Any of a class of rocks that result from the alteration of preexisting rocks in response to changing geological conditions, including variations in temperature, pressure, and mechanical stress.
The preexisting rocks may be igneous , sedimentary , or other metamorphic rocks. The structure and mineralogy reflect the particular type of metamorphism that produced the rock and the composition of the parent rock. Metamorphic rocks are commonly classified by type of facies, predictable mineral assemblages associated with certain temperature and pressure conditions (see, e.g., granulite facies ).