3.1.3. Metamorphic Rocks

The word metamorphic means ‘changed form’. The rocks, originating at or near the surface of the Earth are sometimes subjected to tremendous heat and pressure. This can change the original properties of rocks such as their colour, hardness, texture and mineral composition. Such changed rocks are called metamorphic rocks. Both igneous and sedimentary rocks can change into metamorphic rocks. The special feature about the origin and formation of metamorphic rocks is that they remain in their original position and change under the impact of internal and external forces. Metamorphism can be of thermal and dynamic origin.

1. In the case of thermal metamorphism, the original rocks are changed under the influence of high temperature inside the Earth’s crust. For example, limestone is converted into marble, sandstone into quartzite, shale into slate and coal into graphite.

2. In the case of dynamic metamorphism, the original rocks are changed under the influence of pressure at great depths inside the Earth’s crust. For example, granite is converted into gneiss and shale into schist.