• • Downwarped basin filled with sedimentary rocks. The sediments are thickest in the middle and become thinner toward the edges of the basin.

Sedimentary rocks

• • Rocks formed by the deposition and cementation of sediment grains.

Seiche

• • A periodic wave set up in a lake or bay by a seismic disturbance.

Seismic ray

• • A convenient geometrical representation of the direction of travel of seismic waves.

Seismic sea wave

• • A sea wave caused by an earthquake. Popularly, but incorrectly, known as a tidal wave.

Seismic tomography

• • A technique for constructing a three-dimensional seismic velocity model of Earth's interior.

Seismicity

• • The tendency of a region to be prone to the occurrence of earthquakes.

Seismograph

• • A device designed to detect vibrations from earthquakes or man-made sources.

Seismology

• • The study of earthquakes and the travel of vibration waves through the Earth and other planets.

Sensible heat

• • The heat transferred as a mass of air moves from one place to another, carrying its thermal energy with it.

Shales

• • Sedimentary rocks formed from mud.

Shepherd moons

• • Small moons associated with the ring system of Jupiter that may be responsible for maintaining the integrity of some of the rings.

Shot point

• • The point at which an artificially-induced seismic wave is introduced into the ground.

Solar flares

• • An extremely large release of energy originating in a localized region of the solar corona.

Solar prominences

• • Arches and streamers extending far beyond the Sun's disk, consisting of material ejected during solar flares and guided by the Sun's magnetic field.

Solar wind

• • A flux of charged particles streaming outward from the Sun.

Sonar

• • A device that measures distance to the hard surface of the Earth by measuring the time for a pulse of sound to travel that distance, be reflected off the surface, and return to the instrument.

Source rock

• • The rock in which a hydrocarbon deposit forms.

Southern oscillation

• • The shift of a major persistent low-pressure cell in the western Pacific Ocean, probably linked to the occurrence of the El Niño phenomenon.

Spectrograph

• • A device designed to display or record the spectrum of a light source.

Spectroheliograph

• • A telescopic device that allows photographs to be made in the light of a single emission line.

Spectroscopy

• • The study of emission and absorption lines in spectra.

Spectrum analysis

• • The separation of a complex waveform into its constituent sinusoidal components, similar to the separation of a musical chord into its constituent notes.

Spicules

• • Sharp protuberances in the upper boundary of the chromosphere that give a furry appearance to the edge of the Sun.

Spreading ridge

• • The manifestation in the ocean basins of a diverging plate boundary, where new ocean floor is being created in the process of sea floor spreading.

Spring tide

• • A larger-than-average tidal amplitude, caused by additive effects of the Sun and Moon.

Squall line

• • A series of storms organized ahead of an advancing storm front.

Stable eddy

• • A persistent cyclone in a planetary atmosphere. These are especially prevalent on Jupiter.

Strata

• • Plural of stratum.

Stratosphere

• • The layer of the atmosphere between roughly 10 and 50 km in which the temperature rises with elevation.

Stratum

• • A single sedimentary rock unit that may be distinguished in its features from other rock units above or below it.

Structural trap

• • A fold, fault, or other structural feature of strata that forms a trap for migrating oil and gas.

Subduction

• • The process by which plates move toward one another and overlap, with the lowermost bending down into the mantle and being consumed.

Substorms

• • See Magnetospheric substorms.

Suture zone

• • The zone that marks an ancient collision between continents.

Swell

• • Long-wavelength sea waves that may travel long distances in the open ocean.

Synoptic map

• • Large-scale maps giving a synopsis of weather conditions at a particular time.

Taconite ores

• • Low-grade iron ores.

Tar sands

• • Porous sandstones that contain heavy oil.

Tectonics

• • The study of the deformation and movement of the crust.

Terrane

• • A region characterized by a particular rock type or origin.

Terrestrial planets

• • Mercury, Venus, Earth, and Mars -- the innermost planets that are rocky, dense, relatively small, and deficient or lacking in satellites and ring systems.

Terrigenous sediments

• • Sediments that originate on the landmasses.

Tethys Sea

• • The ocean that separated India and Africa from Asia at the time that Pangea existed.

Thermocline

• • A region of rapid temperature change in the oceans that divides the surface waters from the deep waters.

Thermonuclear fusion

• • The process by which four hydrogen atoms are fused to form one helium atom. A large amount of energy is released in the process.

Thermosphere

• • A region of the atmosphere above the mesosphere in which temperature rises rapidly with elevation above about 80 km.

Tidal heating

• • Frictional heating of a planet's or satellite's interior due to the flexing caused by the passage of tides raised by the gravity field of its partner.

Tillite

• • Lithified glacial deposits.

Titan

• • The largest satellite of Saturn.

Trace elements

• • Elements present in very small amounts.

Trade winds

• • Tropical prevailing winds that blow toward the equator from the northeast in the northern hemisphere and from the southeast in the southern hemisphere.

Transition zone

• • A narrow region separating the chromosphere from the solar corona in which the temperature rises rapidly.

Transform fault

• • A fault characterized by side-to-side motion (no gap or overlap formed) that connects two or more active plate boundaries.

Travel-time curves

• • A timetable that describes how long it takes various seismic waves to travel from a source to a destination.

Trench

• • The deepest parts of the oceans, associated with subduction zones.

Triple junction

• • The place where three plate boundaries come together.

Triton

• • A satellite of Neptune.

Tropopause

• • The boundary between the troposphere and the stratosphere.

Troposphere

• • The lowest portion of the atmosphere, in which the temperature drops with increasing elevation. Weather is largely confined to the troposphere.

Tsunami

• • The Japanese term for a seismic sea wave.

Turbidity current

• • A mass of water and sediment that flows down slopes in ocean or lake bottoms, sometimes attaining considerable force and speed.

Typhoon

• • Hurricane

Umbra

• • The dark inner portion of sunspots.

Uniformitarianism

• • The principle that states that no ancient geological process may be invoked that could not be observed to be in operation in historical times.

Ultramafic

• • Dark and dense rocks composed largely of mafic minerals.

Volcanic dust veil effect

• • Atmospheric cooling produced by decreased solar radiation arriving at the surface due to dust injected into the atmosphere by a volcanic eruption.

Volcanic island arc

• • An arc of generally andesitic island volcanoes situated behind an ocean trench and above the downgoing lithospheric slab in a subduction zone, when the uppermost slab is oceanic.

Wavelength

• • The distance measured between successive crests of a wave.

Westerlies

• • Prevailing winds from the west that are found in northern and southern mid-latitudes.

White smokers

• • Sea-floor vents emitting warm water that is cloudy due to the presence of chemosynthetic bacteria.

Zooplankton

• • Marine animals that live in a floating state and drift with the ocean currents.