Metamorphic rocks form through the transformation of other rocks. This change is caused by heat and pressure acting upon the original rock (igneous, sedimentary, or metamorphic) and altering the rock's mineral structure and/or composition.

1. regional metamorphism - The large-scale transformation of rock that results from mountain-building and other forms of crustal movement. Often results in foliation.
   foliation - alignment of crystals into layers (examples: slate, pyllite, schist, gneiss).
2. contact metamorphism - Alteration of rocks that result when heat, hot liquids, and gases associated with magma bakes and/or reacts with the surrounding rock. This metamorphism is less extensive and does not cause foliation.