Most metamorphic rocks are formed from regional metamorphism. When two plates collide, it causes the release of large amount of pressure, this causes a deformation (folding and faulting) of the rock. As a result of the collision, the denser plate may be forced under the less dense plate thereby causing the rock to fall deep where they are subjected to high pressure and temperature.
Regional metamorphism produces green schist facies, amphibolite facies and granulite facies.
