Contact and regional metamorphism
Also called: Thermal and dynamic metamorphism · Topic: Minerals, Rocks and the Rock Cycle · NCERT: Beyond NCERT
Meaning
These are two kinds of metamorphism, named by the size of the area affected.
- Contact metamorphism happens when hot magma pushes into cooler rock and bakes the rock right next to it. Heat is the main agent, and only a narrow zone around the magma body, such as a dyke, is changed.
- Regional metamorphism changes rocks over very large areas. It happens during mountain building, when rocks are buried deep and squeezed by both great heat and great pressure.
Example
When the Himalaya rose, regional metamorphism turned huge areas of older rock into schist and gneiss. Contact metamorphism, by contrast, may change only a few metres of rock along the sides of a single dyke.
Don't confuse with
- Thermal and dynamic metamorphism: these names refer to the main agent, not the area. Thermal metamorphism is mainly heat, and dynamic metamorphism is mainly pressure crushing the rock. Contact metamorphism is mostly thermal, but regional metamorphism involves both heat and pressure.