| Term | Definition |
|---|
| andesite | A volcanic rock type intermediate in composition between rhyolite and basalt. The extrusive equivalent of diorite. |
| basalt | A fine-grained, dark, mafic igneous rock composed largely of plagioclase feldspar and pyroxene. The extrusive equivalent of gabbro. |
| batholith | A great irregular mass of coarse-grained igneous rock that covers at least 100 square km; the largest pluton. |
| concordant intrusion | An intrusive igneous rock whose boundaries lie parallel to layers of preexisting bedded rock. |
| country rock | The rock surrounding an intrusive igneous rock. |
| dacite | A light-colored, fine-grained extrusive igneous rock with the same general composition as andesite. The extrusive equivalent of grandiorite. |
| decompression melting | Melting that occurs when mantle material rises to an area of lower pressure at a mid-ocean ridge. As the mantle material rises and the pressure decreases below a critical point, solid rocks melt spontaneously, without the introduction of any additional heat. |
| dike | A tabular igneous intrusion that cuts across layer of bedding in country rock. |
| diorite | A coarse-grained intrusive igneous rock with composition intermediate between granite and gabbro. The intrusive equivalent of andesite. |
| discordant intrusion | An intrusive igneous rock that cuts across the layers of the country rock that it intrudes. |
| felsic rock | A light-colored igneous rock that is poor in iron and magnesium and rich in high-silica minerals such as quartz, orthoclase feldspar, and plagioclase feldspar. |
| fluid-induced melting | Melting that takes place when water-laden sediments on a subducting oceanic plate are carried downward into the subduction zone. The increase in pressure squeezes water out of the minerals in the outer layers of the descending slab. The water rises buoyantly into the mantle wedge about the slab. Because water lowers the melting temperature of rock, it induces melting in the mantle wedge. |
| fractional crystallization | The process by which the crystals formed in a cooling magma are segregated from the remaining liquid at progressively lower temperatures. |
| gabbro | A dark gray, coarse-grained intrusive igneous rock with an abundance of mafic minerals, particularly pyroxene. The intrusive equivalent of basalt. |
| granite | A felsic, coarse-grained, intrusive igneous rock composed of quartz, orthoclase feldspar, sodium-rick plagioclase feldspar, and micas. The intrusive equivalent of rhyolite. |
| granodiorite | A light-colored, coarse-grained intrusive igneous rock similar to granite in having abudant quartz, but whose predominant feldspar is plagioclase, not orthoclase. The intrusive equivalent of dacite. |
| hydrothermal vein | A vein filled with minerals that contain large amouns of chemically bound water and are known to crystallize from hot-water sollutions. |
| intermediate igneous rock | An igneous rock midway in composition between mafic and felsic, neither as rich in silica as felsic rocks nor as poor in it as mafic rocks. |
| mafic rock | A dark-colored igneous rock containing minerals (such as pyroxenes and olivines) rich in iron and magnesium and relatively poor in silica. |
| magma chamber | A magma-filled cavity in the lithosphere that forms as ascending drops of melted rock push aside surrounding solid rock. |
| magmatic differentiation | A process by which rocks of varying composition can arise from a uniform parent magma. Various minerals crystallize at different temperatures, and the composition of the magma changes as it is depleted of the chemical elements withrdrawn to make the crystallized minerals. |
| obsidian | A dense, dark, glassy volcanic rock, usually of felsic compositon. |
| ophiolite suite | An unusual assemblage of rocks, characteristic of the seafloor but found on land, consisting of deep-sea sediments, submarine basaltic lavas, and mafic igneous intrusions. The assemblage comprises fragments of oceanic crust that were transported by seafloor spreading and then raised above sea level and thrust onto a continent in a later episode of plate collisions. |
| partial melting | The incomplete melting of a rock that occurs because the minerals that compose it melt at different temperatures. |
| pegmatite | A vein of extremely coarse grained granite, crystallized from a water-rich magma in the late stages of solidification, that cuts across a much finer grained country rock. They provide ores of many rare elements, such as lithium and beryllium. |
| peridotite | A coarse-grained ultramafic intrusive igneous rock composed of olivine with small amounts of pyroxene and amphibole. The dominant rock in Earth's mantle and the source rock of basaltic melts. |
| pluton | A large igneous intrusion ranging in size from a cubic km to hundreds of cubic kms, formed at depth in the crust. |
| porphyry | A lava of mixed texture in which large crystals (phenocrysts) "float" in a predominantly fine crystalline matrix. |
| pumice | A frothy mass of volcanic glass with a great number of holes (vesicles) that remain after trapped gas has escaped from the solidifying melt. |
| pyroclast | A volcanic rock fragment ejected into the air during an eruption. |
| rhyolite | A light-brown to gray, fine-grained extrusive igneous rock with a felsic composition. The extrusive equivalent of granite. |
| sill | A sheetlike concordant intrusion formed by the injection of magma between parallel layers of preexisting bedded rock. |
| stock | An irregular mass of coarse-grained igneous rock less than 100 square km in area. |
| tuff | Any volcanic rock lithified from pyroclasts. |
| ultramafic rock | An igneous rock consisting primarily of mafic materials and containing less than 10 percent fledspar. |
| volcanic ash | Extremely small fragments, usually of glass, that form when escaping gases force a fine spray of magma from a volcano. |
| arkose | A sandstone containing more than 25 percent feldspar. |
| bedding sequence | A pattern of interbedded and vertically stacked layers of sandstone, shale, and other sedimentary rock tyes. |
| bioclastic sediment | A shallow-water sediment consisting primarily of two calcium carbonate minerals - calcite and aragonite - in variable proportions. |
| biological sediment | A sediment that forms near its place of deposition as a result of mineral precipitation within organisms as they grow. |
| bioturbation | The process by which organisms rework existing sediments by burrowing through muds and sands. |
| carbonate environment | A marine setting where calcium carbonate, principally of biochemical origin, is the main sediment. |
| carbonate platform | An extensive flat, shallow area where both biological and nonbiological carbonates are deposited. |
| carbonate rock | A sedimentary rock formed from the accumulation of carbonate minerals precipitated organically or inorganically. |
| carbonate sediment | A sediment formed from the accumulation of carbonate minerals precipitated organically or inorganically. |
| cementation | A major chemical diagenetic change in which minerals are precipitated in the pores of sediments, forming cements that bind clastic sediments and rocks. |
| chemical and biological sedimentary environment | New chemical substances that form by precipitation when some of a rock's components dissolve during weathering and are carried in river waters to the sea. |
| chemical sediment | The dissolved product of weathering precipitated from water by chemical reactions and formed at or near its place of deposition. |
| chemical weathering | The weathering that occurswhen the minerals in a rock are chemically alterted or dissolved. |
| chert | A sedimentary rock made up of chemically or biochemically precipitated silica. |
| clastic particle | A physically transported rock fragment produced by the weathering of preexisting rocks. |
| clastic sediment | An accumulation of clastic particles laid down by running water, wind, or ice and forming layers of sand, silt, or gravel. |
| clay | The most abundant component of fine-grained sediments and sedimentary rocks, consisting largely of clay minerals. The particles are less than .0039 mm in diameter. |
| claystone | A rock made up exclusively of clay-sized particles. |
| coal | A biochemically produced sedimentary rock composed almost entirely of organic carbon formed by the diagenesis of swamp vegetation. |
| compaction | A decrease in the volume and porosity of a sediment that occurs when the grains are squeezed closer together by the weight of overlying sediment. |
| conglomerate | A sedimentary rock composed of pebbles, cobbles, and boulders. The lithified equivalent of gravel. |
| continental shelf | A broad, flat, sand- and mud-covered platform that is a slightly submerged part of a continent and extends to the edge of the continental slope. |
| cross-bedding | A sedimentary structure consisting of bedded material deposited by currents of wind or water and inclined at angles as large as 35 degrees from the horizontal. |
| diagenesis | The physical and chemical changes - including pressure, heat, and chemical reactions - by which buried sediments are lithified into sedimentary rocks. |
| dolostone | An abundant carbonate rock composed primarily of dolomite and formed by the diagenesis of carbonate sediments and limestones. |
| evaporite rock | A sedimentary rock formed from evaporite sediments. |
| evaporite sediment | An accumulation of materials precipitated inorganically from evaporating seawater and from water in arid-region lakes that have no river outlets. |
| flexural basin | A basin that develops within zones of tectonic convergence, where one lithospheric plate pushes up over the other and the weight of the overriding plate causes the overridden plate to bend or flex downward. |
| foraminifera | One of a group of tiny single-celled organisms that live in surface waters and whose secretions and calcite shells account for most of the ocean's carbonate sediments. |
| gas | A fluid organic sediment formed by the diagenesis of organic material in the pores of sedimentary rocks, mainly sanstones and limestones. |
| graded bedding | A bed that formed horizontal or nearly horizontal layers at the time of deposition, in which the coarsest particles are concentrated at the bottom and grade gradually upward into fine silt. |
| gravel | The coarsest clastic sediment, consisting of particles larger than 2 mm in diameter including pebbles, cobbles, and boulders. |
| graywacke | A sandstone composed of a heterogeneous mixture of rock fragments and angular grains of quartz and feldspar |
| iron formation | A sedimentary rock that usually contains more than 15 percent iron in the form of iron oxides and some iron silicates and iron carbonates. |
| limestone | A biochemical sedimentary rock lithified from carbonate sediments and composed mainly of calcium carbonate in the form of the mineral calcite. |
| lithic sandstone | A sandstone that contains many fragments derived from fine-grained rocks, mostly shales, volcanic rocks, and fine-grained metamorphic rocks. |
| lithification | The process that converts sediments into solid rock by compaction or cementation. |
| mud | A clastic sediment, mixed with water, in which most of the particles are less than .062 mm in diameter. |
| mudstone | A blocky, poorly blended, fine-grained sedimentary rock produced by the lithification of mud. |
| oil | An organic fluid formed by the diagenesis of buried organic material that migrates into reservoirs in porous crustal rocks. |
| organic sedimentary rock | A sedimentary rock that consists entirely or partly of organic carbon-rich deposits formed by the decay of once-living material that has been buried. |
| peat | A rich organic material that contains more than 50 percent carbon. |
| phosphorite | A chemical or biochemical sedimentary rock composed of calcium phosphate precipitated from phosphate-rich seawater and formed diagenetically by the interaction between muddy or carbonate sediments and the phosphate-rich water. |
| physical weathering | Weathering in which solid rock is fragmented by mechanical processes that do not change its chemical composition. |
| porosity | The percentage of a rock's volume consisting of open pores between grains. |
| quartz arenite | A sandstone made up almost entirely of quartz grains, usually well sorted and rounded. |
| reef | A moundlike or didgelike organic structure constructed of the carbonate skeletons of millions of organisms. |
| rift basin | A deep, narrow, elongate basin with thick successions of sedimentary rocks and also extrusive and intrusive igneous rocks. |
| ripple | A sedimentary structure consisting of a very small dune of sand or silt whose long dimension is at right angles to the current. |
| salinity | The total amound of dissolved substances in a given volume of water. |
| sand | A clastic sediment consisting of medium-sized particles, ranging from .062 to 2 mm in diameter. |
| sandstone | A clastic rock composed of grains of quartz, feldspar, and rock fragments, bound together by a cement of quartz, carbonate, or other minerals, or by a matrix of clay minerals. The lithified equivalent of sand. |
| sedimentary basin | A region of considerable extent where the combination of deposition and subsidence has formed thick accumulations of sediment and sedimentary rock. |
| sedimentary environment | A geographic location characterized by a particular combination of climate conditions and physical, chemical, and biological processes. |
| sedimentary structure | Any kind of bedding or other surface (such as cross-bedding, graded bedding, or ripples) formed at the time of deposition. |
| shale | A fine-grained clastic rock composed of silt plus a significant component of clay, which causes it to break readily along bedding planes. |
| siliciclastic sedimentary environment | Those environments dominated by silciclastic sediments. The yinclude the desert, lake, glacial, deltas, beaches, continental, deep-ocean floor environments. |
| silt | A clastic sediment in which most of the grains are between .0039 and .062 mm in diameter. |
| siltstone | A clastic rock that contains mostly silt and looks similar to mudstone or very fine grained sandstone. The lithified equivalent of silt. |
| sorting | The tendency for variations in current velocity to segregate sediments according to size. |
| subsidence | A depression or sinking of the crust induced partly by the additional weight of sediments on the crust but driven mostly by tectonic mechanisms, such as regional down-faulting or cooling of the lithosphere. |
| terrigenous sediment | Sediment eroded from the land surface. |
| thermal subsidence basin | A basin produced in the later stages of rifting, when newly formed continental plates are drifting away from each other. The lithosphere that was thinned and heated during the earlier rifting stage cools, leading to an increase in density, which in turn leads to subsidence below sea level, where sediments can accumulate. |
| weathering | The general process that breaks up rock into fragments of various sizes by a combination of physical fracturing and chemical decomposition. |
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