abrasion The mechanical wearing away of a rock by friction, rubbing, scraping, or grinding.

absolute age Age of a rock or surface measured in Earth years in contrast to relative time, which involves only the sequence of events.

accretion The growth of planets from smaller bodies by collisions. In our solar system this occurred over 4.5 billion years ago.

achondrite A stony meteorite that lacks chondrules. Some have igneous textures and compositions and are similar to basalts; they may be derived from lava flows on the surface of asteroids.

albedo The brightness of an object or planetary surface. Bright objects have high albedos and dark ones have low albedos. The albedo is a measure of the reflectance of an object.

alluvial fan A fan-shaped deposit of sediment built by a stream, where it emerges from an upland into a broad valley or plain. Alluvial fans are common in arid and semiarid climates on Earth and on Mars.

alpha particle The nucleus of a helium atom, consisting of two neutrons and two protons.

amphibole An important group of iron- and magnesium-rich silicates. Amphibole crystals are constructed from double chains of silicon--oxygen tetrahedra.

andesite A fine-grained igneous rock composed mostly of plagioclase feldspar with mafic minerals. Andesite has an intermediate silica content and is commonly erupted above subduction zones on Earth, but is not restricted to these settings. Stratovolcanoes commonly erupt andesite.

anomaly  A deviation from the norm or average.

anorthosite A coarse-grained intrusive igneous rock composed primarily of calcium-rich plagioclase. Anorthosite is an important rock type in the lunar highlands.

anticline A fold in which the limbs dip away from the axis.

antipode A point on the surface of a planet, or sphere, exactly opposite another point.

aphanitic texture A rock texture in which individual crystals are too small to be identified without the aid of a microscope.

aquifer A permeable horizon or zone below the surface of a planet through which groundwater moves.

ash fall An accumulation of volcanic fragments erupted explosively that fell to the ground without subsequent flow.

ash flow A blend of unsorted pyroclastic fragments mixed with hot gases erupted explosively from a fissure or crater that flows across a planet's surface.

ash-flow shield A large shieldlike volcano composed of ash-flow tuff that usually surrounds a central collapse caldera. The caldera may be up to 100 km across. Highly silicic magmas, such as rhyolites, commonly erupt from such volcanoes on Earth. Such volcanoes are found on Earth, Io, and perhaps Mars.

asteroid A small, rocky planetary body orbiting the Sun. Asteroids are numbered in the tens of thousands. Most are located between the orbit of Mars and the orbit of Jupiter. Their diameters range downward from 1000 km.

asteroid belt The region between the orbits of Mars and Jupiter where most asteroids are found.

asthenosphere The zone in a planet directly below its lithosphere that is fluid enough to convect. This material may be partially molten.

astronomical unit (AU) The mean distance from the Earth to the Sun---approximately 150 million km.

atmosphere The mixture of gases surrounding a planet or moon. Atmospheres may be outgassed from the interior of a planet, or they may have been trapped gravitationally from the nebula out of which the planets formed.

atom The smallest unit of an element. Atoms are composed of protons, neutrons, and electrons.

axis An imaginary straight line about which a body rotates.


ballistic trajectory The path followed by an unpowered projectile; typically a high arch from the launching or ejection point to the point where it collides with the surface of the planet.

barchan dune A crescent-shaped dune where the tips or horns point downwind. Barchan dunes form in deserts where sand is scarce.

basalt A dark-colored, fine-grained igneous rock composed of plagioclase feldspar and pyroxene. Olivine may or may not be present. Basalt is the most common volcanic rock on the inner planets.

basaltic plains Volcanic regions with many small low-shields and fissure eruptions of basaltic lavas; they are common on the inner planets.

basin, impact A large impact crater, usually taken to be larger than 200 km in diameter. Most impact basins have multiple rings forming their perimeters.

basin, structural A circular or elliptical downwarp in a planet's lithosphere.

batholith A large body of intrusive igneous rock exposed over an area of at least 100 km2.

bed A layer of rock 1 cm or more in thickness.

biosphere The totality of life on or near Earth's surface. No other planet in the solar system is known to have a biosphere.

block faulting A type of normal faulting in which segments of the crust of a planet are broken and displaced to different elevations and orientations.

blowout A small depression excavated by wind erosion.

braided stream A stream with a complex of converging and diverging channels separated by bars or islands. Braided streams form where more sediment is available than can be removed by the discharge of the stream.

breccia A general term for rock consisting of angular fragments in a matrix of finer particles. Examples include sedimentary breccias, volcanic breccias, and impact breccias.

butte A somewhat isolated hill, usually capped with a resistant layer of rocks and bordered by fragments. A butte is an erosional remnant of a formerly more-extensive slope.


calcite A mineral composed of calcium carbonate (CaCO3).

caldera A large, more or less circular depression or basin associated with a volcanic vent. Its diameter is many times greater than that of the included vents. Calderas are believed to result from subsidence or collapse and are associated with many types of volcanoes, including shield volcanoes and ash-flow shields.

carbonaceous Containing carbon other than as carbonate.

carbonate mineral A mineral formed by the bonding of carbonate ions (CO3) with ions such as calcium (Ca) or magnesium (Mg).

carbonate rock A rock composed mostly of carbonate minerals; limestone is an example.

cement Minerals precipitated from groundwater in the pore spaces of a sedimentary rock and binding the rock's fragments together.

central peak A hill or group of mounds located in the center of moderately sized impact craters.

chondritic meteorite A meteorite with chondrules, or a meteorite that is chemically similar to those that do.

chondrule Spherical or nearly spherical bodies found in some stony meteorites. They range up to 10 mm in diameter and consist of silicate minerals.

cinder A fragment of volcanic ejecta from 0.5 to 2.5 cm in diameter.

cinder cone A cone-shaped hill composed of loose volcanic fragments surrounding a central vent. Most are composed of basalt.

clastic Pertaining to fragments (such as mud, sand, and gravel) produced by the mechanical breakdown of rocks, or describing the rocks formed chiefly of consolidated fragments.

coma The diffuse, fuzzy head of a comet that consists of gaseous material and surrounds the dense nucleus. The coma is created as a comet nears the Sun and its ice vaporizes from the heat.

comet A relatively small, icy body that orbits the Sun. The orbits of most comets are highly eccentric. When a comet is in the part of its orbit near the Sun, it develops a coma and a long tail. Comets are thought to have formed in the outer solar system.

composite volcano A moderate-size volcanic cone built by extrusion of ash and lava. Synonymous with stratovolcano.

compound A substance composed of two or more chemical elements.

compression A system of stresses that tends to reduce the volume of or shorten a substance.

condensation A chemical reaction in which a solid crystallizes or a liquid precipitates from a gas.

conduction The transfer of heat energy through solids by molecular impact without movement of the material itself.

continent A part of Earth's crust, from 20 to 60 km thick, composed mostly of granitic rock. Continents rise abruptly above the oceanic crust because of their relatively low densities.

convection The transfer of heat energy by movement of material. Fluids convect as a result of density differences produced by heating and consequent expansion, cooling, and contraction.

core The central part of a differentiated planet, usually composed of denser material. The inner planets have cores of iron in a liquid or solid state. The outer planets have cores of silicates and metal. The icy satellites may also have cores of silicates.

core refrigeration A process that takes place in the central parts of large stars that leads to collapse and nova or supernova explosions.

corona Elliptical, strongly deformed terrains found on Venus and Miranda.

crater frequency The number of impact craters of a certain size per unit area; high crater frequencies indicate old surfaces and low crater frequencies indicate young surfaces.

crater, impact A circular depression created by impact of a meteorite or comet; they are usually surrounded by an ejecta blanket.

crater, volcanic An abrupt more-or-less circular depression formed by extrusion of volcanic material or by collapse of part of a volcano.

cross-cutting relations, principle of The principle that a rock body is younger than any rock across which it cuts.

crust The outermost layer, or shell, of a differentiated planet. The crust is defined on the basis of its chemical composition, not its mechanical properties.

crustal warping Gentle bending (upwarping or downwarping) of a planet's crust.

crystal A solid, polyhedral form bounded by naturally formed plane surfaces.

crystallization The process of crystal growth. It occurs as a result of condensation from a gaseous state, precipitation from a solution, or cooling of a melt.

cyclone A storm in the atmosphere of a planet with counterclockwise (in the northern

hemisphere) circulation of winds.


deflation Erosion of loose rock particles by the wind.

deflation basin A shallow depression formed by wind erosion of loose particles.

degradation The general lowering of the surface of the land by processes of erosion or impact.

delta A large, roughly triangular body of sediment deposited at the mouth of a river.

dendritic drainage pattern A branching stream pattern resembling the branching of certain trees.

density The measure of concentration of matter in a substance: mass per unit volume, expressed in grams per cubic centimeter (g cm3).

differentiation, planetary The process by which the materials in a planetary body are separated according to density and chemical affinity, so that an originally homogeneous body is converted into a zoned, or layered, body with a dense core, a mantle, and a crust.

dike A tabular, intrusive igneous rock that cuts nearly vertically across strata or other surrounding rocks.

distributary Any of the numerous stream branches into which a river divides where it reaches its delta.

divergent plate boundary A lithospheric plate boundary formed where the lithosphere splits into plates that drift apart from one another as new lithosphere is created between them. See also midocean ridge.

dome, structural An uplift that is circular or elliptical in map view, with beds dipping away in all directions from a central area.

dome, volcanic A volcano formed from extremely viscous lavas that takes the shape of a steep-sided dome. Rhyolite domes are usually less than 1 km across on Earth.

drumlin A smooth, glacially streamlined hill that is elongate in the direction of ice movement. Drumlins are usually composed of unsorted glacial sediments.

dunes A low mound of fine-grained material that accumulates as a result of sediment transport in a current system on the surface of a planet. Dunes have characteristic geometric forms that are maintained as they migrate.

dust Very fine-grained material, smaller than sand grains, usually carried by the wind.


earthquake A series of elastic waves propagated in a planet, initiated where stress along a fault exceeds the strength of a rock so that sudden movement occurs along the fault.

eccentric orbit A noncircular orbit.

ecliptic The planet Earth, and most of the other planets, orbit about the Sun.

ejecta Rock fragments, glass, and other material thrown out of an impact crater or a volcano.

ejecta blanket Rock material (crushed rock, large blocks, breccia, and dust) ejected from an impact crater and deposited over the surrounding area.

elastic deformation Temporary deformation of a substance, after which the material returns to its original size and shape.

electron A very small elementary particle with a negative electrical charge. An electron normally moves about the nucleus of an atom.

energy The capacity for doing work.

eolian Pertaining to wind.

equilibrium A state of balance between opposing chemical or physical forces.

erosion The processes that loosen sediment and move it from one place to another on a planet's surface. Agents of erosion include water, ice, wind, and gravity.

eruption The ejection of material from a volcano; material may be erupted as lava, as pyroclastic fragments, or as gas.

escape velocity The velocity required for an object to escape the gravitational control of another planetary object.

excavation The stage of impact crater development where material is thrown out from the growing cavity.

extension A system of stresses such that a body is pulled apart by tensional forces.

extrusive rock A rock formed from a mass of magma that flowed out on the surface of a planetary body.


fault A surface along which a rock body has broken and been displaced.

feldspar A mineral group consisting of silicates of aluminum and one or more of the metals potassium, sodium, or calcium. Feldspars are the most common minerals in the crust of Earth, the Moon, and probably other inner planets.

fission, nuclear The breaking or splitting apart of a nucleus; energy is usually released as a result. Some of the heat-producing decay of uranium occurs by fission.

fissure eruption Extrusion of lava along an open crack.

flood lava An extensive flow of lava erupted chiefly from open fissures. Relatively smooth, featureless plains are produced. On the inner planets, these lavas are commonly basaltic; on the moons of the outer planets, they are composed mostly of water.

fluvial Pertaining to a river or stream.

flux The number of objects passing through a given area per unit time; for example, the meteorite flux corresponds to the number of meteorites striking a surface in a given time.

fold A bend, or flexure, in a rock body.

folded mountain belt A long, linear zone of a planet's crust where rocks have been deformed by horizontal stresses. On Earth, they form at convergent plate boundaries.

footwall The block beneath a dipping fault surface.

fossil Naturally preserved remains or evidence of past life, such as bones, shells, casts, impressions, and trails.

frost wedging The forcing apart of rocks by the expansion of water as it freezes in fractures and pore spaces.

fusion, nuclear The combination of two or more nuclei to form a different, usually heavier, element; energy is commonly given off as a result. The Sun produces its energy by nuclear fusion.


galaxy A group of associated stars, nebulas, star clusters, and planets. Our galaxy contains billions of stars and is shaped like a spiral with a massive central condensation.

geologic column A diagram representing divisions of geologic time and the rock units formed during each major period on a given planetary body.

geologic time scale The time scale determined by the geologic column and by absolute dating of rocks and surfaces on planetary objects.

geothermal gradient The rate at which temperature increases with depth in a planetary body.

geyser A thermal spring that intermittently erupts steam and boiling water.

giant planets The large planets in the outer solar system---Jupiter, Saturn, Uranus, and Neptune.

glacier A mass of ice that is thick enough to flow plastically.

gneiss A coarse-grained metamorphic rock with a characteristic layering resulting from alternating layers of mafic minerals.

graben An elongate fault block that has been lowered in relation to the blocks on either side.

granite A coarse-grained igneous rock composed of alkali feldspar, plagioclase feldspar, and quartz, with smaller amounts of mafic minerals. Granite is rich in silica and is common on Earth's continents.

gravity The tendency for matter to attract itself together.

ground ice Frozen water below the surface of a planetary body; it generally occurs in pore spaces of rocks and soil.

groundwater Liquid water below the surface of a planetary body; it generally occurs in pore spaces of rocks and soil.


half-life The time required for half of the radioactive atoms in an object to decay.

hanging wall The block above a dipping fault surface.

head, cometary The nucleus and coma of a comet.

horst An elongate fault block that has been uplifted in relation to the adjacent rocks.

hot spot The expression at the surface of a planet of a mantle plume, or column, of hot, buoyant rock rising in the mantle beneath the lithosphere.

hydrologic system The system of moving water at the surface of a planet.

hydrosphere The waters of a planet, as distinguished from the rocks, the atmosphere, and the biosphere.

hydrostatic pressure The pressure within a fluid (such as water) at rest, exerted on a given point within the body of the fluid.


ice Solids formed of volatile materials, particularly water, methane, ammonia, and nitrogen.

ice sheet A thick, extensive body of ice that is not confined to valley. If an ice sheet is thick enough, it may move by glacial flow. Ice sheets or caps form at the poles of several planets and moons.

igneous rock Rock formed by cooling and solidification of molten minerals (magma). Igneous rocks can be intrusive or extrusive and can consist of a variety of materials, such as silicates on the inner planets or water on the icy satellites.

interstitial Pertaining to material in the pore spaces of a rock. Groundwater and ground ice are interstitial materials. intrusive rock Igneous rock that, while it was fluid, penetrated into or between other rocks and solidified.

ion An atom or combination of atoms that has gained or lost one or more electrons and thus has a net electrical charge.

iron meteorite Meteorite composed mostly of metallic iron. They are thought to have formed as the cores of small differentiated asteroids.

island arc A chain of volcanic islands on Earth formed above a subduction zone.

isostasy A state of equilibrium, resembling flotation, in which segments of a planet's crust stand at levels determined by their thickness and density. Isostatic equilibrium is attained by flow of material in a viscous fashion.

isotope One of the several forms of a chemical element that have the same number of protons in the nucleus but differ in the number of neutrons and thus differ in atomic weight.


jovian planet The giant planets Jupiter, Saturn, Uranus, and Neptune.


karst A landscape characterized by sinks, solution valleys, and other features produced by

groundwater activity.

Kelvin A temperature scale that begins at absolute zero, equivalent to {min}273 degrees Centigrade or Celsius.

kinetic energy Energy of motion; the kinetic energy of a moving object is equal to one-half the product of its mass multiplied by the square of its velocity.

KREEP basalt A type of basaltic rock found on the Moon that has high concentrations of potassium, rare earth elements, and phosphorous thought to be formed from the residue of the Moon's magma ocean.


lag deposit A residual accumulation of coarse fragments that remains on the surface after finer material has been removed by wind.

landform Any feature of a planet's surface having a distinct shape and origin. Collectively, the landforms of a planet constitute the entire surface configuration.

landslide A general term for relatively rapid types of mass movement.

latitude A north--south coordinate on the surface of a planet; it ranges from 0 degrees at the equator to 90 degrees at the pole.

lava Magma that reaches the surface of a planet and erupts to form a stream of molten rock.

lava tube A cylindrical opening in the central part of a lava flow, formed as liquid magma drains out from beneath a solid crust.

light Electromagnetic radiation that is visible to the eye.

light-year The distance light travels in a vacuum in one year, about 9.46 trillion km.

limestone A sedimentary rock composed mostly of calcium carbonate.

liquid The state of matter in which a substance flows freely and lacks crystal structure. Unlike a gas, a liquid retains the same volume independent of the shape of its container.

lithosphere The relatively rigid outer zone of a planetary body that includes the crust and part of the mantle above the softer asthenosphere.

loess Unconsolidated, wind-deposited dust.

longitude An east--west coordinate on the surface of a planet ranging from 0 to 360 degrees, or from 0 to 180 degrees east and west of a central meridian.

longitudinal dune An elongate sand dune oriented in the direction of the prevailing wind.


mafic rock An igneous rock containing more than 50 percent minerals that are rich in iron and magnesium silicates.

magma A mobile melt which can contain suspended crystals and dissolved gases as well as liquid. A magma must have a high temperature relative to the surface temperature of a planet. Thus water is not a magma on Earth, but on the cold, icy moons of the outer solar system, liquid water is much hotter than the surface and behaves like a magma.

magnetic field The volume affected by the magnetism of an object. Planets generate magnetic fields by internal dynamos.

magnetic pole One of several (usually two for a dipole) points on a planet at which the density of magnetic lines of force is the highest. The needle of a compass aligns itself along these lines of force.

magnetic reversal A complete 180-degree reversal of the polarity of a planet's magnetic field.

mantle The zone of a planetary body's interior between the base of the crust and the core.

mare (pl. maria) Any of the relatively smooth, low, dark areas of the Moon. The lunar maria were formed by the extrusion of floods of basaltic lava.

mass A measure of the total amount of material in an object.

mass movement The transfer of rock and soil downslope by direct action of gravity without a flowing medium (such as a river or glacial ice).

mechanical weathering The breakdown of rock into smaller fragments by physical processes such as frost wedging.

melt A substance altered from the solid state to the liquid state.

mesa A flat-topped, steep-sided highland capped with a resistant rock formation. A mesa is smaller than a plateau but larger than a butte.

metamorphic Pertaining to the processes or products of metamorphism.

metamorphism Alteration of the minerals and textures of a rock by changes in temperature and pressure and by a gain or loss of chemical components.

meteor A meteorite in transit through a planet's atmosphere before it strikes the surface; a shooting star.

meteorite Any particle of solid matter that has fallen to Earth, the Moon, or another planet from space.

meteoroid A meteorite before it reaches a planet.

midocean ridge A divergent plate boundary formed in one of Earth's ocean basins where new lithosphere is formed.

molecule Two or more atoms bound together; the smallest particle of a chemical compound.

moon A natural satellite of one of the nine planets in the solar system.

mountain A general term for any landform that stands above its surroundings. In the stricter geologic sense, a mountain belt is a highly deformed part of planet lithosphere created by tectonic processes rather than volcanic or impact processes.

multiring basin Large impact crater surrounded by several concentric rings.


nebula A body of gas and dust residing within a galaxy. Some nebulas are the birthplaces of stars and planets.

neutron An elementary particle that resides in the nucleus of an atom. A neutron has about the same mass as a proton but has no electrical charge.

normal fault A steeply inclined fault in which the hanging wall has moved downward in relation to the footwall.

nova A star that suddenly increases in brightness by hundreds to thousands of times and then fades away gradually. Novas develop as relatively large stars explode from internal nuclear reactions.

nuclear reaction Reaction between or in atomic nuclei; contrasts with chemical reactions that involve the electrons of an atom. Such reactions include fission, fusion, and radioactive decay.

nucleus, atomic The dense central part of an atom composed of neutrons and protons. The nucleus is positively charged and contains most of the mass of an atom.

nucleus, comet The central portion of a comet composed of icy solids that vaporize to form a head when the comet is in the part of its orbit near the Sun.


occultation An eclipse of a star or a planet by another planetary object.

olivine A silicate mineral with magnesium and iron but no aluminum. Olivine is common in the mantles of the inner planets.

Oort cloud A hypothetical group of comets that orbit the Sun at great distances. Chance encounters with stars or other comets change the orbits of these icy bodies to make them enter the inner solar system.

orbit The path followed by one body in its revolution about another. Determined by gravitational interactions, planetary orbits range from circular to extremely elliptical. The

planets follow orbits around the Sun, and satellites have orbits around a planet.

orogenic Pertaining to deformation of a planet's lithosphere to the extent that a folded mountain belt is formed.

outer planets The planets beyond the orbit of Mars---Jupiter, Saturn, Uranus, Neptune, and Pluto.

outflow channels Large channels on Mars thought to be created by catastrophic floods of water released from below the surface of the planet.

oxidation Chemical combination of oxygen with another substance.


p-wave A compressional seismic wave or "primary" wave.

palimpsest, crater An impact crater that has been nearly destroyed by the rebound of the crater floor by isostatic adjustment. Palimpsests are common on the icy surfaces of the moons of Jupiter.

parabolic dunes A dune shaped like a parabola with the concave side toward the wind.

partial melting The process by which minerals with low melting points liquefy within a rock body as a result of an increase in temperature or a decrease in pressure (or both) while other minerals in the rock are still solid.

patera Pertains to low-profile volcanoes found on Mars and Io.

patterned ground Distinctive geometric patterns created by alternate freeze--thaw processes. Polygonal patterns of cracks are one type of patterned ground found in Earth's polar regions and on Mars.

period A time interval; for example, a geological period or the time required to complete one orbit.

permafrost Permanently frozen ground.

physiographic Pertains to the assemblage of landforms or topography in a region on the surface of a planet.

plagioclase feldspar A group of feldspar minerals that range from calcium to sodium aluminum silicates. Plagioclase is common at the surface of the inner planets. planet A relatively large body that orbits a star. Planets are not luminous. Smaller bodies include asteroids and comets.

planetary nebula A shell of gas expanding away from the surface of a small star. For example, the Sun will die when nuclear burning reactions reach its surface, ripping a layer of gas away to create a planetary nebula.

planetesimal Small solid bodies from which the planets accreted.

plate A broad segment of the lithosphere (including the rigid upper mantle plus the crust) that floats on the underlying asthenosphere and moves independently of other plates.

plate tectonics The theory of planetary dynamics in which the lithosphere is broken into individual plates that participate in convection of the upper mantle of a planet. The lithosphere on such a planet is created and destroyed by recycling back into the mantle.

plateau An extensive upland region.

pluton A body of intrusive igneous rock.

pore fluid A fluid, such as groundwater, that occupies pore spaces of a rock.

pore space The spaces within a rock body that are unoccupied by solid material. Pore spaces include spaces between grains, fractures, gas bubbles, and voids formed by dissolution.

potential energy Energy stored in an object that can be converted to another form; for example, the gravitational potential energy of an object increases with height above the surface of a planet and can be converted to kinetic energy if the object is allowed to fall back to the surface.

proton A relatively large elementary particle with a positive charge. Together with neutrons,

protons reside in the nuclei of atoms.

protostar A forming star, before nuclear fusion starts.

pyroclastic Pertaining to fragmental rock material formed by volcanic explosions.

pyroxene A group of silicate minerals composed of single chains of silicon--oxygen tetrahedra. Most are rich in iron and magnesium.


quartz A silicate mineral composed of silicon--oxygen tetrahedra joined in a three-dimensional network.


radar A technique for observing distant objects that uses reflected radio waves.

radiation The process of energy transfer by electromagnetic waves.

radio telescope A telescope designed to examine radio waves emitted or reflected from distant objects.

radioactive decay The spontaneous disintegration of an atomic nucleus with the emission of energy.

radiogenic heat Heat generated by radioactivity.

radiometric dating Determination of the age in years of a rock or mineral by measuring the proportions of an original radioactive material and its decay product.

rampart craters Impact craters surrounded by ejecta that has fluid-flow features. The ejecta is thought to have contained water, helping it flow across the surface of a planet.

rarefaction The process by which the floor of an impact crater returns to the normal pressure of a planet's surface after the passage of a compressive shock wave.

ray crater An impact crater that has a system of rays extending away from the crater rim. Only young craters have rays. recrystallization Reorganization of elements of the original minerals in a rock resulting from changes in temperature, pressure, or the activity of pore fluids.

refractory A material that melts or vaporizes only at very high temperatures. Silicate minerals are refractory compared to ices.

regolith The blanket of soil and loose rock fragments overlying solid rock on a planet's surface.

relative age The age of a rock or an event as compared with some other rock or event.

relative dating Determination of the order of a series of events in relation to one another without reference to their ages measured in years. Relative geologic dating is based primarily on super-position, faunal succession, and cross-cutting relations.

relative time Geologic time as determined by relative dating, that is, by placing events in chronologic order without reference to their ages measured in years.

relief The difference in altitude between the high and the low parts of an area.

remnant magnetism Permanent magnetism of rocks in a planet's lithosphere as opposed to that caused by the planet's magnetic field.

remote sensing Any means of determining the physical or chemical properties of an object from a distance. Telescopes are remote-sensing devices, but so too are the cameras on spacecraft.

resolution The degree to which small objects can be seen on an image or photograph of the surface of a planet. High-resolution images reveal very small objects whereas low-resolution images allow only large features to be detected.

resonance, orbital Pertaining to the orbital periods of satellites that are simple multiples of one another. If one satellite orbits its primary twice for every orbit of a satellite closer to the planet, it displays a 2:1 resonance. Tidal interactions are accentuated by resonance in satellite


retrograde motion An orbital or spin direction opposite to most bodies in the solar system. As viewed from the north, most planets and satellites move in a counterclockwise direction; retrograde motion would thus be clockwise rotation, or revolution.

reverse fault A fault in which the hanging wall has moved upward in relation to the footwall---a high-angle thrust fault.

revolution The orbital motion of one body around another.

rhyolite A fine-grained volcanic rock composed of quartz, alkali feldspar, and plagioclase. It is the extrusive equivalent of a granite.

rift system A system of faults resulting from extension.

rift valley A valley of regional extent formed by block faulting in which tensional stresses tend to pull the crust apart. Synonymous with graben.

rille An elongate trench, or cracklike valley on the Moon's surface. Rilles can be sinuous (lava channels or collapsed lava tubes) or relatively linear structural depressions (grabens).

ring A stream of small particles that orbits one of the outer planets. The particles are usually made of ice.

river system A river with all of its tributaries.

roche limit The orbit closest to a planet where a satellite can withstand the tidal forces exerted by its primary. Inside the roche limit, a moon will be disrupted. The limit varies for bodies of different sizes and different strengths, but it is usually calculated for a satellite with very low strength, like liquid water.

rock An aggregate of minerals that forms the solid part of a planetary body.

rotation Spinning of a body about an axis running through it.


sand Sedimentary material composed of fragments ranging in diameter from 0.0625 to 2 mm. Sand particles are larger than silt but smaller than pebbles. Much sand on Earth is composed of quartz grains, but other materials can also form sand.

sandstone A sedimentary rock composed mostly of sand-size particles, usually cemented by calcite, silica, or iron oxide.

sapping The process by which valleys are created by the erosion and undercutting caused by spring water.

satellite A planetary body that orbits about a larger one; for example, a moon of a planet.

scarp A cliff produced by faulting or erosion.

sea ice Relatively thin, floating masses of ice that form on terrestrial oceans by freezing of sea water, in contrast to icebergs that form when glaciers enter the sea.

seamount An isolated, conical mound rising more than 100 m above Earth's ocean floor. Seamounts are probably submerged shield volcanoes formed above hot spots.

secondary crater A crater formed by the impact of material ejected from another crater.

sediment Material (such as gravel, sand, and dust) that is transported and deposited by wind, water, ice, or gravity; material that is precipitated from solutions that exist at surface temperatures.

sedimentary rocks Rock formed by the accumulation and consolidation of sediment.

seismic Pertaining to earthquakes or to waves produced by natural or artificial earthquakes.

seismograph An instrument used to detect seismic waves.

shale A fine-grained, clastic sedimentary rock formed by compaction of clay and mud.

shield An extensive area of Earth's continents where igneous and metamorphic rocks are exposed and have relatively low relief and flat surfaces. Rocks of the shield are usually old and many formed as the roots of ancient mountain belts that have now been eroded away.

shield volcano A large volcano shaped like a flattened dome and built up almost entirely of

numerous flows of fluid (usually basaltic) lava. The slopes of shield volcanoes seldom exceed 10 degrees, so that in profile they resemble a shield or broad dome.

shock wave A strong compressional wave that lasts for a very short period of time, as, for example, when a meteorite strikes the surface of a planet.

silicate A variety of rock-forming minerals composed of variously arranged tetrahedra of silicon and oxygen plus other elements. Silicate minerals are especially important in the crusts and mantles of the inner planets and the cores of the icy satellites of the outer planets.

slope retreat Progressive recession of a scarp or the side of a hill or mountain by mass movement or stream erosion.

slump A type of mass movement in which material moves along a curved surface of rupture.

solar system The system that includes the Sun, planets, moons, asteroids, comets, and other objects that orbit the Sun.

solar wind The stream of ions flowing away from the Sun.

soil The surface material of the planets, produced by the disintegration of rocks.

solid The state of matter in which substance has a definite shape and volume and some fundamental strength.

spatter cone A low, steep-sided volcanic cone built by accumulation of splashes and spatters of lava (usually basaltic) around a fissure or other vent.

spectral Pertaining to the various wavelengths of electromagnetic radiation; for example, a spectral class of asteroids is defined based on the character of the light reflected from it.

spectrum The variety of wavelengths (colors for visible light) in electromagnetic radiation; a prism separates these colors to show the spectrum.

spin-orbit coupling Pertains to the orbital and rotational movements of a planet. Coupling occurs when the rotation rate of a planet or a moon is a simple multiple (one, two, or three times) of its revolution rate around the Sun or a planet. The spin and the orbit of the Moon are coupled in such a fashion.

spreading center A plate boundary formed by tensional stress along a terrestrial oceanic ridge. Synonymous with midocean ridge, divergent plate boundary.

sputtering The process of ejection of small amounts of material from a planet's surface as the result of an impact of ions from the solar wind. This process is only effective on bodies that lack atmospheres.

stable platform The part of a terrestrial continent that is covered with flat-lying or gently tilted strata and underlain by a complex of igneous and metamorphic rocks. The stable platform has not been extensively affected by crustal deformation.

star A large self-luminous object that creates energy by nuclear fusion. The Sun is the star at the center of our solar system.

star dune A mound of sand with a high central point and arms radiating in various directions.

stony meteorite A meteorite composed mostly of silicate minerals, in contrast to metallic iron. Achondrites and chondrites are stony meteorites.

stony-iron meteorite A meteorite composed mostly of an intimate mixture of silicates and iron metal.

stratigraphy The study of rock strata on or near the surface of a planet.

stratovolcano A volcano built up of alternating layers of ash and lava flows. Synonymous with composite volcano.

stratum (pl. strata) A layer of rock.

stream valley A valley produced by fluvial erosion.

strike-slip fault A fault in which movement has occurred parallel to the trend of the fault.

subaerial Occurring beneath the atmosphere or in the open air, with reference to conditions or processes (such as erosion) that occur there.

subduction zone An elongate zone in which one lithospheric plate descends beneath another,

a fundamental feature of Earth's plate tectonic system.

sublimation The process by which a material changes state from a solid directly to a vapor without passing to a liquid.

subsidence A sinking or settling of a planet's lithosphere with respect to the surrounding parts.

supernova A very large nova, or stellar explosion.

superposition, principle of The principle that, in a series of strata that has not been overturned, the oldest rocks are at the base and the youngest are at the top.

s-wave A seismic "shear" wave.

synchronous rotation The rotation of a satellite or planet that has equal orbital and rotational periods; 1:1 spin--orbit coupling.


T Tauri star A variable star showing rapid and erratic changes in the electromagnetic energy it emits. T Tauri stars are thought to be young stars from which large amounts of matter are swept away during this stage of their development.

tail, comet Gases and particles of solid that stream away from a comet in the inner solar system.

tectonics Regional or global structures and deformational features of a planetary object.

tension Stress that tends to pull materials apart.

terminator The line separating the sunlit from the dark hemisphere of a planetary object.

terra (pl. terrae) A densely cratered highland on the Moon.

terrace A nearly level surface forming narrow shelves on the inside of a crater.

terrestrial planets The planets most like Earth, with lithospheres of silicate minerals---Mercury, Venus, the Moon, Earth, and Mars.

tessera Terrains on Venus that have been intensely modified by tectonic processes. They consist of interlacing ridges and valleys.

thermal energy Heat energy associated with the motion of the particles in a solid, liquid, or gas.

thermo-karst A karstlike terrain created by collapse as volatile cements or rocks (like water ice) sublime, rather than by solution of rock in water.

thermonuclear reaction Fusion under conditions of high temperature such as in stars.

thrust fault A low-angle fault (45 degrees or less) in which the hanging wall has moved upward in relation to the footwall.

tidal heating The process of frictional heating of a planetary object by the alternate growth and decay of a tide in its lithosphere.

tide Periodic deformation of the lithosphere or hydrosphere of a planetary body that is caused by the gravitational attraction of another object such as the Sun, a satellite, or a planet.

topography The shape and form of a planetary body's surface.

trajectory The path followed by a projectile.

transform fault A special type of strike-slip fault forming the boundary between two moving lithospheric plates, usually along an offset of the oceanic ridge. A fundamental feature of Earth's plate tectonic system.

transverse dune An asymmetrical dune ridge that forms at right angles to the direction of prevailing winds.

trench A narrow elongate depression of Earth's oceanic lithosphere oriented parallel to the trend of a continent or island arc. Trenches mark the sites of subduction zones.

tributary A stream flowing into or joining a larger stream.

turbulent flow A type of flow in which the path of motion is very irregular, with eddies and swirls.


undifferentiated Pertains to a planet that has not become internally differentiated and is thus relatively homogeneous.

upwarp An arched or uplifted segment of a planetary lithosphere.


vent The point of extrusion for volcanic materials. Vents may be fissures, craters, cinder cones, etc.

ventifact A pebble or cobble shaped and polished by wind abrasion.

vesicle A small hole formed in a volcanic rock by a gas bubble that became trapped as the lava solidified.

viscosity The tendency within a body to resist flow. An increase in viscosity implies a decrease in fluidity, or ability to flow.

volatile A substance that can be vaporized at a relatively low temperature. Water and carbon dioxide are volatile materials.

volcanic ash Dust-size particles ejected from a volcano.

volcano The accumulation of extrusive igneous rocks around a volcanic vent.

volume A measure of the amount of space occupied by an object.


weathering The process by which rocks are chemically altered or physically broken into fragments as a result of exposure to atmospheric agents and the pressures and temperatures at or near the surface of a planet, with little or no transportation of the loosened or altered materials.

wind shadow The area behind an obstacle where air movement is not capable of moving material.

wrinkle ridge A sinuous, irregular segmented ridge on the surface of the lunar maria, Mars, and Mercury, believed to be the result of deformation of the lava.


yardang An elongate ridge carved by wind erosion.

Hydrogen and helium atoms

The hydrogen and helium atoms. Hydrogen has one positively charged proton (center) and one orbiting, negatively charged electron. Helium has two protons (red), two neutral particles called neutrons (green) and two orbiting electrons. Neutrons and protons contain most of each atom's mass and reside within a central nucleus. As shown here, hydrogen has an atomic mass of 1 amu and helium has an atomic mass of 4 amu.

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