<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2012) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2012) (Learn how and when to remove this template message) </Td> </Tr> <P> 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 (150 megapascals (1,500 bar)), causing profound physical or chemical change . The protolith may be a sedimentary, igneous, or existing metamorphic rock . </P> <P> 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 at great depths within the Earth's crust . Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite . </P>

What is any process that changes a rock's chemical composition