 Metamorphic rock, 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 degrees C and pressure 100 MPa or more, causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. They 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 horizontal pressure, friction and distortion. They are also formed when rock is heated by the intrusion of hot molten rock called magma from the Earth's interior. The study of metamorphic rocks now exposed at the Earth's surface following erosion and uplift provides information about the temperatures and pressures that occur that great depths within the Earth's crust. Some examples of metamorphic rocks are nice, slate, marble, schist, and quartzite.