Metamorphism describes how one type of rock is changed into another. Schist, a metamorphic rock is produced by adding heat and pressure to mudstone or shale, thus it is metamorphic; the extreme pressures during its creation fire the quartz grains, flat minerals and feldspar into large-grained rock. As metamorphism increases, i.e. pressure and temperature, the size of the crystals and how coarse the folding or foliation increases.
