Sunday, June 13, 2010

Metamorphic Rock

A metamorphic rock is a sedimentary rock which has been altered and hardened by exposure to high temperatures and pressure. The rock precursor that is transformed into a metamorphic rock is called protolith. Metamorphic rock is the result of the transformation of this existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is then 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 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 metamorphic rocks provides us with 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.

Gneiss rock