When existing rocks are subjected to high temperature and pressure, metamorphic rocks are formed. Metamorphism is a process by which these metamorphic rocks are formed. It is of three kinds:
Types of Metamorphism
Thermal metamorphism: Metamorphism caused by extreme temperature is called thermal metamorphism. It occurs because of the heat generated by magma or due to the friction caused by two moving rocks. Example: Forming of slate from clay
Dynamic metamorphism: It occurs when changes in rocks take place because of large-scale movements of the Earth’s crust. The rock may get pushed to greater depths under the surface of the Earth resulting in the formation of new rocks. Example: Shale gets converted to hornfels.
Regional metamorphism:Â When metamorphism is caused by large-scale changes such as mountain building, the igneous and sedimentary rocks may get buried to form new rocks. This is known as regional metamorphism. When only a small area is affected, it is called local or contact metamorphism. Example: Sandstone changes into quartzite.
Processes of Metamorphism
Mechanical metamorphism: It occurs when the texture of rocks is changed by crushing and rubbing processes. In this process, shales develop into slaty splits.
Chemical recombination: This process takes place when rocks containing more than one mineral are metamorphosed into one rock.
Chemical replacement: When liquids and gases enter the surface of the Earth through small cracks, they undergo chemical changes and new rocks are formed.
Recrystallization:Â It takes place when minerals present in rocks are changed into large crystals before metamorphism.
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