Metamorphic rocks are one of three classes of rocks, distinguished from both sedimentary and igneous rocks, because of the way they form. The process that forms metamorphic rock is known as metamorphism, a word derived from the Greek "meta" and "morph", meaning "change/beyond" and "shape/form" respectively. Any rock can become a metamorphic rock by the process of metamorphism.
The main agents or causes of metamorphism are heat, pressure and hydrothermal solution. Different types of rock will require different temperatures and pressures or hydrothermic solutions for change to actually occur. The driving cause for all metamorphisis is for the rock to attain mineral stability after some disturbance to the mineral structure caused by the energy (heat/pressure) or chemical changes.
Sources of Heat and Pressure Change
There are various sources of heat and pressure change giving rise to different types of metamorphis. Most commonly there are changes within or on the earth's crust due to volcanic action or plate movements. Contact metamorphis occurs when volcanic magma comes into contact with a rock and starts the changes. This can happen both at the earth's surface and deep underground. Typically areas up to 100 square kilometers can be affected, in contrast to changes that happen over larger areas.
Regional metamorphis occurs when plate movements at the earth's crust cause pressure and heat changes at the boundaries of plates. At the boundary of plate movements rocks are squeezed with incredible force, producing heat. Sometimes one plate of rock is pulled below another and the rock is subject to heat from friction as well as increased heat and pressure from being pulled lower. Heat increases 20 to 30C for every kilometre of depth. The shearing of rocks in cataclytic metamorphism is another example of heat and pressure generated from rock friction.
Occasionally the source of temperature and pressure may come from a meteor or other extraterrestial object that impacts the earth causing explosion, heat and pressure changes in the rock that is hit. This is called shock metamorphsis and the shock wave itself is responsible for a short-term compression change that mimics long term pressure of conventional metamorphic action.
Range of Heat and Pressure
Regardless of mechanism or the exact source of heat/pressure change metamorphsis occurs at different pressures and temperatures for different rocks. The upper limit of metamorphism occurs at the pressure and temperature where melting of the rock in question begins. When the rock reaches its melting point the process of change is igneous rather than metamorphic.
The lower bound for metamorphism is taken to be temperatures higher than 200C and pressures higher than 300MPa (Mega Pascals). Below these temperatures and pressures changes can occur but the process is known as Diagenesis and it typically affects sedimentary rocks at the earth's surface.
Within the range of temperatures and pressures where change can occur the amount of heat/pressure affects what rock is produced. For example, starting with the common sedimentary rock, shale, slight pressure and heat changes will produce slate. Increased conditions push the change onto phyllite, a rock with increasing amounts of mica. Even more heat and pressure will produce schist, gneiss, migmatite or granite. These rocks are associated with the changes caused by regional metamorphic action.
Heat and pressure not only act directly on the rocks that they encounter but there are secondary effects caused by hydrothermal solutions that can also change rocks. While associated with the heat and pressure the hydrothermal process is different. Minerals are dis-sovled and redeposited by hydrothermal solutions, changing the minerals in the rock and the crystallization. Rocks that come in contact with this hydrothermal solution can have their composition altered as a result of this recrystalization. In particular gasses like steam carry minerals that can create new rocks.
The main causes of metamorphsim in rocks are the changes in heat, pressure or chemistry that cause the rock to morph in order to regain internal balance. Different physical processes cause these changes in heat, pressure or chemistry giving metamorphic rocks that are characteristic of the different physical processes that caused the environment producing change.