Metamorphic Rock

Metamorphic Rock, rock which has had its original composition and texture 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-coloured minerals (mainly quartz and feldspar) are evenly distributed among the dark-coloured micas; gneiss, on the other hand, displays distinctive colour banding. Among the other minerals commonly formed by metamorphic recrystallization, aluminium 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-coloured 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, varicoloured rock in which the dolomite or calcite of the parent sedimentary material has been entirely recrystallized.