Amateur Geologist Structured Geological Glossary: Metamorphic Rocks

Structured Geological Glossary: Metamorphic Rocks

metamorphism	The changes of mineralogy and texture imposed on a rock by pressure and temperature in the Earth's interior. Meteoric water
metamorphic	From the Greek meta (change) and morph (form). A rock that has undergone chemical or structural changes produced by increase in heat or pressure, or by replacement of elements by hot, chemically active fluids.Also applies to the conversion of snow into glacial ice.
thermal aureole	Zone of rock around an igneous intrusion that has been altered or metamorphosed by heat from the hot magma. The rock in the zone is baked.
contact metamorphism	Mineralogical and textural changes and deformation of rock resulting from the head and pressure of an igneous intrusion in the near vicinity.
shock metamorphism	Characteristic changes in rock mineralogy caused by the compression of a passing shock wave.
regional metamorphism	Metamorphism occurring over a wide area and caused by deep burial and high internal temperatures of the Earth.
metaconglomerate	Metamorphosed conglomerate.
isograd	A line or curved surface connecting rocks that have undergone an equivalent degree of metamorphism.
metamorphic zones	An area of metamorphic rock that can be mapped by some special characteristic such as the presence of an index mineral.
metamorphic facies	A set of rocks having a particular mineral composistion and having been metamorphosed under similar conditions of heat and pressure.
greenschist	A metamorphic schist containing chlorite and epidote (which are green) and formed by low-temperature, low-pressure metamorphism.
greenstone	A metamorphic rock derived from basalt or chemically equivalent rock such as gabbro. Greenstones contain sodium-rich plagioclase feldspar, chlorite, and epidote, as well as quartz. The chlorite and epidote make greenstone green.
amphibolite	"A family of silicate mineral forming prism or needlelike crystal. Amphibole minerals generally contain iron, magnesium, calcium and aluminum in varying amounts, along with water. Hornblende always has aluminum and is a most common dark green to black variety of amphibole; it, forms forming in many igneous and metamorphic rocks. Actinolite has no aluminum; it and is needle-shaped and light green. Blue amphibole contains sodium and, of course, is bluish in color. "
eclogite facies	A metamorphic facies developed at high pressure (>10kbar) and high temperature (600°C or more), characterized by the metamorphic rock eclogite.
hydrostatic pressure	The pressure at a point in a body of liquid that is due to the weight of liquid above it.
solid state	In metamorphism, indicates the change of mineral identity without melting. All ion migration occurs while the rock (or pre-glacial ice) is still solid.
recrystallization	The growth of new mineral grain in a rock at the expense of old grains, which supply the material.
geotherm	A curving surface within Earth along which the temperature is constant.
isotherm	A line on a chart that connects all points of equal or constant temperature.
heat sink	That portion of a thermodynamic system that absorbs unused heat.
heat conduction	The transfer of the rapid vibrational energy of atom and molecules, which constitutes heat energy, through the mechanism of atomic or molecular impact.
heat engine	A device that transfers heat from a place of high temperature to a place of lower temperature and does mechanical work in the process.
cataclastic rock	Metamorphic rocks that resemble breccia or poorly sorted sandstone and that form by dynamic metamorphism, which breaks and reorients grain.
porphyroblasts	Large mineral grain that grow during metamorphism.
relict	The structure or mineral of the parent rock that can still be seen in a metamorphic rock.
parent rock	The preexisting rock from which a metamorphic rock form.
foliation	Aligned layer of mineral characteristic of some metamorphic rocks. Foliation form in metamorphic rocks when pressure squeezes flat or elongates minerals so that they become aligned. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied.
lineation	Any linear arrangement of features found in a rock.
schistosity	The parallel layering of individual platy or prismtic mineral, usually mica or hornblende, that have grown during metamorphism so that they are easily visible to the naked eye.
slatey cleavage	A parallel form of cleavage that is developed in fine grained rocks when they are subjected to low grade metamorphism.
flow cleavage	In a metamorphic rock, the parallel arrangement of all planar or linear crystal as a result of rock flowage during metamorphism.
quartzite	(1) A very hard, clean, white metamorphic rock formed from a quartz arenite sandstone. (2) A quartz arenite containing so much cement that it resembles ( 1 ). (3) A nonfoliated metamorphic rock formed from pure, dominantly quartz sandstone. (4) Hard, somewhat glassy-looking rock made up almost entirely of quartz. Metamorphosed quartz sandstone and chert are quartzite.
marble	A metamorphic rock of made of calcium carbonate. Marble form from limestone by metamorphic recrystallization.
hornfels	Usually formed by contact metamorphism. A dark, very fine-grained metamorphic rock of uniform grain size showing no foliation. It is produced by the recrystallization, at high-temperature and low-pressure, by heat from a nearby igneous intrusion. From the German, meaning horn rock.
micaceous	A general term for mica-rich rocks.
hornblende schist	A schist rich in hornblende. Generally with abundant plagioclase feldspar as well. Grades into amphibolite.
slate	A fine grained argillaceous rock that has good cleavage (slatey cleavage) that can be easily split into slabs ana thin plate. Most slate is formed by low grade metamorphism.
phyllite	"A very fine-grained, foliated metamorphic rock, generally derived from shale or fine-grained sandstone. Phyllites are usually black or dark gray; the foliation is commonly crinkled or wavy. Differs from less recrystallized slate by its sheen, which is produced by barely visible flakes of muscovite (mica)."
schist	Metamorphic rock usually derived from fine-grained sedimentary rock such as shale. Individual mineral in schist have grown during metamorphism so that they are easily visible to the naked eye. Schists are named for their mineral constituents. For example, mica schist is conspicuously rich in mica such as biotite or muscovite.
blueschist	Metamorphic rock rich in blue amphibole.
gneiss	A coarse-grained, foliated metamorphic rock that commonly has alternating bands of light and dark-colored mineral.
gneissic layering	The tecture typical of gneisses. The foliation is more widely spaced, has less marking, and often more discontinuous than the schistosc texture.
orthogneiss	Gneiss formed by squeezing (deformation and usually some recrystallization) of a granitic igneous plutonic rock.
eclogite	An extremely high pressure metamorphic rock containing garnet and pyroxene.
migmatite	Mixed rock. A metamorphic rock that form in one of two ways. The metamorphic rock may be heated enough to partially melt, but not completely. The molten minerals resolidify within the metamorphic rock, producing a rock that incorporates both metamorphic and igneous features. Migmatites can also form when metamorphic rock experiences multiple injections of igneous rock that solidify to form a network of cross-cutting dike.
preferred orientation	Any deviation from random-hess in the distribution of the crystallographic or grain shape axes of mineral of a rock (including flow cleavage and foliation), produced by deformation and nonuniform stress during crystallization in metamorphic rocks or by depositional currents in sediment.
mylonite	A very fine lithified fault breccia commonly found in major thrust fault and produced by shearing and rolling during fault movement.
granulite	A metamorphic rock with coarse interlocking grain and little or no foliation.
pyroxene granulite	A coarse-grained contact metamorphic rock containing pyroxene, formed at high temperatures and low pressures.
augen	Augen are relatively large, eye-shaped mineral grain in certain types of metamorphic rocks, especially schist and gneiss. (Augen = eyes in German)