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Metamorphic rock

Metamorphic rock is the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism, which means "change in form".

The protolith is subjected to heat and extreme pressure 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.

Note:   The above text is excerpted from the Wikipedia article "Metamorphic rock", which has been released under the GNU Free Documentation License.
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