Metamorphism is when minerals recrystallize in pre-existing, parent, igneous or sedimentary rock. Pressure, temperature, fluids or deformations are some of the causes of this change. It can also occur when the rocks are buried or dragged by an active lithosphere, then brought back to the surface.
Some of the causes of change can occur from inside of the earth, such as heat and pressure. These generally can be found within the first few miles under the earth's surface. Temperature and pressure increase as the depth increases thus creating changes in the surrounding rocks.
When a rock comes into contact with lava or hot magma, they become metamorphosed by contact metamorphism. This can either happen at the Earth's surface or deeper underground. Hot magma fills a space within the crust of the Earth, these areas are called batholiths, when they are large. As the hot magma comes into contact with the rocks, the rocks change. The amount of magma determines how much rocks will change. The area surrounding a batholith is the zone of altered rock, or an aureole. The aureole may cover more than 100 square miles of land.
These changes can also take place above ground when lava erupts from a volcano. Any rocks that it comes into contact with will change. When metamorphism occurs over large areas of land it is called a regional metamorphism. Rocks that are formed in regional metamorphisms are generally foliated, like slates, schists and gneisses. This kind of change also occurs when continental masses run into each other causing folding or thrusting of land.
When a mechanical force is involved in the change of the rock, it is called cataclastic metamorphism. This can happen when two rocks slide past each other, usually in a fault zone. The friction of these two rocks creates heat. The rock then become deformed through the movement and the heat. This type of metamorphism is rare.
When hot temperatures and moderate pressure are present with hot, chemically active mineral-filled water, and it comes in contact with rock, usually called country rock, hydrothermal metamorphism occurs. The temperature and pressure required for this type of change is lower than with most others. The result are deposits rich with ore.
At depths greater than 200 meters the temperatures increase. When other stresses are not present, the appearance of the rock will not appear to change but new minerals grow. Burial metamorphism produced Zeolites. As the temperature and pressure increase this can overlap with regional metamorphism.
Shock metamorphism, or impact metamorphism, occurs when an object from space, such as a comet, comes into contact with the Earth, or if a volcanic eruption generates ultrahigh pressure. The minerals that are produced, such as coesite and stishovite, are only stable at high pressures. Some of the textures that are created are shock lamellae in mineral grains, which appears as parallel lines in the rock..
The type of metamorphism that a rock goes through can be identified by assessing the surrounding area, taking in the pressure and temperature that the rock underwent