Very often the rock is recrystallized. Contact metamorphism is a localized form of metamorphism that is produced by the heat of an igneous intrusion . The zone affected in this way is called a metamorphic aureole. Hydrothermal metamorphism, or metasomatism, is produced by hot aqueous fluids emanating from igneous intrusions. China clay is produced in this way from granite.
Regional metamorphism takes place on a large scale.
The rocks are subjected to heat, deformation, and the action of hot fluids that may affect their chemical composition. More intense regional metamorphism results in the development of schists, rocks with a characteristic wavy foliation. The normal sequence, from lower to higher metamorphic grade, is known as prograde metamorphism. The process can be reversed if, for example, rocks of a high grade are subsequently maintained for a long time at a lower temperature than was reached during the first metamorphism. Alteration from a higher to a lower grade is retrograde metamorphism.