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.
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