Glossary of terms
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A-B | C-D | E-F | G-H | I-K | L-N | O-Q | R-T | U-Z | [Sources]

ravine: a narrow valley with steep sides, eroded by running water. A ravine is larger than a gulley and smaller than a canyon. [AHDOS]

Rayleigh wave: a vertical, ripple-like seismic wave on or just below the surface of the earth. [AHDOS]

Recent: Holocene. [AHDOS]

recessional moraine: a terminal moraine formed when a receding glacier halts temporarily. [AHDOS]

red beds: a series of deep-red sedimentary strata, of a sandy, silty, or shale-like composition, mainly of the Permian or Triassic periods. Red beds often contain gypsum or salt deposits and are a conspicuous formation in the Rocky Mountains. [AHDOS]

regolith: a layer of soil and loose rock fragments overlying solid rock. [AHDOS]

rejuvenate: to restore to a condition characteristic of a younger landform or landscape, as by a drop in the sea level or an uplift of the land that causes a renewal of erosion. [AHDOS]

replacement: the process by which a new mineral grows in the body of an old mineral by undergoing simultaneous capillary solution and deposition. [AHDOS]

resequent: (of a stream) flowing in the same direction as a consequent stream but developing at a lower level than the original slope. [AHDOS]

residual: of or designating material left in place when rock is weathered. [AHDOS]

retinite: any one of various fossil resins, especially one of those derived from brown coal and peat. [AHDOS]

retrograde metamorphism: metamorphism in which relatively low-temperature minerals replace older high-temperature minerals. [AHDOS]

reverse fault: a fault in which one side has moved above and over the other side, as in a thrust fault. [AHDOS]

rhizosphere: the soil immediately surrounding the roots of a plant. [AHDOS]

rhodochrosite: a mineral consisting mainly of manganese carbonate and usually occurring in rose-red crystals. [AHDOS]

rhyolite: a volcanic rock containing quartz and alkalil feldspar, with texture often showing the lines of flow. [AHDOS]

Richter scale: a scale for indicating the magnitude of earthquakes. [AHDOS]

ridge: a long, narrow chain of hills or mountains. [AHDOS]

rift zone: a large area of the earth in which a rift occurs when plates of the earth's crust move away from one another. [AHDOS]

rock crystal: a colorless, transparent variety of quartz, used in optical instruments and as a semiprecious gemstone. [AHDOS]

ruby: a clear, hard, red precious stone. It is a variety of corundum. [AHDOS]

runoff: water that runs off the land in streams; that portion of precipitating water that flows overland to the sea. [AHDOS]

rutile: a reddish-brown or black mineral with a metallic or diamond-like luster, consisting of titanium oxide and often containing some iron. Rutile is polymorphous with anatase and brookite and is a common ore of titanium. [AHDOS]

salina: a playa encrusted with salt; a salt marsh, salt lake, or the like. [AHDOS]

salt dome: a circular structure of sedimentary rocks resulting from the upward movement of a subterranean mass of salt; a diapiric structure. [AHDOS]

salt lake: a lake having a high concentration of salts, found in areas of large salt deposits. Salt lakes are often saltier than oceans. [AHDOS]

salt marsh: a marsh periodically flooded by salt water through the actions of winds or tides. [AHDOS]

samarskite: a black, lustrous, orthorhombic mineral, an oxide containing niobium, uranium, cerium, samarium and other elements. [AHDOS]

sand: earth material consisting of grains of worn-down or disintegrated rock, mainly of silica, larger than silt but smaller and finer than gravel. [AHDOS]

sandbar: a ridge of sand formed in a river or along a shore by the action of tides or currents. [AHDOS]

sand plain: a small sandy plain, susually a flat-topped fill originally formed as a delta by water running out of a glacier. [AHDOS]

sandstone: a sedimentary rock formed by the consolidation of sand and held together by a natural cement such as silica or iron oxide. [AHDOS]

sand wave: a large wavelike structure formed by rapidly moving currents of water. [AHDOS]

saponite: a clay mineral with a soapy feel that occurs in soft, amorphous masses, filling veins and cavities, as in serpentine and traprock. it is a hydrous silicate of aluminum and magnesium. [AHDOS]

sapphire: a clear, hard, bright-blue variety of corundum. Sapphires are valued as semi-precious stones. [AHDOS]

sapphirine: a pale-blue or greenish metamorphic mineral, a silicate of aluminum, magnesium, and iron. [AHDOS]

saprolite: soft, partly decomposed rock rich in clay, and remaining in its original place. [AHDOS]

saussurite: a very compact mixture of albite, zoite, and other minerals, formed by the alteration of feldspar. [AHDOS]

scapolite: any of a group of minerals of variable composition, essentially silicates of aluminum, calcium, and sodium, occurring in tetragonal crystals and also massive. [AHDOS]

schist: a medium-grained metamorphic rock usually composed essentially of mica, that spits easily into layers. [AHDOS]

schistose: of or resembling schist; havng the structure of schist. [AHDOS]

schlieren: irregular, dark or light streaks or clots occurring in plutonic igneous rock because of varying proportions of the minerals present. [AHDOS]

schorl: torumaline, especially iron-rich black tourmaline. [AHDOS]

schreibersite: a phosphide or iron and nickel occurring only in meteoric iron. [AHDOS]

scolecite: a hydrated silicate of calcium and aluminum, belonging to the zeolite family. Scolecite is found in needle-shaped crystals and fibrous or radiated masses. [AHDOS]

scoria: porous, cinderlike fragments of basaltic lava, usually darker and denser than pumice. [AHDOS]

scorodite: a hydrous ferric arsenate, occurring in orthorrhombic crystals and in earthy form, and usually fof a greenish or brown color. [AHDOS]

scorzalite: a phosphate of iron and aluminum, occurring in crystalline or massive form. It is a blue mineral, isomorphous with lazulite. [AHDOS]

scree: a teep amss of loose rocky fragments lying at the base of a cliff or on the side of a mountain. [AHDOS]

sea cave: a cave formed by the action of waves against a rocky shore. [AHDOS]

sea floor: the floor or bottom of a sea or ocean. [AHDOS]

sea-floor spreading: (in plate techtonics) the process by which the sea floor is being continuously formed and spread by upwellings from the earth's mantle along the mid-ocean ridges when crustal plates move apart, and continuously destroyed along the subduction zones, where plates push against each other and the sea floor sinks into the mantle. [AHDOS]

sea level: the surface of the sea, especially when halfway between mean high and low water. Mountains, plains, and ocean beds are measured as so many meters or feet above or below sea level. [AHDOS]

seam: a layer or stratum, especially of a mineral; bed. [AHDOS]

seamount: a tall, cone-shaped hill or mountain arising from the sea bottom with its summit beneath the surface of the sea, and often having a flat top, as a guyot does. [AHDOS]

seaquake: an earthquake originating at th ebottom of the sea. [AHDOS]

secondary wave: an earthquake wave in which rock particles vibrate at right angles to the direction of travel. [AHDOS]

sectile: that can be cut smoothly by a knife but cannot withstand pulverization. [AHDOS]

sediment: earth, stones, and other matter deposited by water, wind, or ice; to deposit as or form sediment. [AHDOS]

sedimentology: the branch of geology that studies the formation and structure of sediments and sedimentary rocks. [AHDOS]

seif: a long, narrow dune found in sandy deserts in which the winds are variable. [AHDOS]

seismic: of or having to do with earthquakes or other movements of the earth's crust; caused by an earthquake or other movement of the earth's crust. [AHDOS]

selenite: a variety of gypsum, found in transparent crystals and foliated masses. [AHDOS]

senile: having reached an advanced stage of erosion; made flat or level, as by the action of water or wiND. [AHDOS]

septarium: a roughly spherical concretion of minerals occurring in layers, as in sand or clay. A septarium is usually of calcium carbonate or carbonate of iron, having a network of cracks filled with calcite and other minerals. [AHDOS]

serac: a large block of pinnaclelike mass of ice on a glacier, formed by the intersection of two or more crevasses. [AHDOS]

serpentine: a greenish rock-forming mineral with an oily luster, consisting chiefly of magnesium. It is sometimes spotted like a serpent's skin. Serpentine is a soft, waxy substance. A fibrous variety of serpentine (chrysotile) is the most improtant type of asbestos. [AHDOS]

shale: a fine-grained sedimentary rock, formed from hardened clay, mud, or silt in thin layers which split easily. [AHDOS]

shatter cone: a cone-shaped rock fragment with distinctive ridges, produced by intense shock forces from the impact of a meteor. [AHDOS]

shearing plane: the plane along which rupture from shearing stress takes place in rocks. [AHDOS]

sheet: a nearly horizontal layer of igneous or sedimentary rock. [AHDOS]

sheet erosion: the washing away of soil in layers form barren, sloping land by rainfall. [AHDOS]

shelf: a ledge of land or rock, especially a submerged ledge or bedrock; continental shelf [AHDOS]

shelf ice: a ledge of ice sticking out into the sea from an ice sheet. [AHDOS]

shell: a hard, thin layer of rock [AHDOS]

shield: a large area of exposed igneous and metamorphic rocks, usually Precambrian, in a craton. [AHDOS]

shield volcano: a low, broad, dome-shaped volcano built up of overlapping basaltic lava flows and often covering a very large area. [AHDOS]

shift: a slight fault or dislocation in a seam or stratum. [AHDOS]

shoal: a sandbank, sand bar, or ledge of rock, coral, or the like, thak makes the water shallow especially one that can be seen at low tide. [AHDOS]

sial: granitelike rock rich in silicon and aluminum. Sial is the chief rock underlying the land masses, as distinct from the ocean basins. [AHDOS]

sierozem: a type of soil found in temperate or cool arid regions, characterized by a brownish-gray surface covering a lighter layer and supporting sparse, shrubby vegetation. It is low in humus and its lime is near or on the surface. [AHDOS]

silica: a common hard, white or colorless compound, silicon dioxide, that resembles glass. Quartz, flint, opal, and sand ore forms of silica. [AHDOS]

silicate: a compound containing silicon with oxygen and a metal; a salt or silicic acid. Silicates constitute the greater number of the minerals that compose the crust of the earth. Mica, garnet, talc, asbestos, and feldspar are silicates. [AHDOS]

sill: an approximately horizontal sheet of instrusive igneous rock, found between older rock beds. [AHDOS]

sillimanite: a silicate of aluminum occurring in orthorhombic crystals, trimophous with andalusite and kyanite. [AHDOS]

silt: very fine particles of earth, sand, clay, etc., carried by moving water and deposited as sediment. Particles of silt measure from .01 to 1 mm in diameter; they are larger than clay particles and smaller than sand particles. [AHDOS]

Silurian: the thrid geological period of the Paleozoic era, after the Ordovician and before the Devonian, characterized by the development of early invertebrate land animals and land plants. The Silurian formerly included what is now Ordovician and what is now Silurian; the rocks formed during this period. [AHDOS]

sima: basaltic rock rich in iron and magnesium and low in silica. Sima is the chief constituent of the ocean floors. [AHDOS]

sinkhole: a funnel-shaped cavity formed in limestone regions by the removal of the rock through action of rain, ground water, or running water. [AHDOS]

sinter: a hard chemical incrustation or deposit formed on rocks by evaporation of hot or cold mineral waters. Sinter may be siliceous (consisting of silica) or calcareous (consisting of calcium carbonate). [AHDOS]

skeleton crystal: an incompletely developed crystal, having an axial framework, but with faces only partially filled in. [AHDOS]

slate: a fine-grained, usually bluish-gray metamorphic rock that splits easily into thin, smooth layers. [AHDOS]

slide: the downward falling or sliding of a mass of earth, snow, or rock; the mass of earth, snow, or rock falling or sliding down from a hill or mountainside. [AHDOS]

slip: a fault in rock due to the sinking of one section; a movement producing such a fault; the amount of such movement, measured by the amount of displacement along the fault plane. [AHDOS]

slope: the inclined surface of a hill, mountain, plain, etc.; any part of the earth's surface that goes up or down at an angle; the angle at which such a surface deviates from the horizontal. [AHDOS]

slump: the slipping or falling or a mass or rock or unconsolidated material, especially the vertical drop of a mass of rock along a cliff face after the supporting material at the base of the cliff has been undercut by erosion; the mass of fallen material. [AHDOS]

soapstone: a soft metamorphic rock composed mostly of talc. [AHDOS]

solfatara: a volcanic vent or area that gives off only sulfurous gases, steam, and the like. [AHDOS]

solifluction: the downward movement of soil and rcok on the face of the earth, caused by the action of the weather. [AHDOS]

spatter cone: a clot or hardened mass of lava in the form of a steep-sloped cone. [AHDOS]

speleology: the scientific study and exploration of caves. [AHDOS]

spahlerite: native zinc sulfide, the main ore of zinc. Sphalerite is dimorphous with wurtzite. [AHDOS]

sphene: a silicate of calcium and titanium commonly found in igneous rocks. [AHDOS]

spenochasm: a deep wedge-shaped depression of the earth's crust between two faults. [AHDOS]

spilite: a basalt altred by seawater to a mixture of low-temperature, hydrous silicates. [AHDOS]

spilosite: a greenish schistose rock resulting from metamorphism of slate. [AHDOS]

spinel: any of a group of very ahrd minerals consisting chiefly of oxides of magnesium and aluminum and occurring in various colors. The magnesium may be replaced in part by ferrous iron, zinc, or manganese, and the aluminum by ferric iron or chromium. [AHDOS]

stalactite: a formation of calcium carbonate, shaped like an icicle, hanging from the roof of a cave, formed by dripping water that contains calcium carbonate. [AHDOS]

stalagmite: a formation of calcium carbonate, shaped like a cone, built up on the floor of cave, formed by water dripping from a stalactite. [AHDOS]

staurolite: a metamorphic mineral, a silicate of aluminum and iron, yellowish-brown to dark-brown in color, found frequently twinned in the form of a cross. [AHDOS]

step fault: a fault in which the displacement is distributed among several parallel planes at short distances from one another instead of being confined to a single plane. [AHDOS]

stibnite: native antimony sulfide, a lead-gray mineral occurring in orthorhombic crystals and in massive forms. It is the most important ore of antimony. [AHDOS]

stone: a piece of fragment of rock. [AHDOS]

stratification: the formation of strata; deposition or recurrence in strata; bedding. [AHDOS]

stratigraphy: the branch of geology that deals with the order and position of strata. [AHDOS]

streak: the color of the fine powder produced when a mineral is rubbed upon a hard surface. Hematite has a red streak, magnetite a black streak. [AHDOS]

stromatolite: a calcareous rock structure consisting of deposits of blue-green algae. [AHDOS]

stylolite: a columnar structure, often with grooved sides, occurring along a thin seam in limestone or certain other rocks, usually at right angles to the plane of stratification. [AHDOS]

syncline: a fold or folds of rock strata sloping downward from opposite directions so as to form a trough or inverted arch. [AHDOS]

tabular: tending to split into flat, thin pieces; covering a large area whiel being relatively thin. [AHDOS]

taconite: a variety of chert which contains iron, occurring especially in the Mesabi Range of northeastern Minnesota. [AHDOS]

talc: a soft, smooth mineral, a hydrous silicate of magnesium, usually consisting of slippery, translucent sheets of white, apple-green, or gray, used in making face powder. [AHDOS]

taphonomy: the processes and conditions to which plants and animals are subjected as they become fossilized. [AHDOS]

tar sand: a type of oil-yielding sandstone or sand formation containing tarry substances. [AHDOS]

tectonic: of or having to do with general changes in the structure of the earth's crust or witht he forces that affect them. [ahdos]

techtonics: a branch of geology dealing with the forces affecting the structure of the earth's crust; the features or characteristics of this structure. [AHDOS]

tektite: any of various rounded, glassy objects of different shapes and weights, found in various parts of different parts of the world and thought to have come from the moon or to have resulted from the impacts of large meteorites on terrestrial rocks. [AHDOS]

terminal moraine: a moraine deposited at the end of a glacier. [AHDOS]

terrestrial: of the land, not water. [AHDOS]

terrigenous: of or having to do with shallow marine sediments deposited from neighboring land. [AHDOS]

Tertiary: the earlier of the two periods making up the Cenozoic era, immediately following the Mesozoic. During this time the grat mountain systems, such as the Alps, Himalayas, Rockies and Andes appeared, and rapid development of mammals occurred. [AHDOS]

tetrahedral: of or having to do with a tetrahedron; having four faces. [AHDOS]

tetrahedrite: native sulfide of antimony and copper, with various elements sometimes replacing one or the other of these, often occurring in tetrahedral crystals. [AHDOS]

texture: the physical structure of a rock; the spatial relationships between the mineral grains making up a rock, including their size, shape and arrangment. [AHDOS]

thermocline: a layer within a large body of water sharply separating parts of it that differ in temperature, so that the temperature gradient through the layer is very abrupt. [AHDOS]

thorite: a mineral, a silicate of thorium, occurring in crystals or in massive form. Thorite varies in color from orange-yellow to brownish-black or black. [AHDOS]

throw: a fault; the extent of vertical displacement produced by a fault. [AHDOS]

thrust fault: a low-angle, reversed fault, produced by horizontal compression. [AHDOS]

thunder egg: a common name for a small, geodelike body formed in tuff or lava and consisting of chalcedony, opal or agate. [AHDOS]

tiger's-eye: a golden-brown mineral with a changeable luster, composed chiefly of quartz colored by iron oxide, and often used as a gemstone. [AHDOS]

time-rock unit: a unit of classification of rocks based on the span of time during which they were deposited. The basic time-rock units are the stage, the series, and the system. [AHDOS]

tonalite: a granualr, plutonic igneous rock, a variety a diorite, containing quartz, plagioclase, and biotite or hornblende. [AHDOS]

tongue: a narrow strip of land jutting out into water; a long, narrow extension of a glacier or lava flow. [AHDOS]

topaz: a hard, transparent or translucent mineral that occurs in crystals of various forms and colors. Transparent yellow, pink or brown varieties are used as gems. [AHDOS]

topsoil: the upper part of the soil; surface soil, as opposed to subsoil; loam or other earth from or in this part of the soil, usually consisting of sand, clay, and decayed organic matter. [AHDOS]

tourmaline: a chemically complex mineral found in granite and pegmatite. Tourmaline is chiefly a silicate of boron and aluminum with varying amounts of other elements, especially iron, calcium, and sodium. It may be black, brown, red, pink, green, blue, or yellow, and some varieties are used as semiprecious gemstones. [AHDOS]

trachyte: a light-colored volcanic rock consisting chiefly of alkali feldspar, and also containing augite, biotite, or hornblende. [AHDOS]

transform fault: one of numerous strike-slip faults between segments of mid-oceanic ridges and rises along which lateral movement of crustal plates occurs. [AHDOS]

trap: a former term for basalt or other fine-grained, dark, igneous rock usually having a sheetlike or teplike structure. [AHDOS]

tremolite: a white or gray mineral, a variety of amphibole, consisting chiefly of a silicate of calcium and magnesium, occurring in fibrous masses or thin-bladed crystals. [AHDOS]

tremor: a small shaking or vibrating movement of the earth, often occurring before or after an earthquake. [AHDOS]

trench: a long, narrow depression in the ocean floor. [AHDOS]

Triassic: the earliest period of the Mesozoic era, beofre the Jurassic, characterized by the appearance of dinosaurs and primitive mammals, the domination of the earth by reptiles, and much volcanic activity; the rocks formed during this period. [AHDOS]

tripoli: any of severalsoft, porous, siliceous earths or rocks, often composed of the shells of diatoms, radiolarians, infusorians, etc., or of chalcedonic chert. [AHDOS]

tufa: any of various porous rocks formed of powdery matter consolidated and often stratified, especially a form of limestone deposited by springs, lakes, etc. [AHDOS]

tuff: a rock produced by the consolidation of volcanic ash and other volcanic fragments. [AHDOS]

turbidity current: an underwater stream of silt, mud, or the like, usually along the bottom of a slower-moving body of water. [AHDOS]