Sep 15 2014
Metamorphic rocks are formed at a depth of 12 to 16 km beneath the Earth’s surface, where high temperature, pressure and chemical alterations cause transformation of pre-existing rocks into another type of rock.
They are formed through a process called metamorphism. The original rock, i.e., protolith such as igneous rock, sedimentary rock or another old metamorphic rock is subjected to high pressure of about 1500 bars and temperature of over 200°C, which causes a physical or chemical change in the rock.
Metamorphic rocks constitute a large portion of the Earth’s crust. They are classified based on their texture and metamorphic facies into foliated and non-foliated metamorphic rocks. Foliated metamorphic rocks have a layered or banded surface caused by the exposure to high heat and pressure. Examples of foliated rocks include slate, schist, phyllite and gneiss. Alternatively, non-foliated rocks such as quartzite and marble lack layered surface.
The action of shear and stress forces, compression and plate tectonics on the existing rock for a long period of time easily deforms the rock causing them to be compacted into a small volume of space. As a result, metamorphic rocks are generally denser than their original material and also less susceptible to erosional damage. With the movement of Earth’s plates, a plate containing sedimentary or igneous rock may be subducted below another plate. The weight of the material above the plate enables the rock to undergo metamorphism. Sometimes, the rock may be melted slightly by the heat from the Earth's interior in a process called contact metamorphism.
The crystals of the metamorphic rocks are aligned in layers when the rocks are subjected to pressure. Slate formed from shale is a suitable example for this. In some cases, metamorphic rocks contain fossils if they are formed from sedimentary rocks.
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