Metamorphism is a process which occurs within the crust of the Earth.
This occurs under fairly tight pressure conditions, as CO 2 is not lost during the heating. Rather the CO 2 acts as an enhancer of crystallization. Dolomite marble is a higher pressure product. Slate, an economically important metamorphic rock, whose name is often improperly used in soft coal mines for roof shales, is a product of largely dynamic metamorphism. Parent rock, usually shale or less often volcanic ash, is heated under conditions of directed pressure.