Page "Rock (geology)" Paragraph 11

Metamorphic rocks are formed by subjecting any rock type ( including previously formed metamorphic rock ) to different temperature and pressure conditions than those in which the original rock was formed.

These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals ( e. g. by recrystallization ).