These pre-existing rocks get baked by the heat from the cooling magma. The stresses build until movement occurs, generating an earthquake. Increasing stress in rocks along faults can cause high-pressure metamorphism to occur. These extremely strained rocks are called mylonites . Relationships between processes and metamorphic rock characteristics Metamorphic textures.
High temperatures exist in most settings where metamorphism occurs.
Extensive metamorphism does not occur unless the temperature of rock is greater than about 300 o C. High temperatures result in recrystallization , a growth of individual minerals that causes the rock to develop an interlocking crystalline texture. When rocks are metamorphosed under a directed stress, minerals grow in preferred orientation as they recrystallize. The resulting rock may show foliation , an alignment of platy and elongate minerals in the rock. Mineral composition of metamorphic rocks. Metamorphic rocks may contain a variety, or assemblage , of different minerals.