Type of rock that arise from the transformation of existing rock types, in a process called metamorphism.
The original rock (protolith) could be sedimentary, igneous, or existing metamorphic rock which is subjected to heat (temperatures greater than 150 to 200 °C) and pressure (100 megapascals (1,000 bar) or more), causing profound physical or chemical change.
See also
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petroleum Geology + Petrophysics / Rock