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).