| Term | Definition |
|---|
| amphibolite | A usually nonfoliated metamorphic rock made up mainly of amphibole and plagioclase feldspar. Foliated amphibolites can be produced by deformation. |
| blueschist | A metamorphic rock formed under conditions of high pressure and moderate temperature, often containing glaucophane, a blue amphibole. |
| contact metamorphism | Changes in the mineralogy and texture of rock resulting from teh heat and pressure in a small area, such as the rocks near and in contact with an igneous intrusion. |
| eclogite | A metamorphic rock formed under very high pressure and moderate to high temperature, typically containing minerals such as coesite (a very dense, high-pressure form of quartz). |
| exhumation | A process in which subducted metamorphic rocks rise to the surface because of buoyancy and cirulation in the subductions zone. |
| foliated rock | A metamorphic rock that displays foliation. Includes slate, phyllite, schist, and gneiss. |
| foliation | A set of flat or wavy parallel planes produced by deformation. |
| gneiss | A light-colored, poorly foliated rock with coarse bands of segregated light and dark minerals througout, produced by high-pressure, high-temperature metamorphism. |
| granoblastic rock | A nonfoliated metamorphic rock composed mainly of crystals that grow in equant shapes, such as cubes and spheres, rather than in platy or elongate shapes. Include hornfels, quartzite, marble, greenstone, amphibolite, and granulite. |
| granulite | A medium to coarse-grained granoblastic rock formed under conditions of relatively high pressure and temperature. |
| greenschist | A schist containing chlorite and epidote (which are green) and formed by low-pressure, low-temperature metamorphism of mafic rocks. |
| greenstone | A granoblastic rock produced by the low-grade metamorphism of mafic rock. Chlorite accounts for the greenish cast. |
| high pressure metamorphism | Metamorphism that occurs at high pressure (8 to 12 kbar) and ultra-high pressure (greater than 28 kbar). |
| hornfels | A granoblastic rock of uniform grain size that has undergone little or no deformation. Usually formed by contact metamorphism at high temperatures. |
| low grade (burial) metamorphism | Metamorphism in which buried sedimentary rocks are altered by the progressive increase in pressure exerted by overlying sediments and sedimentary rocks and by the increase in heat associated with increased depth of burial in the Earth. |
| marble | A granoblastic rock produced by the metamorphism of limestone or dolostone. |
| melange | A heterogeneous mixture of rock materials produced by high-pressure, low-temperature metamorphism, found where a plate carrying a continent on its leading edge converges with a subducting oceanic plate. |
| metamorphic facies | Groupings of metamorphic rocks of various mineral compositions formed under different grades of metamorphism from different parent rocks. |
| metamorphic P-T path | The history of the changing conditions of pressure and temperature that occured during metamorphism. |
| metamorphic rock | A rock formed by the transformation of preexisting solid rocks under the influence of high pressure and temperature. |
| metasomatism | The transformation of preexisting solid rocks under the influence of high pressure and temperature. |
| migmatite | A mixture of igneous and metamorphic rock produced by incomplete melting. They are badly deformed and contorted, and are penetrated by many veins, small pods, and lenses of melted rock. |
| orogeny | Mountain building, particularly by the folding and thrusting of rock layers, often with accompanying magmatic activity. |
| phyllite | A foliated rock that is intermediate in grade between slate and schist. Small crystals of mica and chlorite give it a more or less glossy sheen. |
| porphyroblast | A large crystal surrounded by a much finer grained matrix of other minerals in a metamorphic rock. |
| quartzite | A very hard, white granoblastic rock derived from quartz-rich sanstones. |
| regional metamorphism | Metamorphism caused by high pressures and temperatures that extend over large regions, as happens where plates collide. |
| schist | A metamorphic rock characterized by the pervasive coarse, wavy foliation known as schistosity. |
| seafloor metamorphism | Metamorphism associated with mid-ocean ridges, in which changes in a rock's bulk chemical composition are produced by fluid transport of chemical components into or out of the rock. |
| shock metamorphism | Metamorphism that occurs when minerals are subjected to the high pressures and temperatures of shock waves generated when a meteorite collides with Earth. |
| slate | The lowest grade of foliated rock, easily split into thin sheets; produced primarily by the metamorphism of shale. |
| stress | The force per unit area acting on any surface within a solid body. It consists of confining pressure and directed pressure. |
| zeolite | A class of silicate minerals containing water in cavities within the crystal structure and formed by metamorphism at very low temperatures and pressures. |
| anticline | A large upfold or arch of layered rocks. |
| axial plane | An imaginary suface that divides a fold as symmetrically as possible, with one limb on either side of the plane. |
| basin | A bowl-shaped depression of rock layers in which the beds dip rapidly toward a central point. |
| brittle material | A material that undergoes little deformation under increasing force, until it breaks suddenly. |
| compressive force | A force that squeezes together or shortens a body. |
| dip | The amount of tilting of a rock layer; the angle at which the bed inclines from the horizontal. |
| dip-slip fault | A fault on which there has been relative movement of the block up or down the dip of the fault plane. |
| dome | A broad circular or oval upward bulge of rock layers. |
| ductile material | A material that undergoes smooth and continuous plastic deformation under increasing force and dos not spring back to its original shape when the deforming force is released. |
| faulting | The process by which tectonic forces cause the lithosphere to break and slip along a fault. |
| folding | The process by which crustal forces deform an area of crust so that layers of rock are pushed into folds. |
| geologic map | A map representing the rock formations exposed at Earth's surface. |
| geologic cross section | A diagram showing the geologic features that would be visible if vertical slices were made through part of the crust. |
| joint | A crack in a rock along which there has been no appreciable movement. |
| normal fault | A dip-slip fault in which the rocks above the fault plane move down relatie to the rocks below the fault plane, extending the structure horizontally. |
| shearing force | A force that pushes two sides of a body in opposite directions. |
| strike | The compass direction of a rock layer, fault, or other planar structure where it intersects with a horizontal surface. |
| strike-slip fault | A fault on which the movement has been horizontal, parallel to the strike of the fault plane. |
| syncline | A large downfold whose limbs are higher than its center. |
| tensional force | A force that stretches a body and tends to pull it apart. |
| thrust fault | A low-angle reverse fault - one with a dip of less than 45 degrees - so that the overlying block moves mainly horizontally. |
| absolute age | The actual number of yeas elapsed from a geologic event until now |
| eon | The largest division of geologic time, embracing several eras |
| epoch | A subdivision of a geologic period, often chosen to correspond to a stratigraphic sequence. Also used for a dvision of time corresponding to a paleomagnetic interval. |
| era | A division of geologic time including several periods, but smaller than an eon. |
| formation | A distinctive set of rock layers in a region that can be recognized and mapped as a unit, having the same physical properties and possibly containing the same assemblage of fossils. |
| half-life | The time required for one-half of the original number of radioactive atoms in an element to decay. |
| isotopic dating | The use of naturally occuring radioactive elements to determine the ages of rocks. |
| period | A division of geologic time represending one subdivision of an era. |
| principle of faunal succession | The principle that the layers of sedimentary rocks in an outcrop contain fossils in a definite sequence. |
| principle of original horizontality | The principle that sediments are deposited as essentially horizontal beds. |
| principle of superposition | The principle that each layer of sedimentary rock in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. |
| relative age | The age of one geologic event in relation to another. |
| stratigraphic succession | A vertical set of rock strata that provides a chronological record of the geologic history of a region. |
| stratigraphy | The description, correlation, and classification of strat in sedimentary rocks. |
| unconformity | A surface between two rock layers that were not laid down in an unbroken sequence. Represents the passage of time. |
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