Ask-a-Geologist Q&A #16: What types of rocks were changed by high temperature and high pressure?

Cory wrote to us and asked: What rocks are changed by high pressure and high temperature?

Metamorphic rocks are the rocks that change either physically or chemically by heat and pressure. The term metamorphic comes from the Greek words “meta” which means change and “morph” which means form. Metamorphism is a solid state change meaning that the minerals within the rock recrystallize in response to heat, pressure and the chemical reaction with hot fluids without melting the original rock.

Metamorphism happens deep in the earth’s crust. There are many types of metamorphism that geologists talk about but four general types that you might want to know are:

Contact metamorphism which occurs along the edges of cooling magma. The change occurs mostly from heat and circulating waters along the edges of the cooling magma chamber. The existing rocks along the edges of the magma are altered; therefore, this type of metamorphism happens over a small area.

Regional metamorphism occurs over a huge area due to high heat and high pressure. This type metamorphism occurs very deep underground and often along plate boundaries and can also occur from the burial of rock below many layers or rock and sediment. Regional metamorphism often happens over huge areas that are greater 1,000 square miles and can be associated with mountain building.

Cataclastic metamorphism occurs along fault zones from intense pressure. This type of metamorphism involves brittle deformation which is where the rocks break into small pieces.

Hydrothermal metamorphism occurs when hot, mineral rich waters alter the existing rock. Generally this type of metamorphism occurs in areas of low temperature and low pressure.

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