metamorphic rocks

metamorphic rock An aggregate of minerals formed by the recrystallization of pre-existing rocks in response to a change of pressure, temperature, or volatile content. Metamorphic rocks can generally be divided into four types: (a) regional metamorphic rocks, formed in response to changes leading to high temperature and high pressure (shearing stress and hydrostatic pressure: see HYDROSTATIC STRESS) accompanying orogenic events (see OROGENY; REGIONAL METAMORPHISM); (b) contact metamorphic rocks, formed in response to changes leading to high temperature (with low pressure) around an igneous intrusion (see THERMAL METAMORPHISM); (c) cataclastic or dynamic metamorphic rocks, formed in response to an increase in directed pressure (shearing stress) particularly in fault and thrust zones (see CATACLASITE); and (d) burial metamorphism, formed in response to changes leading to high pressure (with low temperature).

metamorphic rock Broad class of rocks that have been changed by heat or pressure from their original nature – sedimentary, igneous, or older metamorphic. The changes characteristically involve new crystalline structure, the creation of new minerals, or a radical change of texture. For example, the metamorphic rock slate is made from sedimentary shale.