Metamorphic rocks arise from the transformation of existing
rock types, in a process called
metamorphism, which means "change in form". The original rock (
protolith) is subjected to heat (temperatures greater than 150 to 200 °C) and pressure (1500
bars), causing profound physical and/or chemical change. The protolith may be a
sedimentary rock, an
igneous rock or another older metamorphic rock.