Metamorphic Rock

Metamorphic Rock :,Rock, the original composition and texture of which has been altered by heat and pressure deep within the earth's crust. Metamorphism that is a result of both heat and pressure is referred to as dynamothermal, or regional; metamorphism produced by the heat of an intrusion of igneous rock is termed thermal, or contact. Four common varieties of metamorphic rock can be traced to a parent sedimentary or igneous rock, because rocks display varying degrees of metamorphism, depending on how much heat and pressure they have endured. Thus, shale is metamorphosed to slate in a low-temperature environment, but if heated to temperatures high enough for its clay minerals to recrystallize as mica flakes, shale becomes metamorphosed into a phyllite. At even higher temperatures and pressures, shale and siltstone completely recrystallize, forming schist or gneiss, rocks in which the alignment of mica flakes produces a laminated texture called foliation. In schist, the light-colored minerals (mainly quartz and feldspar) are evenly distributed among the darkcolored micas; gneiss, on the other hand, displays distinctive color banding. Among the other minerals commonly formed by metamorphic recrystallization, aluminum silicates such as andalusite, sillimanite, and kyanite are pervasive enough to be considered diagnostic. Among the nonfoliated metamorphic rocks, quartzite and marble are the most common. Quartzite is typically a

tough, hard, light-colored rock in which all the sand grains of a sandstone or siltstone have been recrystallized into a fabric of interlocking quartz grains. Marble is a softer, more brittle, varicolored rock in which the dolomite or calcite of the parent sedimentary material has been entirely recrystallized.

Metamorphic rock: Rock altered in structure and composition by pressure, heat, or chemically active fluids after original formation. (If heat is sufficient to melt the original rock, technically it becomes an igneous rock upon cooling.) The term was coined in 1833 by Scottish geologist Charles Lyell (1797- 1875). The mineral assemblage present in a metamorphic rock depends on the composition of the starting material (which may be sedimentary or igneous) and the temperature and pressure conditions to which it is subjected. There are two main types of metamorphism. Thermal metamorphism, or contact metamorphism, is brought about by the baking of solid rocks in the vicinity of an igneous intrusion (molten rock, or magma, in a crack in the Earth's crust). It is responsible, for example, for the conversion of limestone to marble. Regional metamorphism results from the heat and intense pressures associated with the movements and collision of tectonic plates. It brings about the conversion of shale to slate.