Visualizing Geology Expanded Glossary
Shared by: bmo99796
Visualizing Geology: Expanded Glossary Term Pg# Definition aa 167 Viscous, slow-moving basaltic lava that solidifies into a rough, clinkery volcanic rock. ablation 394 Loss of ice from a glacier. ABRASION See Illustrated Key Word Glossary. abyssal plain 94 The main, flat part of the ocean floor. accessory mineral 47 A mineral that is present in such small quantity that it does not determine the properties of the rock. accretionary wedge 236 Sediment that is scraped off a subducting plate and piled up at the front of the over-riding plate; see mélange, ophiolite. accumulation 394 Addition of snow and ice to a glacier. acid rain 188 Rain with higher than normal acidity (pH less than about 5), usually the result of human activities. active layer 401 The layer of ground above permafrost, which melts in summer and freezes in winter. active (volcano) 162 Describes a volcano that is erupting or has erupted within recorded history. aerated zone 325 See zone of aeration. aerobic (metabolism) 422 Describes metabolism that occurs in the presence of free oxygen. aerosol 161 Extremely tiny liquid droplet or solid particle. aftershock 123 Earthquake that follows a large earthquake, in the same area or along the same fault system. agglomerate 156 Pyroclastic rock consisting of large volcanic fragments (tephra) cemented together; see tuff. aggregate 51 Collection of mineral grains intergrown or cemented together to form a rock. A horizon 195 Horizon that underlies the O horizon in a soil profile; dark- colored mix of mineral and organic matter; see topsoil. AIR See Illustrated Key Word Glossary. alluvial fan 228 Sedimentary deposit, usually fan-shaped, that forms where a stream leaves a steep mountain valley and flows onto a flatter lowland area. ALLUVIUM See Illustrated Key Word Glossary. amino acid 422 A group of carbon-based organic compounds essential to life processes. amorphous (solid) 39 Describes solids that lack a crystal structure. amphibolite 286 Metamorphic rock containing abundant amphibole. anaerobic (metabolism) 422 Describes metabolism that occurs in the absence of oxygen. andesite 170 Intermediate-silica volcanic rock with abundant feldspar, mixed with darker minerals; extrusive equivalent of diorite. ANGIOSPERM See Illustrated Key Word Glossary. angle of repose 383 The steepest angle, measured from the horizontal, at which a pile of sedimentary particles is stable. angularity 214 Characteristic of a sediment particle that has rough or sharp edges and corners; see roundness. angular unconformity 66 Erosional surface in which older sedimentary strata, below, are oriented at an angle to younger strata, above. anion 35 Ion with a negative electric charge. Antarctic Bottom Water 347 Cold, salty, dense seawater that sinks near Antarctica and (ABW) flows northward, part of the deep ocean thermohaline circulation. anthracite 459 Highest rank of coal, formed by metamorphism of bituminous coal. ANTHROPOGENIC See Illustrated Key Word Glossary. ANTICLINE See Illustrated Key Word Glossary. aphanitic 168 Describes an igneous rock in which the mineral grains are so small that they can only be seen clearly with magnification. apparent polar 97 The apparent shifting of Earth’s pole positions in the past, as wandering determined by paleomagnetism, and reflecting the history of lithospheric plate motion relative to Earth’s actual pole positions. aqueous 280 Watery (describes a fluid or a solution). AQUICLUDE See Illustrated Key Word Glossary. AQUIFER See Illustrated Key Word Glossary. Archaea 424 Prokaryotic organisms, typically extremophiles. Archean 69 The second eon of geologic history, one of the three eons of Precambrian time; see Hadean, Proterozoic, Phanerozoic. arête 397 Sharp-crested ridge, where two cirques on opposite sides of a mountain meet. arid (land) 379 Land in which annual precipitation is less than 250 mm; desert. arroyo 385 Steep-sided canyon formed by intermittent downpours in desert landscapes. artesian well 329 Well drilled into a confined aquifer, in which the water rises above the level of the water table or the ground surface without pumping. Arthropoda 434 Phylum that includes modern crabs, spiders, centipedes, and insects – the most diverse phylum on Earth, and the first creatures to make the transition from sea to land. asbestiform (habit) 42 Growth habit in which the mineral resembles asbestos, that is, occurring in long, thin fibers; see fibrous. ash (volcanic) 216 The smallest particles of volcaniclastic sediment or tephra. assemblage 51 See mineral assemblage. ASTHENOSPHERE See Illustrated Key Word Glossary. asymmetrical (fold) 262 Describes a fold in which one limb dips more steeply than the other. atmosphere (a unit of 277 One atmosphere is defined as being equal to the air pressure pressure) at sea level on Earth’s surface. ATMOSPHERE See Illustrated Key Word Glossary. ATOM See Illustrated Key Word Glossary. atomic number 34 Number of protons in the nucleus of an atom. atomic substitution 38 Substitution of one atom for another within a crystal structure. aureole 290 Zone of contact metamorphic rock surrounding an igneous intrusion. aurora borealis 139 Northern lights, caused by charged particles from the Sun entering Earth’s atmosphere at high latitudes along magnetic field lines; the corresponding phenomenon in the Southern Hemisphere is aurora australis. Australopithecus 438 Earliest hominid known to have walked upright. axial plane 260 Imaginary plane that divides a fold as symmetrically as possible. axial trace 260 Line of intersection of the axial plane of a fold with a horizontal plane. axis (of a fold) 260 Line where the axial plane of a fold intersects a fold; see hinge. back-arc basin 237 Arc-shaped basin that forms as a result of crustal thinning behind a magmatic arc. bacteria 424 Prokaryotic microorganisms. bajada 385 Broad alluvial apron that is formed when adjacent alluvial fans coalesce. BANDED IRON See Illustrated Key Word Glossary. FORMATION barchan dune 384 A mobile, crescent-shaped dune that is formed where the wind regularly blows in one direction; the “horns” of the crescent point downwind. BARRIER ISLAND See Illustrated Key Word Glossary. barrier island coast 357 Series of long, narrow sandy islands lying offshore and parallel to an extensive region of low-lying coast; see lowland beach. barrier reef 361 A coral reef that is separated from the land by an inlet or lagoon. basal sliding 397 Mechanism of glacier movement in which the ice at the bottom slides across the underlying rock or sediment. basalt 16 Low-silica, mafic volcanic rock with abundant olivine and pyroxene; extrusive equivalent of gabbro; the most common volcanic rock on Earth, of which oceanic crust is composed. basalt plateau 153 Extensive plain of basaltic lava, formed by the extrusion of basaltic magma from fissures; see flood basalt. base flow 318 Normal (non-storm) flow of water in a stream, contributed by groundwater. basin 262 Depression in the land surface, usually caused by circular synclinal folding, or downwarping, of the crust. BATHOLITH See Illustrated Key Word Glossary. bay barrier 360 Ridge of sand or gravel that blocks the mouth of a bay. BEACH See Illustrated Key Word Glossary. BEACH DRIFT See Illustrated Key Word Glossary. bed (of a stream) (1) 198 (1) Rock and sediment that forms the bottom of a stream channel. bed (of a glacier) (2) 397 (2) Rock or sediment on which a glacier rests. bed (sedimentary) (3) 218 (3) Sedimentary layer or stratum. BED LOAD See Illustrated Key Word Glossary. BEDDING See Illustrated Key Word Glossary. BEDDING SURFACE See Illustrated Key Word Glossary. bedrock 263 Solid rock that underlies surface soil or alluvium, or is exposed at surface outcrops. bench 357 See wave-cut bench. Benioff zone 107 Zone of shallow- to deep-focus earthquakes that marks a subduction zone. B horizon 195 Horizon that lies below the A horizon (or E, if one is present) in a soil profile; typically brownish or reddish in color because of iron oxides. biogenic 214 Produced by biologic processes, or composed mainly of plant or animal remains. BIOGENIC SEDIMENT See Illustrated Key Word Glossary. biologic resource 454 A natural resource derived from plant or animal waste or remains. BIOMASS ENERGY See Illustrated Key Word Glossary. BIOSPHERE See Illustrated Key Word Glossary. bipedal (animal) 438 Habitually walking upright on two legs. bituminous 459 Describes coal of a rank between lignite (lower) and anthracite (higher). “black smoker” 100 A rock “chimney” that is formed at a seafloor hydrothermal vent where superheated water emerges from the crust, carrying dissolved minerals that precipitate upon hitting the cold ocean water. blueschist 286 Metamorphic rock that forms in conditions of high pressure and low to moderate temperature, and contains a blue-colored amphibole. BODY WAVE See Illustrated Key Word Glossary. bomb (volcanic) 216 Largest clasts of volcaniclastic sediment or tephra. BOND See Illustrated Key Word Glossary. braided (channel) 308 Stream channel with a tangled network of branching and reuniting tributaries, separated by point bars and islands. breaker 355 A wave that is approaching a shoreline, and collapses into whitewater surf when the front of the wave becomes too steep to support the advancing water. breccia 222 Coarse-grained sedimentary rock in which the clasts are angular; see conglomerate. briny 325 Describes water that contains abundant dissolved mineral salts, rendering it unfit for human consumption. BRITTLE See Illustrated Key Word Glossary. DEFORMATION brittle-ductile transition 250 Depth below which the ductile properties of rock become more important than the brittle properties. BURIAL See Illustrated Key Word Glossary. METAMORPHISM butte 381 Isolated hill with steep sides formed from soft sedimentary rock, topped by a flat layer of more resistant rock. calcerous ooze 231 Biogenic marine sediment that consists largely of the calcium carbonate remains of microscopic shelled organisms. caldera 156 Roughly circular, steep-walled basin that forms as a result of collapse of the roof of a magma chamber following a volcanic eruption. caliche 196 A hard, white, crusty soil layer, formed by the precipitation of dissolved calcium carbonate; see hardpan. Cambrian explosion 432 The rapid development of many new life forms, whose fossils now mark the beginning of the Cambrian Period and the Phanerozoic Eon; Cambrian radiation. Cambrian radiation 432 See Cambrian explosion. cap rock 462 Impermeable stratum immediately overlying a reservoir rock, which stops the upward migration of oil and gas; see petroleum trap. carbonaceous chondrite 82 Rare type of carbon-bearing stony meteorite thought to be an unaltered mixture of materials dating from the formation of the solar system. carbonate minerals 47 A group of minerals based on the complex (CO3)2– anion. catchment 313 Watershed or drainage basin. cation 35 Ion with a positive electric charge. CAVE See Illustrated Key Word Glossary. cavern 333 System or series of caves. CELL See Illustrated Key Word Glossary. cement 54 A substance that holds pieces together; in sedimentary rocks the common cements are quartz, calcite, and iron oxide. CEMENTATION See Illustrated Key Word Glossary. Cenozoic 69 The current era, one of the three eras that comprise the Phanerozoic Eon; see Mesozoic, Paleozoic. chain reaction 468 Chemical reaction in which the fissioning (splitting) of an atom produces atomic particles that cause other atoms split. CHANNEL See Illustrated Key Word Glossary. channelization 321 Modification of a natural river channel for flood control, erosion control, transportation, or some other purpose. characteristic crystal 38 The regular, orderly geometric packing of atoms that is unique structure to crystals of each type of mineral; one of the requirements for a substance to be classified as a mineral. chemical 289 Chemical changes leading to changes in the composition of recrystallization minerals or growth of new minerals during metamorphism; see mechanical deformation. CHEMICAL SEDIMENT See Illustrated Key Word Glossary. CHEMICAL See Illustrated Key Word Glossary. WEATHERING chemosynthesis 422 Process by which some bacteria use energy from inorganic materials to make organic molecules; see photosynthesis. chert 224 Sediment composed of tiny particles of silica; often biogenic, resulting from the accumulation of siliceous remnants of marine microorganisms. chitin 434 Tough organic compound that forms the hard outer layer of many arthropods. chordates 435 Phylum of animals that have at least a primitive version of a spinal chord (called a notochord); humans are chordates. C horizon 195 Horizon that lies below the B horizon in a soil profile; the lowest soil horizon, also called the subsoil, below which lies unweathered parent rock. cinder cone 153 Cone-shaped pile of very fine, loose volcanic particles, typical of Strombolian eruptions; see shatter cone. cirque 397 Bowl-shaped basin on a mountainside, with the open side pointing downslope, formed mainly by frost wedging and glacial abrasion. cirque glacier 390 Glacier that occupies a bowl-shaped depression on a mountainside (a cirque). clast 214 Fragment; individual minerals grains or rock fragments in a sediment. clastic 214 Describes a rock or sediment that consists mainly of fragmented rock and mineral debris. CLASTIC SEDIMENT See Illustrated Key Word Glossary. CLAY See Illustrated Key Word Glossary. cleavage 43 Tendency for a mineral to break in preferred directions. climate 368 Average weather pattern of a region over a long period. closed cycle 305 See closed system. closed system 7 System whose boundaries permit the exchange of energy but not of materials with its surroundings; see open system, isolated system. COAL See Illustrated Key Word Glossary. coalification 224 Process whereby plant matter is converted to peat and then to lignite and bituminous coal. coastal desert 379 Desert on the western margin of a continent, which is formed when cold seawater cools the air flowing onshore, thus reducing rainfall. collision zone 104 Convergent margin where two continental plates collide and thicken. COMPACTION See Illustrated Key Word Glossary. composite volcano 153 See stratovolcano. composition 165 The chemical elements that comprise a material; generally expressed in terms of percent by weight. COMPOUND See Illustrated Key Word Glossary. COMPRESSION See Illustrated Key Word Glossary. compressional stress 257 See compression. COMPRESSIONAL See Illustrated Key Word Glossary. WAVE computer-aided 136 Technique that utilizes computers to produce a three- tomography dimensional image of the interior of an object, based on a series of two-dimensional images that are like “slices” through the object; see seismic tomography. CONDENSATION See Illustrated Key Word Glossary. conduction 108 Transmission of heat through a material by direct contact and flow of heat from one molecule to another, without deforming the material. cone of depression 330 Conical depression in the water table, immediately surrounding a pumped well. confined aquifer 328 Aquifer bounded by aquicludes; see unconfined aquifer, confining layer. confining layer 328 Aquiclude. confining stress 247 Stress that is equal in all directions; see uniform stress. conformable 65 Describes strata that have been deposited layer on layer, with no depositional gaps. CONGLOMERATE See Illustrated Key Word Glossary. CONTACT See Illustrated Key Word Glossary. METAMORPHISM CONTINENTAL See Illustrated Key Word Glossary. CRUST continental divide 314 A divide, or ridge, separating streams that flow toward opposite sides of a continent, usually into different oceans. CONTINENTAL DRIFT See Illustrated Key Word Glossary. continental interior 379 Inland desert that gets little rain because it is so far from the desert ocean. continental rise 94 Sediment-covered transition from a continental slope to an abyssal plain. continental shelf 93 Submerged border of a continent, which slopes gently away from the land. continental slope 93 Steep slope, beyond the continental shelf, that leads down to the deep ocean floor. contour lines 265 Lines of equal elevation on a topographic map. CONVECTION See Illustrated Key Word Glossary. convection cell 108, The space involved in one cycle of convection, in which hot 366 material rises, loses its heat while moving laterally, sinks as it becomes colder and denser, and then repeats the cycle. convection current 366 Flow within a material that is undergoing heat transfer by convection. converge 104 To move toward one another. convergence 378 Movement of two or more things toward one another. CONVERGENT See Illustrated Key Word Glossary. MARGIN coprolite 430 Fossilized fecal droppings. coral bleaching 371 Whitening of corals; when the water gets too warm, coral polyps expel the symbiotic algae (zooxanthellae) that normally give them their bright colors, and eventually die. coral reef 357 Reef composed mainly of the calcareous colonies of many individual coral polyps. CORE See Illustrated Key Word Glossary. CORIOLIS EFFECT See Illustrated Key Word Glossary. CORRELATION See Illustrated Key Word Glossary. covalent bonding 37 Very strong type of bonding in which electrons from different atoms “pair up”. crater (1) 156 (1) Funnel-shaped depression, opening upward, at the top of a volcano, from which gases, fragments of rock, and lava are or were ejected. crater (2) 14 (2) Depression excavated by the high-speed impact of a meteorite with a planetary surface. CRATON See Illustrated Key Word Glossary. CREEP See Illustrated Key Word Glossary. crest (1) 318 (1) Peak flow or discharge during a flood. crest (2) 353 (2) Highest point of a wave; see trough. crevasse 395 Deep, gaping fissures in the upper surface of a glacier. cross bedding 219 Sedimentary beds that are inclined with respect to a thicker stratum within which they occur. cross bed 383 See cross bedding. crude oil 462 Petroleum, prior to the distillation process. CRUST See Illustrated Key Word Glossary. CRYOSPHERE See Illustrated Key Word Glossary. crystal 41 A solid with an orderly internal arrangement of atoms; see crystal structure. crystal face 41 Smooth, planar growth surfaces of a crystal. crystal flotation 171 Differentiation process in which the first crystals to solidify from a melt are less dense than the melt, and thus rise to the surface. crystal settling 171 Differentiation process in which the first crystals to solidify from a melt are more dense than the melt, and thus sink to the bottom. CRYSTAL See Illustrated Key Word Glossary. STRUCTURE crystalline 40 Describes a solid that possesses an internal crystal structure. CRYSTALLIZATION See Illustrated Key Word Glossary. cubic (habit) 42 Mineral habit in which crystals grow in the shape of cubes. Curie point 137 Temperature above which permanent magnetism in a given material is not possible. cut bank 310 Steep stream bank, formed by lateral erosion around a meander. cyanobacteria 424 Single-celled, prokaryotic organisms that contain chlorophyll and release oxygen during photosynthesis; formerly called blue-green algae. cycle 10 Recurring sequence of events or processes with no beginning or ending point. daughter atoms 75 Atoms or isotopes that are formed as a result of the radioactive decay of parent atoms. debris avalanche 161, Granular flow of mixed regolith or volcaniclastic debris, 202 moving downslope at a high velocity. decompression melting 165 Melting induced by a decrease in pressure, as when a rock rises toward Earth’s surface. deductive (reasoning) 136 Solving a problem by starting from a general principle or model and moving to a specific or inescapable conclusion; see inductive. deep zone (of the 345 In oceans and large lakes, the relatively cold, high-density ocean) water at depths greater than the thermocline; below about 100 m, in the oceans; see surface layer. DEFLATION See Illustrated Key Word Glossary. DELTA See Illustrated Key Word Glossary. density 45 Mass per unit of volume; see specific gravity. DEPOSITION See Illustrated Key Word Glossary. depositional 224 A physical environment on land (such as a desert, streambed, environment or wetland) or in the ocean (such as a continental shelf or submarine canyon) where deposition of sediment occurs. DESERT See Illustrated Key Word Glossary. desert pavement 382 Surface layer of coarse particles, concentrated mainly by the removal of finer material by deflation, in arid environments. DESERTIFICATION See Illustrated Key Word Glossary. diagenesis 220 Chemical, physical, and biologic changes undergone by sediment from the moment it is deposited, up to and including lithification. diameter 139 Length of a straight line that passes from edge to edge through the center of a circle or sphere. differential stress 247 Stress that is not equal in all directions. differentiation (1) 15 (1) Segregation of a planetary body into zones of varying density (and, typically, composition), with the highest-density material in the center or core, and the lowest-density material, the crust, on the outside. differentiation (2) 171 (2) Igneous processes that result in the development of more than one rock type from a common magma. dike 65, A tabular, parallel-sided sheet of intrusive rock that cuts 174 across the layering of the intruded rock. diorite 170 Intermediate-silica plutonic rock with abundant feldspar mixed with darker minerals; intrusive equivalent of andesite. DIP See Illustrated Key Word Glossary. DISCHARGE (1) See Illustrated Key Word Glossary. DISCHARGE (2) See Illustrated Key Word Glossary. disconformity 66 Surface of erosion in which the older strata below are parallel to the younger strata above. discontinuity 135 A boundary across which the properties of Earth materials change abruptly; see seismic discontinuity. DISSOLUTION See Illustrated Key Word Glossary. dissolved load 198 Matter dissolved in stream water. diverge 104 To move away from one another. divergence 379 Movement of two or more things away from each other. DIVERGENT MARGIN See Illustrated Key Word Glossary. DIVIDE See Illustrated Key Word Glossary. DNA See Illustrated Key Word Glossary. dolostone 224 Sedimentary rock composed chiefly of the mineral dolomite. DOMAIN See Illustrated Key Word Glossary. dome 262 Circular or elliptical anticlinal structure or upwarping of the crust, in which the layers slope gently away in all directions from the center. dormant (volcano) 162 Describes a volcano that has not erupted in recorded history but still shows signs of volcanic activity. downstream 310 Near the mouth of a stream or the terminus or a glacier; the direction toward which a stream or glacier is flowing. DRAINAGE BASIN See Illustrated Key Word Glossary. DUCTILE See Illustrated Key Word Glossary. DEFORMATION DUNE See Illustrated Key Word Glossary. Earth resource 454 Useful and/or valuable things that can be extracted from Earth. Earth system science 7 Application of a systems approach to the study of Earth and the relationships among its component parts. earthquake 107 Sudden shaking of Earth caused by the abrupt release of energy from accumulated strain. eccentricity 404 Departure from circularity (of Earth’s orbit around the Sun, in this case); see Milankovitch cycles. eclogite 286 High-grade metamorphic rock that contains garnet and pyroxene. economic geologist see Geologist who searches for new mineral deposits and 484 estimates how long they are likely to last. Ediacara fauna 426 A group of very early multicellular marine eukaryotic organisms known only from imprints in the uppermost strata of the Proterozoic Eon. E horizon 195 Soil horizon that sometimes occurs between the A and B horizons in a soil profile; typically gray with little humus, most common in soils of evergreen forests. elastic 119 Describes a reversible or non-permanent change in a substance subjected to squeezing or stretching forces; deformation followed by rebound once the deforming stress is removed; also describes a material that deforms elastically; see ductile, brittle. ELASTIC See Illustrated Key Word Glossary. DEFORMATION elastic limit 248 The limit of elastic deformation; if stresses exceed this limit, permanent (non-elastic) deformation will occur. ELASTIC REBOUND See Illustrated Key Word Glossary. THEORY electron 35 Subatomic particle with a negative electric charge and no mass. ELEMENT See Illustrated Key Word Glossary. El Niño 369 A coupled atmosphere-ocean phenomenon in which the trade winds weaken, causing warm water to pool along the equator in the eastern part of the Pacific, inhibiting the upwelling of Antarctic Bottom Water, and causing anomalous weather conditions in many parts of the world. end moraine 397 See recessional moraine. energy resource 457 A resource of energy derived from a source other than human muscle power. eolian 379 Wind-deposited. EOLIAN SEDIMENT See Illustrated Key Word Glossary. eon 69 The longest division of geologic time; we are currently in the 4th eon (Phanerozoic); see era, period, epoch. ephemeral 316 Short-lived; describes a lake, stream, or wetland that is only intermittently or occasionally filled with water. EPICENTER See Illustrated Key Word Glossary. epoch 72 A division of geologic time that is shorter than a period. era 69 A division of geologic time that is shorter than an eon and longer than a period. EROSION See Illustrated Key Word Glossary. erratic 399 See glacial erratic. eruption 152 Volcanic activity accompanied by the emergence of materials such as lava and/or pyroclastics from a vent. esker 400 Long, narrow, often sinuous ridge of stratified drift, deposited by the meltwater from a glacier. ESTUARY See Illustrated Key Word Glossary. EUKARYOTE See Illustrated Key Word Glossary. eutrophication 316 Process whereby a body of water becomes over-enriched in nutrients, leading to algal and other plant over-growth and decay, with consequent depletion of oxygen; naturally occurring, but exacerbated by human activity. EVAPORATION See Illustrated Key Word Glossary. EVAPORITE See Illustrated Key Word Glossary. events 4 Things that happened in the geologic past, such as the formation of a mountain chain, extinction of a species, or opening of an ocean basin; see historical geology. EVOLUTION See Illustrated Key Word Glossary. exchanging gases 432 The ability to have gases pass in and out through the skin, a necessity for organisms that live surrounded by air see internal aquatic environment; structural support; and moist environment for the reproductive system. exfoliation 186 Shedding or spalling of successive shells, like the layers of an onion, around a solid rock core; see sheet jointing. exponential Geometric (rather than arithmetic) growth or decay. extinct (volcano) 162 Describes a volcano that shows no signs of volcanic activity and is deeply eroded. extremophile 422 A microorganism, usually single-celled, that lives and thrives in conditions of high temperature, salinity, acidity, pressure, etc.; see archaea. extrusive 153 Volcanic rock; describes lava that emerges from a volcano, or the rock that solidifies from it; see intrusive. facies See sedimentary facies, metamorphic facies. fall 202 Sudden vertical drop of rock fragments or debris. FAULT See Illustrated Key Word Glossary. felsic 169 Describes an igneous rock that contains abundant feldspar and quartz; usually light-colored, with about 70% SiO2 by weight; see mafic, intermediate. fermentation 422 Metabolic process through which energy is obtained from nutrients in the absence of oxygen. Ferrel cells 368 Mid-latitude atmospheric circulation cells, between polar cells and Hadley cells. fibrous (habit) 42 Growth habit in which the mineral occurs in long, thread-like fibers; see asbestiform. filter pressing 171 Differentiation process in which crystals become segregated from a melt when the magma is pressed through a small opening by tectonic forces. fissile 222 Describes a rock that is easily split along closely spaced planes. fission 468 Splitting of an atom; see chain reaction. fissure (1) 153 Long, narrow crack in the crust, through which steam and/or lava may extrude. fissure (2) 123 Long, narrow crack that results from rupture of the ground surface during an earthquake. fjord 391 Valley that is partly filled by an arm of the sea. fjord glacier 391 Glacier that flows along a fjord to the sea, often giving rise to icebergs. flanks 260 The limbs of a fold. flash flood 385 Localized flooding, typically of short duration and lag time, and high volume. FLOOD See Illustrated Key Word Glossary. flood basalt 153 See basalt plateau. flood frequency curve 320 Graph showing the number of times per year a flood of specific magnitude occurs in a particular location. FLOODPLAIN See Illustrated Key Word Glossary. FLOW See Illustrated Key Word Glossary. FOCUS See Illustrated Key Word Glossary. foggara 455 Aqueducts in North Africa built by the Garamantes people. FOLD See Illustrated Key Word Glossary. fold-thrust belt 268 A mountain belt that consists of a series of large folds and nappes, separated by huge thrust faults. FOLIATION See Illustrated Key Word Glossary. footwall block 256 Block that underlies a tilted fault plane; see hanging-wall block. foreshock 123 Earthquake that precedes a (usually) larger earthquake. formation 265 Unit of rock that can be distinguished and mapped on the basis of rock type and recognizable geologic contacts above and below. FOSSIL See Illustrated Key Word Glossary. FOSSIL FUEL See Illustrated Key Word Glossary. FRACTIONAL See Illustrated Key Word Glossary. CRYSTALLIZATION FRACTIONAL MELT See Illustrated Key Word Glossary. FRACTIONATION See Illustrated Key Word Glossary. fracture 256 Crack in a rock, characteristic of brittle deformation. fringing reef 361 Coral reef built on the continental shelf that forms the coast of a landmass. frost wedging 186 Freezing of water in a confined space in a rock, causing the rock to be pried apart; a form of mechanical weathering. fuel cell 468 Electrochemical device that produces electricity from a fuel, typically natural gas or hydrogen, and an oxidizing agent, without combustion. fumarole 157 Volcanic vents that emit gases. gabbro 170 Low-silica, mafic plutonic rock that contains abundant olivine and pyroxene; intrusive equivalent of basalt. general circulation 407 Computerized, three-dimensional representation of Earth’s model (GCM) atmospheric and oceanic conditions and circulation patterns, used to simulate weather and climate. gene 429 A portion of the DNA of an organism, on which is encoded the characteristics of the organism that can be inherited by its descendants. Geographic Information 321 Computer-based system for managing and analyzing spatial System (GIS) data. GEOLOGIC COLUMN See Illustrated Key Word Glossary. geologic contact 265 Boundary between two distinct rock units. GEOLOGIC CROSS See Illustrated Key Word Glossary. SECTION geologic hazard 22 Earthquakes, volcanic eruptions, floods, landslides, and other Earth processes that can be damaging to human interests. GEOLOGIC MAP See Illustrated Key Word Glossary. geologist 4 Scientist who studies geology, that is, Earth and its materials, processes, and past events. GEOLOGY See Illustrated Key Word Glossary. geosphere 10 Earth’s four sub-systems: lithosphere, hydrosphere, atmosphere, and biosphere. geothermal energy 470 Earth’s internal or terrestrial energy. geothermal gradient 163 Increase in temperature with depth beneath Earth’s surface. geyser 157 Thermal spring that intermittently erupts water and steam. glacial erratic 399 Large boulder that has been carried by a glacier and deposited in an area of different bedrock type. glacial grooves 397 Sub-parallel scratches on a rock surface, formed by rock debris embedded in the basal ice of a glacier; deeper and wider than glacial striations. glacial striations 397 Sub-parallel scratches on a rock surface, formed by rock debris embedded in the basal ice of a glacier; see glacial grooves. glacial surge 397 Infrequent, very rapid downslope movement of a glacier by basal sliding, possibly driven by a buildup of meltwater on the glacier bed. GLACIATION See Illustrated Key Word Glossary. GLACIER See Illustrated Key Word Glossary. glacier ice 392 Ice in a glacier, formed by the compaction and recrystallization of snow. glassy 168 Describes a volcanic rock that solidified quickly, with insufficient time to crystallize individual mineral grains; see volcanic glass. Global Positioning 101 Network of satellites and receivers that allow for very precise System (GPS) identification of location; used to track plate motion. global warming 402 Gradual increase in global average surface temperature; often refers to recent climatic changes thought to be caused by human activity. GNEISS See Illustrated Key Word Glossary. graben 257 Trench-like structure formed by the down-dropping of rock between two steeply dipping normal faults; see horst. graded bed 219 Sedimentary layer in which the grain size varies smoothly from coarsest at the bottom to finest at the top. GRADIENT See Illustrated Key Word Glossary. gradualism 429 The idea that evolution is always an extremely slow, gradual process; see punctuated equilibrium. granite 170 Felsic, high-silica plutonic rock with abundant quartz and feldspar; intrusive equivalent of rhyolite. granular flow 202 Flow in which the regolith is not water-saturated, so the weight of the flowing mass is supported by grain-to-grain contact, or by repeated collisions between grains; see slurry flow. gravel 214 Sediment in which the individual grains are pea-sized or larger. greasy (luster) 41 Mineral luster that resembles a thin film of oil. GREENHOUSE See Illustrated Key Word Glossary. EFFECT greenhouse gas 364 Atmospheric gas that absorbs long-wavelength radiation, causing warming, mainly water vapor, carbon dioxide, and methane; see greenhouse effect. greenschist 286 Low-grade metamorphic rock rich in the green mineral chlorite; the product of low-grade metamorphism of basalt. groundmass 169 Aphanitic matrix of a porphyritic igneous rock. GROUNDWATER See Illustrated Key Word Glossary. groundwater mining 330 Withdrawal of groundwater at a rate that is faster than the rate of replenishment of the aquifer. Gulf Stream 347 A narrow, well-defined surface ocean current that transports heat from the tropics in the Gulf of Mexico to the poles, flowing along the eastern coast of North America and across the northern Atlantic Ocean. GYMNOSPERM See Illustrated Key Word Glossary. HABIT See Illustrated Key Word Glossary. Hadean 69 The earliest eon of geologic history, and one of the three eons that comprise Precambrian time; see Archean, Proterozoic, Phanerozoic. Hadley cells 368 Low-latitude atmospheric circulation cells on either side of the Equator, which dominate wind systems in tropical and sub- tropical regions; as descending cold air in the Hadley circulation returns to the equator it is deflected toward the west by the Coriolis effect, creating the trade winds. HALF-LIFE See Illustrated Key Word Glossary. hanging-wall block 256 Block on the overhanging side of a fault plane; see footwall block. HARDNESS See Illustrated Key Word Glossary. hardpan 196 A hard, crusty soil layer, formed by the precipitation of dissolved minerals; see caliche. Hawaiian (eruption) 153 A relatively quiet, non-explosive volcanic eruption, typically involving voluminous, low-viscosity flows of basaltic lava; characteristic of shield volcanoes. head 394 Source or uppermost part of a glacier, where accumulation occurs. headland 357 Promontory; a piece of land that juts out into the sea. headwaters 308 The upper reaches of a stream, near its source. HIGH-GRADE See Illustrated Key Word Glossary. high relief 265 Describes an area where there is a large difference between the highest and lowest elevations; mountainous topography. hinge (of a fold) 260 Line where the axial plane of a fold intersects the fold; fold axis historical geology 4 The part of geology that is concerned with the nature and sequence of geologic events from the past, as reconstructed from the rock and fossil record. homeostasis 422 Maintenance of constant or balanced internal conditions; steady state. Hominidae 438 The family of humans. hominid 438 Human-like organisms Homo erectus 439 Thought to be the first species of the human genus (Homo). Homo habilis 439 A species of the genus Homo, so named (habilis = “handy”) because they are thought to have used tools. Homo neandertalensis 439 A species of the genus Homo, which followed and replaced Homo erectus. Homo sapiens 439 The species to which all of today’s humans belong. hoodoo 381 Column or pinnacle of soft sedimentary rock, topped by a remnant of a more resistant layer of rock. horn 397 Sharp, pointed mountain peak bounded on all sides by cirques. horst 257 Elongate, elevated block of crust, bounded by steeply-dipping normal faults; see graben. host ?? The rock that surrounds and encases an inclusion or xenolith; or, a rock from which a partial melt is generated. hot spot 110 Where a hot mantle plume from the mantle reaches the surface and melts through the overlying lithospheric plate, causing volcanism. humidity 362 The amount of water vapor in the atmosphere. HUMUS See Illustrated Key Word Glossary. hydrocarbon 458 A solid, liquid, or gaseous compound that consists primarily of hydrogen and carbon. hydrocarbon trap 462 See petroleum trap. HYDROELECTRIC See Illustrated Key Word Glossary. ENERGY hydrograph 318 Graph showing discharge, flow, and/or other properties (such as depth or velocity) of flowing water with respect to time. HYDROLOGIC CYCLE See Illustrated Key Word Glossary. hydrologic station 318 Location where measurements are taken of the flow or discharge in a stream. hydrology 304 Scientific study of water and its properties and occurrences on and under the ground. hydrolytic weakening 251 Process whereby water can enter strong minerals, such as olivine and quartz, when they are undergoing stress, significantly weakening them. HYDROSPHERE See Illustrated Key Word Glossary. hydrothermal deposit 474 Mineral deposit formed by the deposition of materials by hot watery (aqueous) solutions, typically in vein systems. hydrothermal reservoir 470 Underground system of hot water and/or steam, which can be used to derive geothermal power. hydrothermal vent 423 See seafloor hydrothermal vent, “black smoker”. hypothesis 101 An explanation, as yet unproven but subject to testing and experimentation, that accounts for certain facts, observations, or phenomena; see theory. ice age 404 See glaciation. ice cap 391 Polar ice sheet. ice dam 396 Part of a glacier that blocks the valley along which its meltwater would normally flow, creating a freshwater lake. ice sheet 389 Continent-sized mass of ice that overwhelms nearly all the land surface within its boundaries. ice shelf 392 Thick mass of glacier ice that floats on the sea, but is typically connected to a glacier on land. ice wedge 401 Narrow “dike” of ice, wedge-shaped in cross section, that forms upon the freezing of water in a crack. iceberg 392 Large mass of floating ice. IGNEOUS ROCKS See Illustrated Key Word Glossary. immature (sediment) 222 Describes clastic sediment that is coarse-grained and poorly sorted, indicating that it has not been transported very far from its source. impact crater 15 Depression excavated by the high-speed collision of a meteorite with a planetary surface. inclusion 65 A rock, mineral, or other material that is surrounded and encased by a “host” rock; see xenolith. index mineral 294 Mineral whose first appearance marks the limit of a particular zone of metamorphism; see isograd. inductive (reasoning) 136 Method of reasoning or problem-solving that starts with specific observations and attempts to derive a hypothesis or basic principle to explain them; see deductive. inertia 128 Resistance of a large mass to a sudden movement, change in velocity, or change in direction; the tendency of an object at rest to remain at rest and an object in motion to remain in motion. inertial force 350 The apparent force arising from an object’s inertia. INFILTRATION See Illustrated Key Word Glossary. inner core 143 The central, solid portion of Earth’s Fe-Ni core. inorganic process 38 Chemical process that does not involve organic compounds or living matter; formation by inorganic processes is one of the requirements for a substance to be classified as a mineral. inselberg 205 Outcrop of resistant rock, isolated by erosion of the surrounding plain; monadnock. interstices 49 Spaces between closely packed objects. interbasin transfer 321 Intentional transfer of surface water from one drainage basin to another. interglacial 404 The period of time between glaciations. intermediate 169 Describes an igneous rock that is between felsic and mafic in color and composition, with a mix of dark- and light-colored minerals, and about 60% SiO2 by weight. internal aquatic 432 The ability to retain water within the body, a requirement for environment organisms to live successfully away from water; see exchanging gases; moist environment for the reproductive system; and structural support. internal creep (in a 395 Process of glacial movement, whereby ice crystals deep glacier) within the glacier slide slowly past one another along crystal planes. interstices 49 Pores; spaces between mineral grains in a rock. intertropical 368 Zone of rising warm air and low atmospheric pressure near convergence zone the equator, where the descending air of the Hadley cells returns to the equator from both north and south. intrusive 173 Describes a magma or plutonic rock body that cuts across a pre-existing body of rock; see extrusive. ion exchange 188 Transfer of ions between a mineral and a solution. ionic bonding 37 Type of bonding in which one atom transfers an electron to another atom. iron meteorite 140 Meteorite that consists almost entirely of metallic iron and nickel; see stony meteorite, stony-iron meteorite. island arc 106 Chain of volcanic islands that results from the subduction and consequent melting of an oceanic plate along an ocean-ocean convergent margin; see volcanic arc; magmatic arc. isobar ?? Line of equal pressure. isograd 294 Line of equal (metamorphic) grade; a line on a map connecting the points of first appearance of a specific metamorphic index mineral, thus defining the associated metamorphic zone. isolated system 7 System whose boundaries permit the exchange of neither materials nor energy with its surroundings; see closed system, open system. ISOSTASY See Illustrated Key Word Glossary. isotherm 164 Line of equal temperature on a graph or map. ISOTOPES See Illustrated Key Word Glossary. jade 287 Gem-quality jadeite or nephrite. jetty 356 Wall that extends seaward from the shoreline, at a high angle. JOINT See Illustrated Key Word Glossary. jovian planets 13 The four outer planets of our solar system, which are all massive, gaseous, and icy; Jupiter and Jupiter-like planets Saturn, Uranus and Neptune; see terrestrial planets. karst topography 332 Topographic landforms, such as sinkholes, resulting from the dissolution of bedrock, usually carbonate. kerogen 464 Waxy, organic insoluble material, found in sediments and sedimentary rocks, especially shale. kerosene 460 A liquid produced by the distillation of petroleum, which can be used as a fuel oil. kettle 400 Basin within glacial drift, created by the melting of a large chunk of buried glacial ice. kettle lake 400 Lake that forms in a kettle. kettle pond 400 Small kettle lake. kimberlite pipe 138 Explosive volcanic vent that originates at 100-200 km depth in the mantle. KINGDOM See Illustrated Key Word Glossary. Köppen-Geiger climate 369 A categorization of Earth’s climate zones based mainly on system temperature and precipitation, with six basic climate zones: tropical, dry, temperate-humid, cold-humid, polar, and highland. Kuiper Belt objects 13 Small, icy objects that originate in the Kuiper Belt, a region at the outer edge of the solar system; Pluto is now considered to be a large Kuiper Belt object. laccolith 174 Conformable intrusive body in which the overlying rock has bulged upward, forming a mushroom-shaped sill. lagoon 360 Elongate bay on the inshore side of a barrier island or barrier reef. lahar 161 Hot volcanic mudflow. lake 314 Standing body of water with a flat, open surface. laminar (flow) 198 Fluid flow characterized by the smooth movement of particles in straight, semi-parallel pathways; see turbulent. landslide 202 See slump, slide. lapilli 216 Intermediate-sized tephra. latent heat 348 Heat that is absorbed or released by a substance during a phase change; for example, heat energy is absorbed when water evaporates, and released when water freezes. lateral moraine 397 Ridge or pile of debris that has been transported by a glacier and deposited along its edges. LAVA See Illustrated Key Word Glossary. left-lateral (fault motion) 258 Movement on a strike-slip fault in which the block on the opposite side of the fault from the observer moves toward the left; also called sinistral motion. lignite 459 The lowest rank of coal. limbs (of a fold) 260 See flanks. LIMESTONE See Illustrated Key Word Glossary. liquefaction 127 Process whereby a solid (such as soil) behaves like a liquid. LITHIFICATION See Illustrated Key Word Glossary. LITHOSPHERE See Illustrated Key Word Glossary. LOAD See Illustrated Key Word Glossary. loess 228 Fine-grained, yellow-brown, wind-deposited (eolian) sediment. logarithmic 132 Growth pattern that follows a geometric rather than arithmetic rule (e.g., population growth); growth by incremental addition of a certain proportion of the existing amount, rather than a fixed amount; similarly, a decay pattern that follows a geometric rule, decreasing incrementally by proportion rather than by an absolute amount (e.g., radioactive decay). long-term forecasting 123 Methods aimed at predicting the occurrence of a natural hazard years or even decades in advance. longitudinal dune 382 Parallel dune ridges that form in deserts with a meager sand supply and/or bi-directional winds. LONGSHORE See Illustrated Key Word Glossary. CURRENT LOW-GRADE See Illustrated Key Word Glossary. low relief 265 Describes an area with little difference between the highest and lowest elevations; flat topography. lowland beach 357 A beach that forms along a flat, low-lying coast; see barrier island coast. LUSTER See Illustrated Key Word Glossary. mafic 169 Describes an igneous rock that contains abundant dark- colored minerals rich in iron and magnesium; usually with less than 50% SiO2 by weight; see felsic, intermediate. MAGMA See Illustrated Key Word Glossary. magma chamber 163 Reservoir of magma underground. magmatic arc 163 Line of volcanoes located inland from and parallel to a continent-ocean collision zone, where an oceanic plate is subducting under a continental plate, undergoing partial melting at depth, and generating volcanism. magmatic mineral 476 Mineral deposit in which minerals have been concentrated as deposit a result of magmatic processes such as fractional crystallization or crystal settling. MAGNETIC See Illustrated Key Word Glossary. REVERSAL magnetism 137 A fundamental physical force, created by permanent ferromagnets or moving electric charges. magnitude 132 Size; the magnitudes of geologic events such as earthquakes and cyclones, are usually quantified by measuring the energy released during the event. MANTLE See Illustrated Key Word Glossary. MARBLE See Illustrated Key Word Glossary. marine terrace 357 Wave-cut platform that has been raised above sea level, either by tectonic forces uplifting the land or by global (eustatic) sea level decline. mass 139 The amount of matter in an object. mass balance 394 Describes a situation in which accumulation in one part of a system or reservoir is balanced by loss in another part of the reservoir, thus maintaining a constant total mass. MASS EXTINCTION See Illustrated Key Word Glossary. mass number 34 The number of protons plus the number of neutrons in the nucleus of an atom. MASS WASTING See Illustrated Key Word Glossary. materials (Earth) 4 Part of the focus of physical geology; examples of Earth materials include minerals, rocks, soils, air, and water. matrix 169, Fine-grained material that surrounds and encloses coarser 222 grains in a rock. maturation 462 The series of complex physical and chemical reactions in sedimentary rocks that lead, over time and with increased pressure and temperature, to the formation of liquid and gaseous hydrocarbons. meandering (channel) 308 A stream channel with many twists and bends, marked by cut banks on one side and point bars on the other. mechanical deformation 289 Grinding, crushing, bending, and fracturing of rocks; an important metamorphic process, usually accompanied by some chemical recrystallization. MECHANICAL See Illustrated Key Word Glossary. WEATHERING medial moraine 400 Ridge of material deposited along the centerline where two glaciers have joined, trapping their lateral moraines between them. megathrust earthquake 118 Extremely powerful earthquake on the boundary between a subducting plate and the overriding plate. mélange 236 Chaotic mixture of volcaniclastic sediment, oceanic lithosphere, and rocks metamorphosed under the high- pressure, low-temperature conditions of a subduction zone, which accumulates along a subducting plate margin; see ophiolite, accretionary wedge. membrane 422 Porous and permeable “skin” that holds together the parts of a cell, and maintains an internal condition of homeostasis. mesa 381 Hill topped by a flat layer of resistant sedimentary rock; larger than a butte. mesosphere (1) 143 (1) The mantle from the bottom of the asthenosphere (about 350 km depth) to the core-mantle boundary. mesosphere (2) 363 (2) Thermal layer of the atmosphere above the stratosphere, from about 50 to about 80 km in altitude, in which temperature decreases with altitude. Mesozoic 69 The middle of the three eras that comprise the Phanerozoic Eon; see Cenozoic, Paleozoic. metabolism 422 Mechanisms by which a living organism extracts energy and material sustenance from its environment. metallic bonding 37 Type of bonding in which electrons are shared among several tightly packed atoms. metallic (luster) 41 Mineral luster that resembles that of a polished metal surface. metallogenic province 478 Regions of the crust in which mineral deposits are unusually concentrated. METAMORPHIC See Illustrated Key Word Glossary. FACIES metamorphic mineral 476 Mineral deposit in which minerals have become concentrated deposit by metamorphic or metasomatic processes. METAMORPHIC See Illustrated Key Word Glossary. ROCKS metamorphic zone 294 Rocks of roughly equivalent metamorphic grade, lying between two isograds in a regionally metamorphosed terrain. METAMORPHISM See Illustrated Key Word Glossary. METASOMATISM See Illustrated Key Word Glossary. METEORITE See Illustrated Key Word Glossary. meteoroid 12 Fragment of rock or dust in orbit around the Sun. mica 281 Family of minerals in which silicate anions polymerize as flat sheets, leading to a platy mineral habit. microfracture 256 Extremely tiny fracture in a rock or an individual mineral grain, which may only be visible by microscope. midocean ridge 104 Long, continuous fissure that runs along the centers of Earth’s major oceans, marking locations where oceanic crust is splitting apart and seafloor spreading is occurring. migmatite 281 A composite rock, part igneous and part metamorphic, which has undergone partial melting and thus contains pockets or veins of solidified magma. Milankovitch cycles 404 Variations in the eccentricity, tilt, and precession of Earth’s axis on different time scales, causing variations in the amount of solar energy that reaches Earth’s surface, the combined effects of which correspond closely to 20,000-, 40,000-, and 100,000-year glacial-interglacial cycles. MINERAL See Illustrated Key Word Glossary. mineral assemblage 51 The types and relative proportions of minerals that constitute a rock. mineralization 430 Process in which organic materials (such as bones, wood, teeth) are gradually replaced by inorganic minerals. mineralization (2) (2) Deposition of economically important minerals by hydrothermal fluids. mineraloid 39 An amorphous solid; an inorganic solid that is not a true mineral because it lacks an orderly internal crystal structure. mineral resource 457 An occurrence, concentration, or deposit of useful or potentially valuable minerals, which is (at least theoretically) extractable; see ore. moai 454 Large statues carved from volcanic rock by natives of Rapa Nui (Easter Island). modified Mercalli 132 Scale (from I = weakest to XII = strongest) for quantifying the intensity scale magnitude of an earthquake based on felt and observed ground shaking and damage. Mohorovičić 141 Seismic discontinuity that marks the boundary between the discontinuity (“Moho”) crust and the mantle, within the lithosphere. Mohs relative hardness 43 Scale used to determine the hardness of a mineral by scale comparison to a series of samples of known hardness, from 1 = softest (talc) to 10 = hardest (diamond). moist environment for 432 A requirement for all sexually reproducing organisms that live the reproductive system in a land-based environment; see internal aquatic environment, exchanging gases, and structural support. mold 430 Imprint left in soft sediment by an animal or plant; see trace fossil. MOLECULE See Illustrated Key Word Glossary. MOMENT See Illustrated Key Word Glossary. MAGNITUDE monadnock 205 See inselberg. monocline 259 Simplest type of fold; a local steepening in otherwise uniformly dipping strata. monomineralic 288 Describes a rock that consists almost entirely of one type of mineral, such as quartzite (quartz), marble (calcite), or dunite (olivine). Moon 14 Earth’s natural satellite. moon 12 A natural satellite; a small object that is gravitationally locked in orbit around a planet. MORAINE See Illustrated Key Word Glossary. mouth (of a stream) 310 Downstream, where the water in a stream or river flows into another surface water body. mud crack 225 Sedimentary feature that indicates deposition in an intermittently wet environment. MUDSTONE See Illustrated Key Word Glossary. mummification 430 Mechanism of fossilization whereby the soft parts of the organism dry and harden before they have decompose. mutation 429 Accidental deletion or transposition or substitution of one amino acid for another in the heritable traits encoded in the genes on an organism’s DNA. naked-seed plant 433 See gymnosperm. nappe 267 Very large, usually recumbent, anticlinal fold. NATURAL GAS See Illustrated Key Word Glossary. natural levee 311 Ridge-like deposit of coarse sediment that builds up along the banks of a stream channel; deposited when the stream flows out of its channel and onto the floodplain, and drops the coarsest part of its load near the banks. NATURAL See Illustrated Key Word Glossary. RESOURCES NATURAL See Illustrated Key Word Glossary. SELECTION naturally occurring solid 38 One of the requirements for a material to be classified as a mineral. nebula 13 Rotating cloud of interstellar gas and dust. nebular theory 13 The concept that our solar system coalesced from a swirling cloud of interstellar dust and gas; see nebula. neutron 34 Sub-atomic particle that is electrically neutral; part of the nucleus of an atom. nonconformity 66 Surface of erosion that separates younger sedimentary strata above from older igneous or metamorphic rocks below. NONRENEWABLE See Illustrated Key Word Glossary. RESOURCES NORMAL FAULT See Illustrated Key Word Glossary. normal polarity 79 Magnetic polarity (north-south directionality) that is the same as Earth’s current polarity; see reversed polarity. North Atlantic Deep 346 A cold, salty water mass in the northern part of the Atlantic Water (NADW) Ocean, which sinks because of its density, driving the Atlantic portion of the thermohaline circulation in the ocean’s deep zone; see Antarctic Bottom Water. nuclear energy 468 Energy derived from fission, that is, from splitting an atom. nucleus (pl. nuclei) 34 The core of an atom, consisting of protons and neutrons. NUMERICAL AGE See Illustrated Key Word Glossary. OCEANIC CRUST See Illustrated Key Word Glossary. oceanic trench 104 The deepest parts of the ocean, marking the locations of subduction zones. O horizon 195 Accumulation of organic matter that overlies the A horizon in many soil profiles. OIL See Illustrated Key Word Glossary. OIL SHALE See Illustrated Key Word Glossary. open system 9 System whose boundaries permit the exchange of materials and energy with its surroundings; see closed system, isolated system. ophiolite 237 Fragments of oceanic lithosphere, scraped off a subducting plate and accreted onto a continental margin, typically consisting of peridotite overlain by gabbro dikes, pillowed oceanic basalt, and seafloor sediment; see accretionary wedge, mélange. ORE See Illustrated Key Word Glossary. organelle 425 The small internal structures of a cell. OROGEN See Illustrated Key Word Glossary. outcrop 263 Bedrock exposed at the surface. outer core 143 The outermost, liquid portion of Earth’s Fe-Ni core. overland flow 308 Water that flows downslope over the land surface; see sheet flow. overturned 255 Rock layer or fold limb that has been tilted so steeply that it has tipped past the vertical. oxbow lake 310 Lake that forms when a meander is cut off at a narrow part of its loop by a new channel, leaving a curved section of the former meander isolated from the main channel. oxidation 189 Chemical reaction involving the addition of oxygen or removal of hydrogen; the reverse of reduction. oxide minerals 47 A group of minerals based on the simple oxide anion O2–. oxidized, oxidizing 227 Oxygen-rich; describes a chemical reaction involving the addition of oxygen or removal of hydrogen; the reverse of reduction. OZONE LAYER See Illustrated Key Word Glossary. pahoehoe 167 Runny, low-viscosity basaltic lava that solidifies into a smooth, ropy volcanic rock. paleoclimatology 404 Study of ancient climates. PALEOMAGNETISM See Illustrated Key Word Glossary. PALEONTOLOGY See Illustrated Key Word Glossary. PALEOSEISMOLOGY See Illustrated Key Word Glossary. paleosol 404 Ancient soil horizons. Paleozoic 69 Earliest of the three eras that comprise the Phanerozoic Eon; see Cenozoic, Mesozoic. Pangaea 92 Wegener’s name for a “supercontinent” that existed 300 million years ago, when the present-day continents were joined in one major landmass. parabolic dune 384 Dune that forms in coastal regions where there is sufficient moisture for some vegetation to grow; the arms of the dunes point upwind. parent atoms 75 The original atoms of a radioactive source material; see daughter atoms. partial melting 280 Process in which a rock begins to melt, but only a small amount of melt is generated before conditions change, the rock solidifies, or the melt coalesces and rises away from the host rock. patterned ground 401 Periglacial landform that results when ice wedges are so numerous that they become interconnected, their tops forming a hexagonal pattern on the surface. pause 364 A boundary between two thermal layers in the atmosphere, marked by a reversal in the temperature-altitude trend; see tropopause. pearly (luster) 41 Mineral luster that resembles that of a pearl. PEAT See Illustrated Key Word Glossary. pegmatite 169 Plutonic rock with unusually coarse mineral grains (larger than 2 cm). PERCOLATION See Illustrated Key Word Glossary. peridotite 142 Mafic plutonic rock that consists mainly of the minerals olivine and pyroxene, thought to be representative of the composition of the upper part of the mantle. periglacial 401 Near or around a glacier. period 72 The smaller sub-divisions of the three eras (Paleozoic, Mesozoic, and Cenozoic) of the Phanerozoic Eon. PERMAFROST See Illustrated Key Word Glossary. PERMEABILITY See Illustrated Key Word Glossary. petrified wood 430 Wood that has undergone mineralization, that is, in which the organic matter has been replaced by minerals carried in solution by groundwater. PETROLEUM See Illustrated Key Word Glossary. petroleum trap 462 A geologic situation with a source rock, a reservoir rock, and a cap rock. phaneritic 168 Describes an igneous rock (usually plutonic) in which the mineral grains are large enough to be seen without magnification. Phanerozoic 69 The fourth and current eon of geologic history; see Hadean, Archean, Proterozoic. phenocryst phosphate minerals 47 A group of minerals based on the complex (PO4)3–anion. photodissociation 420 Splitting or breakdown of a molecule as a result of interaction with sunlight. PHOTOSYNTHESIS See Illustrated Key Word Glossary. photovoltaic 467 Device that turns solar energy into electricity; a solar cell. phreatic (saturated) 326 See zone of saturation. zone PHYLLITE See Illustrated Key Word Glossary. physical geology 4 The part of geology that is concerned with processes at or beneath the surface of Earth, and the materials on which those processes operate. piedmont glacier 391 Glacier that flows out of the mountains and spreads out onto the surrounding lowlands. pillow basalt 237 Balloon-like volcanic structures that form when lavas are extruded under water; see ophiolite. placer deposit 478 Mineral deposit in which heavy minerals become concentrated as a result of the winnowing action of waves, currents, or winds. planet 12 One of the eight large objects in orbit around the Sun (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune). planetary accretion 14 A 20th-century supplement to the nebular theory, which says that the planets formed as a result of collisions of condensed rocky, metallic, and icy debris 4.56 billion years ago, shortly after the formation of the Sun. plastic deformation 248 See ductile deformation. plate 104 An enormous fragment of lithosphere; see plate tectonics. PLATE TECTONICS See Illustrated Key Word Glossary. playa 228 Dry or seasonally dry lake bed. Plinian (eruption) 153 The most explosive type of volcanic eruption, typically involving the ejection of enormous ash columns 20 km or more into the stratosphere, and deadly pyroclastic flows; typical of stratovolcanoes. plume 109 Long, thin blob or column of hot rock in the mantle, which is buoyant and rises toward the surface. plunge 260 Degree of dip (measured from the horizontal) of a line, such as a fold axis. plunging (fold) 260 Fold in which the axis is not horizontal. PLUTON See Illustrated Key Word Glossary. PLUTONIC ROCK See Illustrated Key Word Glossary. point bar 310 Linear ridge of sediment deposited along the banks of a stream channel, on the inner curve of a meander; see bar. polar cells 368 High-latitude atmospheric circulation system in which cold air descends and flows away from the poles and is deflected toward the west by the Coriolis effect, creating the polar easterlies. polar desert 379 Polar regions that receive very little annual precipitation, and are therefore technically deserts even though they are cold and snow- or ice-covered. polar easterlies 368 Winds associated with the returning (cold) limb of the polar cells, deflected by the Coriolis effect such that they flow toward the west along the polar fronts. polar front 368 Boundary between polar cells and Ferrel cells. polar glacier 389 High-latitude glaciers where the mean annual temperature is below freezing, and little or no seasonal melting occurs; see temperate glacier. polarity 78 North-south directionality of a magnetic field. polymerization 49 Chemical process whereby simple structural units bond together to form larger, more complex units (polymers). polymerize 422 Undergo (or cause to undergo) polymerization. polymorphs 49 Two (or more) minerals with the same chemical formula but different crystal structures. polyp 361 Individual tiny coral animals, which live in colonies in the ocean. pongidae 438 The family of apes. poorly-sorted 215 Sediment in which there is a wide range of particle sizes; see (sediment) sorting. pores (or pore spaces) 185 Small spaces between mineral grains in a rock or sediment. pore fluid 280 Fluid in the pore spaces of rocks; commonly aqueous. POROSITY See Illustrated Key Word Glossary. porphyritic 168 Rock texture with large mineral grains (phenocrysts) in an aphanitic or glassy matrix. P (primary) wave 129 Compressional wave that travels quickly through the subsurface; the first seismic body wave to arrive at a seismic station following an earthquake; see S (secondary) wave. Precambrian 72 The first three eons of geologic history: Hadean, Archean, and Proterozoic Eons; geologic time prior to the onset of the Phanerozoic Eon, Paleozoic era, and Cambrian Period, 542.0 million years ago. precession 404 “Wobbling” of Earth’s rotational axis; see Milankovitch cycles. PRECIPITATION See Illustrated Key Word Glossary. precursor phenomena 123 Anomalous activity or events leading up to a major or catastrophic geologic event; for example, swarms of foreshocks may be observed as precursors to a major earthquake; can be used to aid in short-term prediction of hazards. PRESSURE See Illustrated Key Word Glossary. primary atmosphere 419 Earth’s original (or primordial) atmosphere; the gaseous envelope that surrounded early Earth and was stripped away by the solar wind, meteorite impacts, or both; see secondary atmosphere. primary hazard (or 123 Direct effect of a hazardous process or event, causing a threat effect of a hazard) to human interests; for example, ground motion is a primary hazard associated with earthquakes; see geologic hazard, secondary hazard. primitive 140 Describes meteorites that have been unaffected by melting or other geologic processes since the origin of the solar system. principle of cross- 62 One of the four key principles of stratigraphy; a stratum must cutting relationships always be older than any feature that cuts or disrupts it; by extension, a xenolith or inclusion within another rock must pre-date the rock that surrounds it. principle of floral and 68 New species of plants (flora) and animals (fauna) succeed faunal succession earlier species as they evolve, leading to stratigraphic ordering of fossil assemblages. principle of lateral 65 One of the four key principles of stratigraphy; sediments are continuity deposited in horizontally continuous layers. principle of original 62 One of the four key principles of stratigraphy; water-laid horizontality sediments are deposited in horizontal strata. principle of stratigraphic 62 One of the four key principles of stratigraphy; in any superposition undisturbed sequence of strata, each stratum is younger than the stratum below it and older than the stratum above it. processes (Earth) 4 Part of the focus of physical geology; Earth processes include volcanism, plate tectonics, flooding, and erosion. prograde ? Metamorphic changes that occur in a rock as temperature (metamorphism) and/or pressure are increasing toward a peak condition; see retrograde metamorphism. PROKARYOTES See Illustrated Key Word Glossary. Proterozoic 69 The third eon of geologic history, and one of the three eons that comprise Precambrian time; see Hadean, Archean, Phanerozoic. proton 34 Sub-atomic particle with a positive electric charge; part of the nucleus of an atom. pumice 168 Glassy volcanic rock with many vesicles; see vesicular basalt. punctuated equilibrium 429 The idea that species can persist largely unchanged for a very long time, with occasional periods of very rapid evolutionary change; see gradualism. P-wave shadow zone 136 A “shadow” on the side of Earth opposite to an earthquake, caused by the liquid outer core refracting P waves, thus slowing their transmission. pyroclast 156 Fragment of rock ejected during a volcanic eruption; see tephra. PYROCLASTIC FLOW See Illustrated Key Word Glossary. QUARTZITE See Illustrated Key Word Glossary. radioactive 75 Describes an isotope that spontaneously undergoes a change involving the release of energetic particles from its nucleus. radioactive decay 75, The spontaneous process of decay, whereby an isotope 468 releases energetic particles from its nucleus. RADIOACTIVITY See Illustrated Key Word Glossary. RADIOMETRIC See Illustrated Key Word Glossary. DATING rainshadow desert 379 Desert that forms where a mountain range blocks moist air flowing inland from the ocean, removing moisture from the air and causing a zone of low precipitation on the downwind side. rank (of coal) 459 Grade of coal; the higher the carbon content, the greater the rank; the lowest rank is lignite, followed by bituminous coal and anthracite, the highest. rate of cooling 168 Speed at which a magma or lava cools; one of the factors that controls the grain sizes in the resulting igneous rock. recessional moraine 397 A terminal moraine that forms when a glacier melts and recedes; an end moraine. RECHARGE See Illustrated Key Word Glossary. RECRYSTALLIZATION See Illustrated Key Word Glossary. recumbent (fold) 262 Fold that is so strongly overturned that the limbs are almost lying flat, with one limb essentially upside down. recurrence interval 320 Average time interval between two events (such as floods) of the same magnitude. reduced, reducing, 227 Oxygen-poor; describes a chemical reaction involving the reduction removal of oxygen or addition of hydrogen; the reverse of oxidation. REEF See Illustrated Key Word Glossary. REFLECTION See Illustrated Key Word Glossary. REFRACTION See Illustrated Key Word Glossary. REGIONAL See Illustrated Key Word Glossary. METAMORPHISM REGOLITH See Illustrated Key Word Glossary. RELATIVE AGE See Illustrated Key Word Glossary. relative motion 258 Movement of one lihtospheric plate in comparison to an adjacent plate, which may also be in motion. relief 265 Topographic variations; the shape of the land, including differences in elevation. remote sensing 134 Collecting information about an object from a distance. RENEWABLE See Illustrated Key Word Glossary. RESOURCE replication 422 Process by which a living organism creates a more-or-less accurate copy of itself. reservoir (1) 316 A lake that is deliberately created by damming its outlet. reservoir (2) 307 A place in a system or cycle where material “resides” or is stored for a period of time, its residence time. reservoir rock 462 Porous rock unit in which oil and gas accumulate; see cap rock, source rock. residence time 307 Average length of time spent by a unit of material in a particular reservoir. residual deposit 478 Mineral deposit in which minerals are concentrated as a result of weathering-related processes. resinous (luster) 41 Mineral luster that resembles that of dried tree gum or resin. resurgent dome 157 Dome-shaped or cone-shaped mass of magma that forms inside a caldera as a result of renewed volcanic activity. retrograde (1) (1) Reverse, or backwards; refers to metamorphic changes that occur in a rock as temperature and/or pressure are decreasing from a peak condition; see prograde metamorphism. retrograde 15 Reverse, or backwards; refers to the rotation of planetary objects (such as Venus) that rotate east-to-west, that is, opposite to the sense of rotation of most objects in the solar system. REVERSE FAULT See Illustrated Key Word Glossary. reversed polarity 79 Magnetic polarity (north-south directionality) that is the opposite to Earth’s current polarity; see normal polarity. rhyolite 170 High-silica, felsic volcanic rock with abundant quartz and feldspar; volcanic equivalent of granite. RICHTER See Illustrated Key Word Glossary. MAGNITUDE SCALE rift 104 Where a continental or oceanic plate is thinning and splitting apart; spreading center or divergent margin. RIFT VALLEY See Illustrated Key Word Glossary. right-lateral (fault 258 Movement on a strike-slip fault in which the block on the motion) opposite side of the fault from the observer moves toward the right; also called dextral motion. rip current 355 Strong, sheet-like current along the bottom of the ocean in nearshore areas, by which much of the water of waves breaking onshore is returned seaward. ripple mark 225 Sedimentary feature that indicates deposition in a near-shore, shallow-water environment. river 308 Stream of significant volume. river flow 345 Stream flow. RNA 422 See Illustrated Key Word Glossary. ROCK See Illustrated Key Word Glossary. ROCK CYCLE See Illustrated Key Word Glossary. rock-forming minerals 47 The common minerals, of which there are about 30, which comprise the bulk of Earth’s crust – about 99 percent by volume. rocky (cliffed) coast 357 Steep, rocky coastline, usually characterized by wave-cut cliffs and wave-cut benches. rotational 202 Describes downslope movement along a curved (rather than flat) slip face. roundness 215 Characteristic of a sediment particle that is smooth; see angularity. ruby 44 Gem-quality red corundum. runoff 308 See surface runoff. run-up (height) 352 Highest elevation reached by an incoming tide; also used to quantify tsunami and storm surges. rupture 118 Break suddenly. saline 307 Salty. SALINITY See Illustrated Key Word Glossary. SALTATION See Illustrated Key Word Glossary. SAND See Illustrated Key Word Glossary. SANDSTONE See Illustrated Key Word Glossary. sapphire 44 Gem-quality blue corundum. saturated zone 325 See zone of saturation. scarp 256 Cliff-like landform resulting from vertical motion along a fault that breaks the ground surface. SCHIST See Illustrated Key Word Glossary. SCHISTOSITY See Illustrated Key Word Glossary. seafloor hydrothermal 423 Opening or fissure where superheated water emerges at the vent sea floor; see “black smoker”. SEAFLOOR See Illustrated Key Word Glossary. SPREADING sea ice 392 Ice that forms by direct freezing of sea water. seam 460 Layer or stratum of coal. seamount 110 Raised topographic feature that marks the location of an underwater volcano; most seamounts are the extinct remnants of hot-spot volcanic islands. sea wall 356 Wall constructed parallel to a coastline; a sea wall that is a short distance offshore is called a breakwater. secondary atmosphere 420 The atmosphere that evolved by volcanic degassing, after Earth’s primary atmosphere was stripped away by the solar wind early in Earth history. secondary hazard (or 123 Hazardous event or process that is a threat to human effect of a hazard) interests, itself triggered by a damaging or catastrophic event; for example, a landslide can be triggered by the ground shaking associated with an earthquake; see primary hazard, geologic hazard. sediment 51 Rock and mineral matter that has been fragmented, transported, and deposited. sedimentary deposit 475 Mineral deposit, such as an evaporite deposit, in which the minerals become concentrated as a result of sedimentary or near-surface processes. sedimentary facies 232 A set of sedimentary features that reflects the characteristics of the depositional environment. SEDIMENTARY See Illustrated Key Word Glossary. ROCKS seed 434 An evolutionary feature of vascular plants, in which the male and female reproductive bodies are contained in a protective coating that includes moisture and food for the plant embryo. seedless plants 433 Plants, such as mosses and ferns, that reproduce via spores rather than seeds; the first plants were seedless. SEISMIC See Illustrated Key Word Glossary. DISCONTINUITY seismic gap 125 Fault segment that has been seismically inactive for a long time, even though tectonic movement and stress along the fault are still occurring and strain energy is building up; used in the long-term forecasting of earthquakes. seismic tomography 136 Technology that allows seismologists to superimpose a sequence of two-dimensional seismic “snapshots” to build a three-dimensional image of Earth’s interior; see computer- assisted tomography. SEISMIC WAVE See Illustrated Key Word Glossary. SEISMOGRAM See Illustrated Key Word Glossary. SEISMOGRAPH See Illustrated Key Word Glossary. SEISMOLOGY See Illustrated Key Word Glossary. seismometer 128 See seismograph. semiarid 379 Land in which the annual precipitation ranges between 250 and 500 mm. shadow zone 135 See S-wave shadow zone, P-wave shadow zone. SHALE See Illustrated Key Word Glossary. shatter cone 153 Cone-shaped pile of loose fragmental volcanic debris, typical of Strombolian eruptions; see cinder cone. SHEAR See Illustrated Key Word Glossary. SHEAR WAVE See Illustrated Key Word Glossary. sheet flow 308 Water that flows downslope over the land surface in thin, broad sheets; see overland flow. sheet jointing 186 See exfoliation. shelf break 93 Abrupt change in slope that marks the transition from continental shelf to continental slope. shield (continental) Craton. SHIELD VOLCANO See Illustrated Key Word Glossary. shock metamorphism 292 Metamorphism that occurs in conditions of extremely high but short-lived pressure, such as during a meteorite impact. short-term prediction 123 Methods aimed at predicting the time, magnitude, and location of occurrence of a natural hazard, such as an earthquake, to provide an opportunity for authorities to issue an early warning. silica (complex anion) 47 A strongly bonded anionic complex of silica + oxygen; (SiO4)4– the basic structural unit of silicate minerals. silicate minerals 47 A group of minerals with a structure based on the complex silica (SO4)2– anion; the most common family of rock-forming minerals. siliceous ooze 231 Biogenic marine sediment that consists largely of the remains of microscopic silica-secreting marine organisms; see calcareous ooze. sill 174 Tabular, parallel-sided sheet of intrusive rock that is parallel to the layering of the intruded rock. silt 194 Sediment in which the individual grains are finer than sand, on average. siltstone 214, Rock that results from the lithification of silt. 222 sinkhole 332 Dissolution cavity, like a cave, but with a wider opening to the sky. SLATE See Illustrated Key Word Glossary. SLATY CLEAVAGE See Illustrated Key Word Glossary. slide 202 Rapid displacement of a mass of rock or sediment, straight down a steep or slippery slope. slip face (1) 202 (1) Surface along which a slump, slide, or other slope failure occurs. slip face (2) 383 (2) Steep leeward face of a dune. SLOPE FAILURE See Illustrated Key Word Glossary. slump 202 Rapid rotational movement of a mass of rock or sediment downslope along a curved slip face. slurry flow 202 Flow in which the regolith is water-saturated; see granular flow. SOIL See Illustrated Key Word Glossary. SOIL HORIZON See Illustrated Key Word Glossary. soil moisture 325 Pore water in the zone of aeration. SOIL PROFILE See Illustrated Key Word Glossary. solar energy 466 Energy from the Sun that reaches Earth’s surface. solar system 12 The Sun and planets, moons, asteroids, meteoroids, Kuiper Belt objects, and other natural objects in orbit around the Sun. solifluction 202 A relatively slow slurry (water-saturated) flow, common where soil cover is thin over bedrock or permafrost. sorting 215 The degree to which a collection of sedimentary particles has been winnowed and separated by size; see well-sorted, poorly-sorted. source (of a stream) 308 Headwaters; where small tributaries come together to form a stream. source rock 462 Rock unit in which organic material has been converted into oil and natural gas; see reservoir rock, cap rock. spatter cone 153 Cinder cone. SPECIES See Illustrated Key Word Glossary. specific chemical 38 A specific, constrained (rather than variable) chemical composition composition; whether simple or complex, one of the requirements for a substance to be classified as a mineral. specific gravity 45 The weight of a mineral in air (WA) compared to the weight of the mineral in water (WW); specifically, the ratio WA/(WW +WA); see density. spelunker 334 Caver, or cave explorer. sphericity 215 Characteristic of a sediment particle that is approximately the same length and width; see angularity, roundness. spit 360 Elongate ridge of sand or gravel that projects from land into the open water of an embayment along the coast. spore 433 Reproductive bodies of seedless plants. spreading center 104 Where a continental or oceanic plate is thinning and splitting apart; a rifting or divergent margin. SPRING See Illustrated Key Word Glossary. S (secondary) wave 129 Shear body wave that travels relatively slowly through the subsurface, arriving at a seismic station after the P waves generated by the same earthquake; see P (primary) wave. stalacite 333 Deposits that hang from the ceiling of a cave, consisting of calcium carbonate and other dissolved minerals that precipitated from groundwater as it dripped from the ceiling. stalagmite 333 Deposits that project upward from the floor of a cave, consisting of calcium carbonate and other dissolved minerals that precipitated from groundwater as it dripped to the floor. star dune 384 Stationary, star-shaped dune that forms in arid areas where the wind regularly blows in several different directions. steady state Homeostasis. Steno’s Law 41 The rule that the angle between any corresponding pairs of crystal faces of a given mineral species is constant no matter what the overall shape of size of the crystal might be. stock 173 Pluton that is smaller than a batholith (less than 10 km across). stomata 432 Adjustable openings in the leaves of plants, through which gases can be exchanged with the surrounding environment. stony-iron meteorite 140 Meteorite that consists of a (typically) heterogeneous mixture of metallic iron and nickel with rock; see stony meteorite, iron meteorite. stony meteorite 140 Meteorite that consists almost entirely of rock, and shows signs of having undergone melting, differentiation, and other geologic processes; see iron meteorite, stony-iron meteorite. storm runoff 318 During a flood, the “extra” water (above normal base flow) in the channel, contributed mainly by precipitation. storm surge 319 An inflow of water from the ocean (or a large lake) during a storm, leading to coastal flooding. straight (channel) 308 A (typically) short stretch of a stream channel that almost forms a straight line; most common near a stream’s headwaters, where the gradient is high. STRAIN See Illustrated Key Word Glossary. strain rate 250 The speed at which deformation of a material occurs, which can influence deformational behavior. strata (sing. stratum) 62 Depositional layers or units of sedimentary rock. stratigraphic time scale 69 Geologic Column; a diagram showing the succession of all know strata, fitted together in chronological order. STRATIGRAPHY See Illustrated Key Word Glossary. stratosphere 365 Thermal layer of the atmosphere from about 10-15 km to about 50 km in altitude, in which temperature increases with altitude; see troposphere. STRATOVOLCANO See Illustrated Key Word Glossary. STREAK See Illustrated Key Word Glossary. streak plate 45 Unglazed fragment of porcelain, used to test the streak of a mineral sample. STREAM See Illustrated Key Word Glossary. streamflow 308 Water flow that is concentrated in well-defined channels. STRESS See Illustrated Key Word Glossary. STRIKE See Illustrated Key Word Glossary. STRIKE-SLIP FAULT See Illustrated Key Word Glossary. stromatolite 424 Mound-like structures consisting mainly of layered calcium carbonate, built by colonies of cyanobacteria. Strombolian (eruption) 153 Volcanic eruption that is more explosive than a Hawaiian eruption, typically involving the ejection of incandescent showers of lava or rock and the formation of shatter cones or cinder cones. STRUCTURAL See Illustrated Key Word Glossary. GEOLOGY structural support 432 A requirement for all land-based organisms; see exchanging gases, internal aquatic environment, and moist environment for the reproductive system. subduction 107 Process whereby an oceanic plate plunges into the asthenosphere, sliding beneath another oceanic or continental plate along a convergent plate boundary. SUBDUCTION ZONE See Illustrated Key Word Glossary. subsidence 320 A drop in ground surface elevation. subsoil 195 Soil horizon that lies below the B horizon in a soil profile; also called the C horizon, below which lies unweathered parent rock. subtropical (desert) 379 The world’s most extensive deserts, associated with the two belts of low rainfall near the 30° N and S latitudes; includes the Sahara, Kalahari, and Great Australian Deserts. sulfate minerals 47 A group of minerals based on the complex (SO4)2– anion. sulfide minerals 47 A group of minerals based on the simple S2– anion. SURF See Illustrated Key Word Glossary. SURFACE CREEP See Illustrated Key Word Glossary. surface current 346 Shallow ocean currents (like the Gulf Stream) that are set in (oceanic) motion by the prevailing winds. surface layer (or zone) 345 In oceans and lakes, the layer of water at the surface that consists of relatively warm, low-density water; depth to about 100 m, in the oceans; see deep zone, thermocline. SURFACE RUNOFF See Illustrated Key Word Glossary. SURFACE WAVE See Illustrated Key Word Glossary. surf zone 355 Narrow zone of turbulent water where waves break onshore, between the line of breakers and the shore itself. surge See glacial surge; storm surge. SUSPENDED LOAD See Illustrated Key Word Glossary. SUSPENSION See Illustrated Key Word Glossary. sustainable farming 197 A farming movement that incorporates care for soils and erosion-prevention strategies, in addition to minimizing the use of agro-chemicals. swash 356 Uprushing water of waves on a beach; (the opposite is “backwash”). S-wave shadow zone 136 A “shadow” on the side of Earth that is opposite to an earthquake, caused by the liquid outer core blocking the transmission of S waves. SYNCLINE See Illustrated Key Word Glossary. SYSTEM See Illustrated Key Word Glossary. tar 464 Oil that is semisolid – too viscous to be readily pumped; also called bitumen. TAR SAND See Illustrated Key Word Glossary. TECTONIC CYCLE See Illustrated Key Word Glossary. temperate glacier 389 Low- or mid-latitude glacier in which the ice is near its melting temperature, and ice and meltwater co-exist; see polar glacier. temperature 165 A measure of the heat content of a material. TENSION See Illustrated Key Word Glossary. tensional (or 256 See tension. extensional) stress tephra 156 Pyroclastic ejecta from a volcano, ranging in size from very fine (ash) to car-sized (bombs). terminal moraine 397 Ridge or pile of debris that has been transported by a glacier and dropped at its terminus. terminus 397 The end, or farthest extent, of a glacier. terrestrial planets 13 The four innermost planets of our solar system, which are all small, rocky, and metallic; Earth and the Earth-like planets Mercury, Venus, and Mars; see jovian planets. tertiary hazard (or effect 157 A long-lasting and/or indirect threat to human interests or of a hazard) permanent environmental change, caused by a damaging or catastrophic event; for example, a volcanic eruption can destroy vegetation and render an area uninhabitable; see primary hazard, secondary hazard, geologic hazard. texture (of a rock) 168 The appearance of a rock, as determined by the grain sizes, shapes, arrangement, and interrelationships of its constituent minerals. theory 92 A hypothesis that has been tested and is strongly supported by experimentation, observation, and scientific evidence. thermal spring 156 Hot spring; naturally heated groundwater that emerges at the surface in a spring. thermocline 345 Transitional zone between the surface layer and the deep zone in oceans and large lakes; zone in which temperature changes rapidly with depth. thermohaline circulation 348 Deep ocean “conveyor belt” global circulation, driven by differences in the temperature and salinity of the water; see North Atlantic Deep Water, Antarctic Bottom Water. thermosphere 364 The outermost thermal layer of the atmosphere, above about 80 km, in which temperature increases with altitude. thin section 172 A very thin slice of rock (0.03 mm thick), cut and polished for the purpose of examination under a microscope with transmitted light. THRUST FAULT See Illustrated Key Word Glossary. tidal bore 352 Where coastal topography constrains the incoming water of the high tide, causing it to rush into a long, narrow bay. tidal inlet 360 An opening between the ocean and an inland lagoon, that forms where erosion breaches a barrier island. tidal range 352 The difference in level between high tide and low tide in a particular location. TIDES See Illustrated Key Word Glossary. TILL See Illustrated Key Word Glossary. tilt 404 Departure from verticality (of Earth’s rotational axis, in this case); see Milankovitch cycles. tombolo 360 Spit-like ridge of sand or gravel that joins an island to the mainland. TOPOGRAPHIC MAP See Illustrated Key Word Glossary. topographic profile 264 Graph that shows the variations in elevation of the ground surface in a particular area, derived from the contour lines on a topographic map. topography 265 Relief; differences in elevation; the shape of the ground surface. topsoil 195 The A horizon in a soil profile; a dark-colored layer of mixed mineral and organic matter, within reach of plant roots. TRACE FOSSIL See Illustrated Key Word Glossary. trade wind 368 The returning limb of Hadley cells, deflected by the Coriolis effect into strong easterly winds, just north and south of the Equator. TRANSFORM FAULT See Illustrated Key Word Glossary. transitional zone 345 See thermocline. TRANSPIRATION See Illustrated Key Word Glossary. transverse dune 384 Dune that forms where the supply of sand is copious, causing barchan dunes to merge; oriented perpendicular to the prevailing winds. triangulation 130 Method for determining the location of an earthquake’s epicenter; each of three seismic stations determine their distance from the epicenter on the basis of the difference in arrival times of P and S waves, and plot this on a map as a circle of radius d = distance; where the three circles intersect is the epicenter. tributary stream 313 Small stream that combines with other tributaries to form a larger stream. tropopause 362 Thermal boundary between the troposphere and the stratosphere; in the troposphere, temperature decreases with altitude, whereas the trend is reversed in the stratosphere; see pause. troposphere 362 Lowest thermal layer of the atmosphere, generally below about 10-15 km in altitude, in which temperature decreases with altitude; see stratosphere. trough 353 Lowest point of a wave; see crest. tsunami 116 Long-wavelength, high-velocity water wave resulting from a sudden vertical displacement of the sea floor, generally caused by an earthquake, volcanic eruption, or underwater landslide; a seismic sea wave. tuff 156 Pyroclastic rock consisting of fine volcanic fragments (tephra) cemented together; see agglomerate. tundra 401 Treeless northern landscape with long winters, short summers, poorly developed soils, and low, scrubby vegetation; characterized by the presence of permafrost. turbidite 231 Graded layer of sediment deposited by a turbidity current. TURBIDITY CURRENT See Illustrated Key Word Glossary. turbulent (flow) 198 Fluid flow that is complex and energetic, characterized by many swirls and eddies; see laminar. unconfined aquifer 328 Aquifer that is not bounded by aquicludes, but is connected to the atmosphere via pore spaces in the overlying rock or sediment, and in which water is free to rise to its natural level; see confined aquifer. UNCONFORMITY See Illustrated Key Word Glossary. unconventional 465 A variety of hydrocarbon deposits, including tar sands and oil (hydrocarbons) shales, which are generally more difficult to extract than conventional petroleum. uniform stress 247 See confining stress. UNIFORMITARIANISM See Illustrated Key Word Glossary. unifying theory 19 A well-established theory that accounts for a wide variety of observations about natural physical processes; the theory of plate tectonics is a unifying theory in geology. upstream 308 Near or toward the source or headwaters of a stream, or the head of a glacier. U-shaped valley 398 Classic glacially-scoured mountain valley with a rounded base and steep walls. vadose (aerated) zone 325 See zone of aeration. valley glacier 390 Glacier that forms in a mountainous area, and flows down a stream channel or mountain valley. Van der Waals bonding 37 Weak attraction between electrically neutral molecules that have asymmetrical electric charge distributions; the positive end of one molecule is attracted to the negative end of another molecule. vascular plants 432 Plants that have structural support from stems and a set of vessels or tissues through which water and dissolved elements are transferred from the roots to the leaves. veins 280 Stringers that criss-cross a rock, consisting of minerals that were dissolved and have precipitated from fluids passing through the rock. ventifact 381 Bedrock surface or stone that has been abraded and shaped by wind-blown sand. vesicle 168 Holes that remain when a magma with abundant gas bubbles solidifies. vesicular basalt 168 Basaltic volcanic rock with many vesicles. VISCOSITY See Illustrated Key Word Glossary. vitreous (luster) 41 Mineral luster that resembles that of glass. volcanic arc 106 Island arc or magmatic arc. volcanic ash 156 See ash. volcanic glass 39 Naturally occurring amorphous solid that forms when magma ejected from a volcano cools and solidifies too quickly for a crystal structure to be developed. volcanic neck 174 Volcanic pipe that has been exposed by erosion. volcanic pipe 174 Remnant of a channel that originally fed magma to a volcanic vent. VOLCANIC ROCK See Illustrated Key Word Glossary. volcanic tremor 158 Seismic activity associated with volcanoes, caused by the movement of magma underground. volcanic vent 174 Opening through which lava and other volcanic materials are extruded. volcaniclastic 216 Sediment in which the clasts are of volcanic origin. sediment VOLCANO See Illustrated Key Word Glossary. Vulcanian (eruption) 153 Volcanic eruption that is more explosive than a Strombolian eruption, typically involving the ejection of large ash plumes up to 10 km in height, and the generation of hot pyroclastic flows; typical of stratovolcanoes. water cycle 10 See hydrologic cycle. WATER TABLE See Illustrated Key Word Glossary. water vapor 362 The gaseous phase of water (H2O). water-scarce 321 See Illustrated Key Word Glossary. watershed 7, 313 Drainage basin or catchment; can include the physical drainage basin as well as the ecosystem, flora, fauna, and even climate and human activities of a particular drainage basin. wave-cut bench 357 Platform cut into a coastline by wave erosion; see wave-cut cliff. WAVE-CUT CLIFF See Illustrated Key Word Glossary. wave refraction 355 Bending or change in direction of a line of waves as it approaches the shoreline at an angle; also applies to seismic waves or light waves; see refraction. weather 368 Short-term, local variations in wind, rain, snow, sunshine, and other atmospheric characteristics. WEATHERING See Illustrated Key Word Glossary. well sorted (sediment) 215 Sediment in which all of the particles are approximately the same size; see sorting. wetland 316 Bog; an area that has little or no free water surface, or has standing water on a seasonal basis. world ocean 342 The Pacific, Atlantic, Indian, and Southern Oceans, with a number of smaller bodies including the Arctic Ocean, the Mediterranean Sea, Hudson Bay, and the Persian Gulf; collectively, they cover 71% of Earth’s surface. xenolith 65, “Foreign rock”; a rock that is encased within another rock; see 137 inclusion. zone of aeration 325 Subsurface zone in which at least some of the pore spaces in the rock or sediment are filled with air, rather than water; see vadose zone. zone of saturation 325 Subsurface zone in which all of the pore spaces in the rock or sediment are filled with water, that is, the subsurface below the water table; see phreatic zone.