A

Alkali:
Any strongly basic substance, such as hydroxide or carbonate of an alkali metal (e.g. sodium, potassium).

Alluvial Fan:
A fan-shaped deposit of sediment built by a stream where it emerges from an upland or a mountain range into a broad valley or plain. Alluvial fans are common in arid and semiarid climates.

Alpine Glacier:
A glacier occupying a valley, usually high in mountainous terrain.

Amphibole:
An important rock-forming mineral group of ferromagnesian silicates. Amphibole crystals are constructed from double chains of silicon-oxygen tetrahedra.

Andesite:
A fine-grained extrusive igneous rock composed mostly of plagioclase feldspar, with amphibole and biotite. Andesite and basalt represent 98% of all volcanic rocks.

Anorthosite:
A granular plutonic igneous rock composed almost wholly of the mineral plagioclase.

Anticline:
Folded rocks, generally convex upward, in which the core contains geologically older rocks.

Aquifer:
A permeable strata or zone below a planetary surface through which a fluid moves.

Arcuate:
Curved or bent.

Arete:
A narrow, sharp ridge separating two adjacent glacial valleys.

Ash:
Volcanic fragments the size of dust particles.

Ash Flow:
A turbulent blend of unsorted pyroclastic material (mostly fine-grained) mixed with high-temperature gases ejected explosively from a fissure or crater.

Ash-flow Tuff:
A rock composed of volcanic ash and dust, formed by deposition and consolidation of ash flows.

Asteroid:
One of the many small bodies revolving around the Sun, with orbits generally between Mars and Jupiter.

Asteroid Belt:
A region of space between Mars and Jupiter where most asteroids are found.

Asthenosphere:
A weak spherical shell below the lithosphere.

Atmosphere:
The body of gases surrounding or comprising any planet or other celestial body.

Aureole (Mars):
Zone of chaotic terrain surrounding large volcanos on Mars.

Aurora:
Bands of light in the night sky produced by the ejection of charged particles, usually seen at high latitiudes.

B

Bajada:
The series of coalesced alluvial fans, often forming a debris apon alaong the base of a mountain.

Ballistic Trajectory:
The flight path of a projectile moving through space.

Barchan Dune:
A crescent-shaped dune, the tips or horns of which point downwind. Barchan dunes usually form in desert areas where sand is scarce.

Basalt:
A dark-colored, fine-grained igneous rock composed of plagioclase (over 50%) and pyroxene. Olivine may or may not be present. Basalt and andesite represent 98% of all volcanic rocks.

Basin (Impact):
Term applied to impact structures larger than about 100 kilometers across.

Basin (Structural):
A circular or elliptical downwarp. After erosion, the youngest beds are exposed in the central part of the structure.

Batholith:
A large body of intrusive igneous rock exposed over an area of at least 100 square kilometers.

Biotite:
An important rock-forming ferromagnesian silicate with silicon-oxygen tetrahedra arranged in sheets.

Breccia:
Rock composed of broken rock fragments of various sizes surrounded by finer-grained materials.

Bright Ray Crater:
A meteorite crater that has a system of rays extending like splash marks from the crater rim.

Butte:
A somewhat isolated hill, usually capped with a resistant layer of rock and bordered by talus. A butte is an erosional remnant of a formerly more extensive slope.

C

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 may or may not be related to explosive eruptions.

Cambrian:
A period of geologic time from approximately 570 to 500 million years ago.

Carbonate Rock:
Rock composed mostly of carbonate minerals.

Carboniferous:
A period of geologic time from approximately 345 to 280 million years ago.

Cenozoic:
An era of geologic time from the beginning of the Tertiary period to the present, or from 65 million years ago to the present.

Central Peak:
A central, high area produced in an impact crater by inward and upward movement of underlying material.

Chaos:
Distinctive area of broken terrain.

Chasma (pl. chasmata):
A deep, elongated, steep-sided depression.

Cinder Cone:
A cone-shaped hill composed of loose volcanic fragments, commonly found along fissures away from the central vent.

Cirque:
An amphitheater-shaped depression at the head of a glacial valley, excavated mainly by ice plucking and frost wedging.

Cirroform Anvil:
Circular deck of clouds that flow out from the top of the eye wall in a hurricane.

Clast:
A rock fragment produced by weathering of a larger rock, which has been incorporated in another rock.

Composite Volcano:
A large volcanic cone built by extrusion of alternating layers of ash and lava. Synonymous with stratovolcano.

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

Confluence:
The point where two or more streams meet.

Convection:
Mass motions within a fluid resulting in transport and mixing of the properties of that fluid.

Convergence Angle:
The angle between the spacecraft (as it took the first image), the center of the image, and the spacecraft (as it took the second image).

Convergent Plate Boundary:
The zone where the leading edges of converging lithospheric plates meet. Convergent plate boundaries are sites of considerable geologic activity and are characterized by volcanism, earthquakes, and crustal deformation.

Core (of a planet):
The central zone, or nucleus of a planet or satellite, consisting of higher-density material.

Coriolis Force:
The acceleration that a body in motion experiences when observed in a rotating frame. This apparent force acts at right angles to the direction of the angular velocity.

Corona (Sun) (pl. coronae):
The bright, irregular-shaped envelope of ionized gas around the Sun.

Corona (Venus) (pl. coronae):
Oval-shaped feature on the surface of Venus.

Crater:
An abrupt circular depression formed by extrusion of volcanic material, by collapse, or by the impact of a meteorite.

Creep:
The slow but continuous deformation of a material under constant load or prolonged stress.

Cretaceous:
A period of geologic time from approximately 135 to 65 million years ago.

Crust:
The chemically distinct outermost solid layer of a planet or satellite.

D

Dacite:
A fine-grained extrusive rock with the same general composition as andesite but having less plagioclase and more quartz.

Debris Apron:
A continuous deposit of sediment at the base of a mountain front or around an isolated massif. On Earth, debris apron is generally synonymous with bajada.

Debris Avalanche:
The very rapid and usually sudden sliding and flowage of incoherent, unsorted mixtures of soil and weathered bedrock.

Debris Flow:
The rapid downslope movement of debris (rock, soil, and mud).

Debris Slide:
A slow to rapid downslope movement of relatively dry and unconsolidated soil and rock debris that forms an irregular hummocky deposit resembling a moraine.

Decollement:
Detachment structure of strata owing to deformation, resulting in independent styles of deformation in the rocks above and below. It is associated with folding and with overthrusting.

Deformation:
A general term for the process of folding, faulting, shearing, compression, or extension of rocks as a result of various forces.

Dendritic Drainage Pattern:
A branching stream pattern, resembling the branching of certain trees, such as oaks and maples.

Devonian:
A period of geologic time from approximately 400 to 345 million years ago.

Diapir:
A dome or fold created by the upward intrusion of a lighter mass into a denser, crustal rock. The most common diapiric structures on Earth have cores of salt and are called salt domes.

Differential Erosion:
Variation in the rate of erosion on different rock masses. As a result of differential erosion, resistant rocks form steep cliffs, whereas nonresistant rocks form gentle slopes.

Dike:
The intrusion of magma into a crack that cuts across existing rocks.

Divergent Plate Boundary:
A plate margin formed where the lithosphere splits into plates that drift apart from one another. Divergent plate boundaries are areas subject to tension, where new crust is generated by igneous activity. Synonymous with spreading center.

Dome:
An uplift that is circular or elliptical in map view, with beds dipping away in all directions from a central area. Also, a general term for any dome-shaped landform.

Domical:
Shaped like a dome.

Dorsum (pl. Dorsa):
Ridge.

Dune:
A low mound of fine-grained material that accumulates as a result of sediment transport in a current system. Sand dunes are commonly classified according to shape.

E

Effusive Lava Flows:
Nonexplosive extrusions of molten lava onto the surface of a planet or satellite.

Ejecta:
The deposit surrounding an impact crater composed of material thrown from the crater during its formation.

Ejecta Blanket:
Rock material (crushed rock, large blocks, breccia, and dust) ejected from an impact crater or explosion crater and deposited over the surrounding area.

Eocene:
An epoch of geologic time from approximately 57 to 36 million years ago.

Esker:
A long, narrow, sinuous ridge of stratified glacial drift deposited by a stream flowing beneath a glacier in a tunnel or in a subglacial stream bed.

Evaporite:
A rock composed of minerals derived from evaporation of mineralized water. Examples: rock salt, gypsum.

Extension:
The act of stretching or spreading apart.

Extrusion:
The emission of relatively viscous lava onto a planet's surface, or the rock formed by this process.

Extrusive Rock:
A rock formed from a mass of magma that flowed out on the surface of a planet.

Eye Wall:
Cylindrical band of tightly spaced thunderstorms that outline the eye of a hurricane. The strongest winds of a hurricane are generated in the eye wall.

F

Fault:
A fracture or zone of fractures along which the sides are displaced relative to one another.

Fault, Normal:
Fault in which the rocks have been shifted vertically by extensional forces.

Fault, Reverse:
Fault in which the rocks have been shifted vertically by compressional forces. Generally synonymous with thrust fault.

Fault Scarp:
A cliff produced by faulting.

Feldspar:
A mineral group consisting of silicates of aluminum and one or more of the metals potassium, sodium, or calcium.

Ferromagnesium Minerals:
A variety of silicate minerals containing abundant iron and magnesium.

Festoon:
Set of parallel ridges formed on the surface of lava flows.

Fjord:
A glaciated valley flooded by the sea to form a long, narrow, steep-walled inlet.

Fissure Eruption:
An eruption that takes place from an elongate fracture in the lithosphere.

Flatiron:
One of a series of short, triangular ridges forming a saw-toothed pattern on the flank of a mountain. Usually flatirons are made up of steeply dipping, resistant rocks that stand out due to differential erosion.

Floodplain:
The flat, occasionally flooded area bordering a stream.

Fluidize:
To make fluid, usually by the rapid introduction of a liquid such as water.

Fluvial:
Pertaining to river or rivers.

Fold:
A bend, or flexure, in a rock.

Fossa (pl. Fossae):
Long, narrow, shallow depressions.

Fracture:
A general term for any break in a rock, whether or not it causes displacement, due to mechanical failure by stress. Fracture includes cracks, joints, and faults.

Fretted Terrain:
A martian landscape made up of low-lying sparsely cratered plains and adjacent highstanding cratered terrain, separated by escarpments.

Fusion:
The combining of atoms and consequent release of energy.

G

Galilean Satellites:
The four largest satellites of Jupiter: Io, Europa, Ganymede, and Callisto. Synonymous with jovian satellites.

Glacier:
A mass of ice formed from compacted, recrystallized snow that is thick enough to flow plastically.

Glass:
An amorphous igneous rock formed from a rapidly cooling magma.

Gneiss:
A coarse-grained metamorphic rock with a characteristic type of layering resulting from alternating layers of light-colored and dark-colored minerals. Its composition is generally similar to that of granite.

Gondwanaland:
The ancient continental landmass that is thought to have split apart during Mesozoic time to form the present-day continents of South America, Africa, India, Australia, and Antarctica.

Graben:
An elongate crustal depression bounded by normal faults on its long sides.

Granite:
A coarse-grained intrusive igneous rock composed of K-feldspar, plagioclase, and quartz, with small amounts of ferromagnesian minerals.

Gravity:
Viewed from a frame of reference fixed in the Earth, force imparted by the Earth to a mass that is at rest relative to the Earth. By extension, the attraction of any heavenly body for any mass, as martian gravity.

Grooves:
Curvilinear depressions or troughs found on some icy satellites.

H

Highlands (Lunar):
Rugged, heavily cratered, bright terrain on the Moon.

Holocene:
An epoch of geologic time from approximately 10,000 years ago to the present.

Horn:
A sharp peak formed at the intersection of the headwalls of three or more cirques.

Horst:
An elongate block of uplifted crust bounded by normal faults on its long sides.

Hotspot:
The expression at a planet's surface of a mantle plume, or column of hot, buoyant rock rising in the mantle beneath a lithospheric plate.

Hummock:
A small, rounded or cone-shaped, low hill or a surface of other small, irregular shapes. A surface that is not equidimensional or ridgelike.

Hurricane:
A severe tropical cyclone with wind speeds greater than 119 kilometers per hour, characterized by heavy rains. Hurricanes in the western Pacific and Indian Oceans are called typhoons.

I

Ice Cap:
A dome-shaped or plate-like cover of perennial ice and snow covering the summit area of a mountain mass so that no peaks emerge through it (or covering a flat landmass such as an Arctic island) that spreads due to its own weight outwards in all directions, and has an area of less than 50,000 square kilometers.

Ice Field:
An extensive mass of land ice covering a mountain region, consisting of many interconnected alpine and other types of glaciers, covering all but the highest peaks and ridges.

Ice Sheet:
A thick, extensive body of glacial ice that is not confined to valleys. Localized ice sheets are sometimes called ice caps.

Ice Stream:
A current of ice in an ice sheet or ice cap that flows more rapidly than the surrounding ice, usually flowing to the ocean or to an ice shelf and not constrained by exposed rock.

Igneous:
Materials or processes related to magma.

Ignimbrite:
The rock formed by the widespread deposition and consolidation of ash flows and nuees ardentes.

Image Numbers:
A unique identifying number assigned by each mission to each image. Space shuttle image numbers are listed by roll and frame number.

Image Resolution (Full View):
The smallest detail (in meters) that is visible in the image. This is equivalent to the width of an individual picture element (pixel) that makes up the image. This information can be used to estimate the sizes of features in the image. This value is applicable only to the full-sized image.

Image Width:
Horizontal width of the entire image in kilometers (multiply by 0.6 for width in miles). This information can be used to estimate the sizes of features in the image. The image width is applicable to all views except the regional image.

Imbricate:
Overlapping, as tiles on a roof or scales on a bud.

Impact:
In planetology, the collision of objects ranging in size from tiny micrometeoroids to planetesimals.

Inner Planets:
The four planets nearest the Sun: Mercury, Venus, Earth, and Mars.

Intrusion:
Injection of a magma into a preexisting rock, or the rock resulting from this process.

Intrusive Rock:
Igneous rock that, while it was fluid, penetrated into or between other rocks and solidified. It can later be exposed at the Earth's surface after erosion of the overlying rock.

Ionized Gas:
Gas that has been converted in whole or part to ions.

Island Arc:
A chain of volcanic islands. Island arcs are generally convex toward the open ocean.

J

Joint:
Fracture or parting in rock, but without vertical or horizontal displacement of the rocks.

Jovian Satellites:
The four largest satellites of Jupiter: Io, Europa, Ganymede, and Callisto. Synonymous with Galilean satellites.

Jurassic:
A period of geologic time from approximately 190 to 135 million years ago.

K

Kilometer:
Metric unit of length equal to 1000 meters or approximately 0.62 miles.

L

Landslide:
A general term for relatively rapid types of mass movement, such as debris flows, debris slides, rockslides, and slumps.

Lava:
Magma that reaches the surface of a planet or satellite.

Limestone:
A sedimentary rock composed mostly of calcium carbonate.

Lineament:
A topographic feature or group of features having a linear configuration. Lineaments commonly are expressed as ridges or depressions or as an alignment of features such as stream beds, volcanos, or vegetation.

Lithosphere:
The stiff upper layer of a planetary body, including (on Earth) the crust and part of the upper mantle, lying above the weaker asthenosphere; the solid part of a planet.

Lithify:
To change to rock, or to petrify; especially to consolidate from a loose sediment to a solid rock.

Lobate:
Shaped like a lobe.

Location:
Latitude and longitude of the center of the image.

Longitudinal Dune:
An elongate sand dune oriented in the direction of the prevailing wind.

M

Mafic Rock:
An igneous rock containing more than 50% ferromagnesian minerals.

Magma:
Mobile or fluid molten rock material.

Magma Chamber:
Reservoir of molten magma under most volcanic edifices, which is the source of magma to intrusive and extrusive igneous deposits.

Magnetic Field:
A region in which forces are exerted on any magnetized bodies or electric currents present.

Magnetic Field Lines:
A curve whose tangent at any point is in the magnetic-field direction at that point.

Magnetosphere:
The region of space surrounding a planet in which the planet's magnetic field dominates that of the solar wind.

Mantle:
The interior zone of a planet or satellite below the crust and above the core that behaves plastically.

Mantle Plume:
Melted, buoyant rock believed to be rising through the mantle.

Mare (pl. maria):
An area on the Moon that appears darker and smoother than its surroundings, composed primarily of basaltic lava flows.

Mass Wasting:
The movement of rock and soil downslope due to gravity.

Massif:
A large mountain or block of crust that is more resistant than the surrounding rock and usually stands out on the landscape.

Mensa (pl. Mensae):
A flat-topped prominence with cliff-like edges.

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.

Mesozoic:
An era of geologic time from the end of the Paleozoic to the beginning of the Cenozoic, or from about 225 to 65 million years ago.

Metamorphic Rock:
Any rock formed from preexisting rocks within a planet's crust by changes in temperature and pressure and by chemical action of fluids.

Metamorphism:
Solid-state recrystallization, and replacement of less-stable with more-stable mineral phases by heat and/or pressure.

Meteorite:
Extraterrestrial material that survives falling to a planetary surface as a recoverable object.

Meter:
Metric unit of length equal to approximately 3.28 feet or 39.37 inches.

Metric Camera:
Mapping camera flown on the Apollo 15, 16, and 17 missions that was used to obtain moderate-resolution (50 meters), cartographic-quality stereo images of the lunar surface along the spacecraft flight path.

Micrometeorite:
A meteorite or meteoritic particle with a diameter generally less than a millimeter; a meteorite so small that it undergoes atmospheric entry without vaporizing or becoming intensely heated and hence without disintegration.

Mid-Ocean Ridge:
An uninterrupted mountain chain on the floors of the major ocean basins, whose ridgeline marks the boundary between two diverging plates. These mountains can be up to thousands of kilometers wide.

Miocene:
An epoch of geologic time from approximately 24 to 5 million years ago.

Mission:
Spacecraft or mission that obtained the images.

Molten:
In a fluid state or melted.

Mons (pl. Montes):
Mountain or peak.

Moraine:
A general term for a landform composed of an unsorted and unstratified glacial deposit.

Morphology:
External form, structure, or shape.

Mudflow:
A flowing mixture of mud and water.

Multiring Basin:
A large crater (more than 300 kilometers in diameter) containing a series of concentric ridges and depressions.

N

Nadir:
That point on the celestial sphere vertically below the observer.

Nuee Ardente:
A hot cloud of volcanic fragments and superheated gases that flows as a mass because it is denser than air. Upon cooling, it forms a type of rock called tuff, including ash-flow tuff and welded tuff.

O

Oligocene:
An epoch of geologic time from approximately 36 to 24 million years ago.

Ordovician:
A period of geologic time from approximately 500 to 440 million years ago.

Outer Planets:
The planets with orbits larger than that of Mars: Jupiter, Saturn, Uranus, Neptune, and Pluto.

P

Paleocene:
An epoch of geologic time from approximately 65 to 57 million years ago.

Paleozoic:
An era of geologic time from the end of the Precambrian to the beginning of the Mesozoic, or from about 570 to 225 million years ago.

Palimpsest:
A roughly circular albedo spot on icy satellites that lacks topographic structure, and is presumed to mark the site of a former crater and its rim deposit.

Panoramic Camera:
Camera flown on the Apollo 15, 16, and 17 missions that imaged the surface of the Moon directly below the spacecraft at high resolution (1 to 2 meters), and at decreasing resolution away from the center of the image out to 54° in both directions. The long dimension of the panoramic images were perpendicular to the flight path.

Parallel Drainage Pattern:
A drainage pattern in which the streams and their tributaries are regularly spaced and flow parallel or subparallel to one another over a considerable area.

Patera (pl. Paterae):
A crater with irregular or scalloped edges; inferred to be volcanic.

Peak Ring Basin:
Large impact basin featuring a ring of massifs on the basin floor.

Permian:
A period of geologic time from approximately 280 to 225 million years ago.

Pinnacle:
A tall, tower-shaped or spire-shaped pillar of rock.

Pit Crater:
An impact crater containing a central depression rather than a central peak. Also called a central pit crater.

Plagioclase:
A group of feldspar minerals.

Planitia:
Plateau or high plain.

Planum:
A high plateau.

Plastic Flow:
Flow that involves a permanent change in a substance's shape or volume that does not involve failure by rupture.

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

Plate Tectonics:
The theory of global dynamics in which the lithosphere is believed to be broken into individual plates that move in response to convection in the upper mantle. The margins of the plates are sites of considerable geologic activity.

Pleistocene:
An epoch of geologic time from approximately 1.6 million to 10,000 years ago.

Pliocene:
An epoch of geologic time from approximately 5 to 1.6 million years ago.

Precambrian:
All geologic time before the beginning of the Paleozoic era, or from 4.6 billion to 570 million years ago.

Proterozoic:
The more recent of two great divisions of the Precambrian, or from 2.5 billion to 570 million years ago. This is further divided into Early Proterozoic (2.5 to 1.6 billion years ago), Middle Proterozoic (1.6 billion to 900 million years ago), and Late Proterozoic (900 to 570 million years ago).

Pumice:
A rock consisting of frothy natural glass.

Pyroclastic:
Pertaining to fragmental rock material formed by volcanic explosions.

Pyroxene:
A group of rock-forming silicate minerals composed of single chains of silicon-oxygen tetrahedra.

Q

Quadrangle:
Each planet has been divided into roughly rectangular areas called quadrangles. These are used by the U.S. Geological Survey (USGS) and the scientific community to map the planets and refer to the maps shown in the Atlas of the Solar System published by the USGS.

Quartz:
An important rock-forming silicate mineral composed of silicon-oxygen tetrahedra joined in a three-dimensional network. It is distinguished by its hardness, glassy luster, and fracture pattern.

Quaternary:
A period of geologic time from approximately 3 (to 2) million years ago to the present.

R

Radial Drainage Pattern:
A group of streams extending down from a central topographic high point, creating a pattern similar to spokes extending from the center of a wheel.

Rampart Crater:
A martian impact crater whose ejecta deposits terminate in a rampart, instead of progressively grading into the nearby landscape.

Regio:
A large area of distinctive albedo markings.

Regolith:
A layer of fragmentary debris produced by meteoritic impact on the surface of an object.

Resistance:
The ability to withstand breakdown by erosion.

Resurgent Dome:
Dome-shaped upwarp or intrusion, in the middle of a caldera, that formed after the main eruption.

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

Ridge:
A long and narrow topographically high feature.

Rift Valley:
A valley of regional extent formed by normal faulting in which extensional stresses tend to pull the crust apart, usually located along divergent plate margins.

Rille:
Trench or crack-like valleys, up to several hundred kilometers long and 1 to 2 kilometers wide.

Ring Fault:
A steep-sided fault pattern that is cylindrical in outline and is associated with cauldron subsidence.

Ring of Fire:
Name given to a series of volcanos found at the subduction boundary around much of the Pacific lithospheric plate.

Rock Glacier:
A mass of poorly sorted, angular boulders cemented with interstitial ice. It moves slowly by the action of gravity.

S

Salt Dome:
A dome produced in sedimentary rock by the upward movement of a body of salt.

Sandstone:
A sedimentary rock composed mostly of sand-size particles.

Sapping:
The removal of material due to undermining by groundwater flow.

Satellite:
A relatively small, naturally occurring body orbiting a planet. A saatellite is referred to as a moon of the planet it orbits.

Satellite (Man-made):
A man-made object designed to orbit or fly by a celestial body.

Scarp:
A steep slope produced by erosion or faulting.

Sedimentary Rock:
Rock formed by the accumulation and consolidation of sediment, including conglomerate, sandstone, siltstone, shale, and limestone.

Shale:
A fine-grained clastic sedimentary rock formed by consolidation of clay and mud.

Sheeted Dike:
A dike in which the igneous material has been squeezed so that it takes on a layered appearance.

Shield:
An extensive area of a continent where igneous and metamorphic rocks are exposed and have approached equilibrium with respect to erosion and deformation. Rocks of the shield are usually very old (that is, more than 600 million years old).

Shield Volcano:
Volcanic mountain characterized by gently sloping flanks.

Silica:
Chemically resistant silicon dioxide, which is an essential constituent of many minerals.

Silicic:
A term applied to igneous rock or magma containing about two-thirds silicon dioxide. In general, the more silicic a magma, the more explosive the volcanic eruption.

Sill:
A tabular mass of igneous rock that is parallel to the rocks within which it lies.

Silurian:
A period of geologic time from approximately 440 to 400 million years ago.

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

Solar:
Of, pertaining to, or caused by the Sun.

Solar System:
The Sun and other celestial bodies within its gravitational influence, including planets, asteroids, satellites, comets, and meteors.

Spacecraft Altitude:
The vertical altitude of the spacecraft above the surface of the planet.

Stereo Baseline:
The distance in kilometers between the two points where the two images were taken.

Stratosphere:
The region of a planet's atmosphere above the troposphere and below the ionosphere, characterized by a vertical temperature gradient.

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

Striation:
A scratch or groove produced on the surface of a rock by a geologic agent, such as a glacier or stream.

Strike-slip Fault:
A fault in which movement has occurred parallel to the strike of the fault.

Subduction:
Subsidence of the leading edge of a lithospheric plate into the mantle.

Subduction Zone:
An elongate zone in which one lithospheric plate descends beneath another. A subduction zone is typically marked by an oceanic trench, lines of volcanos, and crustal deformation associated with mountain building.

Sublimate:
To change in state directly from solid to gas (or gas to solid).

Subsidence:
Downwarping or sinking to a lower level.

Sulcus (pl. Sulci):
Subparallel furrows and ridges.

Sulfide:
A mineral compound characterized by the linkage of sulfur with a metal or semimetal.

Syncline:
Folded rocks generally concave upward, whose core contains geologically younger rocks.

T

Tectonic:
Deformation of planetary materials, as in faulting of the crust.

Tension:
Stress that tends to pull materials apart.

Terra:
Extensive landmass.

Terrain Softening:
In situ deformation of material resulting in a characteristic "softened" landscape on Mars. Terrain softening is particularly prevalent around martian impact craters.

Tertiary:
A period of geologic time from approximately 65 to 3 (to 2) million years ago.

Tessera (pl. tesserae):
Tile-like or polygonal terrain.

Tholus (pl. Tholi):
Small domical mountain or hill.

Topography:
Relative elevation of surface features.

Transform Fault:
A special type of strike-slip fault forming the boundary between two moving lithospheric plates, usually along an offset segment of the oceanic ridge.

Trench:
A long, deep channel (up to several kilometers deep) in the ocean floor, parallel to a continent or island arc.

Triassic:
A period of geologic time from approximately 225 to 190 million years ago.

Triple Junction:
A point where three lithospheric plates meet.

Tropopause:
The boundary in a planet's atmosphere between the troposphere and the stratosphere, where the temperature gradient goes to zero.

Trough:
A long, narrow depression.

Tuff:
A fine-grained rock composed of volcanic ash.

U

U-shaped Valley:
A valley having a pronounced parabolic cross profile suggesting the form of a broad letter "U," with steep walls and a broad, nearly flat floor. U-shaped valleys are carved by glacial erosion.

Upwarp:
An arched or uplifted segment of the crust.

V

V-shaped Valley:
A valley having a pronounced cross profile suggesting the form of the letter "V," characterized by steep sides and short tributaries. V-shaped valleys result from downcutting by a stream.

Valley Glacier:
A glacier that is confined to a stream valley.

Vallis (pl. Valles):
Valley or canyon.

Vertical Exaggeration:
The degree to which the perception of vertical relief is enhanced in each view. This parameter is a function of when each image was taken. It can also be defined as 5 × (B/H), where B/H is the base to height ratio (given here as B = stereo baseline and H = spacecraft altitude).

Vertical Resolution:
The smallest amount of vertical relief that is visible in the stereo pair. Any object or feature taller than this limit will be detectable as a three-dimensional object.

Viscous:
Relatively resistant to flow.

Volcanic Pits:
Irregularly shaped volcanic depressions.

Volcanism:
The processes by which magma and gasses are transferred from a planet's interior to the surface.

W

Wrinkle Ridge:
A sinuous, irregular segmented ridge on the surface of a lunar mare, believed to be a result of deformation of the lava.

X, Y, Z

Yardang:
An elongate ridge carved by wind erosion.






Some of the definitions given in this glossary were taken from the following:


Bates R. L. and Jackson J. A., eds. (1987) Glossary of Geology, 3rd edition. American Geological Institute, Alexandria, Virginia. 788 pp.

Greeley R. and Batson R. (1997) The NASA Atlas of the Solar System. Cambridge University Press, Cambridge, UK. 369 pp.

Hamblin W. K. (1989) The Earth's Dynamic System: A Textbook in Physical Geology, 5th edition. MacMillan Publishing, New York. 576 pp. Reprinted by permission of Prentice-Hall, Inc., Upper Saddle River, New Jersey.