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 and pressure, (temperatures greater than 150 to 200 C and pressures of 1500 bars) causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and are classified by texture and by chemical and mineral assemblage (metamorphic facies). They may be formed simply by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above it. They can form from tectonic processes such as continental collisions, which cause...'
Contributions by Vsmith, Geologyguy, and 18.104.22.168.