Metamorphic processes

The Rock Cycle

The rock cycle is a collection of processes which shape the surface of the Earth. Rocks are constantly recycled by burial, melting, uplift and erosion. During metamorphic processes, few components (e.g. fluorine, boron), other than water, enter or leave rocks. No overall chemical changes occur; instead, the original chemical constituents are just re-adjusted during recrystallisation and the texture of the rock changes, usually becoming coarser-grained.

The important factors that produce metamorphic changes are

• pressure deep within the crust
• temperature
• strain (shape and volume changes as a result of stress during deformation)
• fluid activity (pressure due to fluids in pore spaces within the original rocks)

Types of metamorphic processes

All of these factors vary widely and result in different types of metamorphism depending upon which factor is dominant. The three main types of metamorphism are:

1. Contact (also known as thermal) metamorphism: this is produced by high temperature, low pressure, low strain and variable fluid pressure. It is normally produced by igneous rocks such as granites intruding into older colder rock sequences within a few kilometres of the Earth's surface.
2. Regional metamorphism: this is produced by variable temperature, high pressure, variable strain and variable fluid flow and is normally created by tectonic processes within the Earth at variable depths.
3. Dynamic metamorphism: this is produced by variable strain, variable pressure, variable temperature and high fluid pressure and normally occurs in active fault zones.

Contact and dynamic metamorphism are usually restricted to localised areas whereas regional metamorphism affects large areas of the crust, sometimes over tens of thousands of square kilometres. Contact metamorphism occurs as zones a few hundred metres wide around large igneous intrusions while dynamic metamorphism is restricted to fault and thrust zones only a few tens of metres thick. All three types of metamorphism can overlap.