(I) On the basis of Agency - Metamorphism is caused by thermal and dynamic agencies.
(a) Thermal Metamorphism - If metamorphism is brought about by heat, it is called Thermal Metamorphism. Heat is received through many ways in this metamorphism.
(i) Hot Magma - It raises high temperature.
(ii) Mutual Friction - The rocks when subjected to movement produce heat by mutual friction.
(iii) Hot gases, vapors and liquids - It has already been mentioned that hot mineralizers (Chlorine, vapors. etc.) produce metamorphism. The extent of metamorphism will however depend upon the nature and amount of mineralizers, the pressure exerted and the porosity of rocks.
(iv) Geothermal heat-Earth's heat alone is capable of bringing about metamorphism. This happens in rocks below the surface of the Earth where the temperature is high. This is not due to the high temperature of magma. For example, clay has changed into shale in the geosyncline of the Alps.
(b) Dynamic Metamorphism - The rocks of the earth are subjected to pressure due to geodynamic forces. Rocks are warped and their temperature rises. If water is available, the minerals are reorganized into new compounds. Mylonite is a mineral which is metamorphosed in this way.
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