From Wikipedia, the free encyclopedia Metamorphic rock is the
result of the transformation of an existing rock type, the
protolith, in a process called metamorphism, which means "change
in form". The 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 horizontal pressure,
friction and distortion. They are also formed when rock is
heated up by the intrusion of hot molten rock called magma from
the Earth's interior. The study of metamorphi