Metamorphic Rock

Metamorphic rocks are the transformation of the existing rocks, the protolith through the process of metamorphism. The Protoliths can be the igneous rock, sedimentary rock or any old metamorphic rock. Due to severe heat and pressure, the protolithis get various chemical or physical changes. These rocks create a huge part of the Earth's crust and these can be classified by the mineral and chemical assemblage and by the textures. Examples of the metamorphic rocks are: slate, gneiss, schist, marble and quartzite.

Formation of the metamorphic rocks:
The metamorphic rocks are created deep beneath the surface of the Earth because of high temperatures and because of the huge pressure of the layers of the rocks above it. Another process of the formation of the metamorphic rocks is the tectonic process like the continental collision which leads to the horizontal pressure, distortion and friction. Metamorphic rocks can also be formed while any rock gets heated up by the invasion of Magma from the interior part of the Earth.

Minerals which can be found in the metamorphic rocks:
Due to the high pressure and high temperature, minerals are usually found in the metamorphic rocks. The minerals are known as the index minerals and it includes: kyanite, silimanite, staurolite, some garnet and the andalusite. Some other minerals like olivines, amphiboles, pyroxenes, mica, quartz and feldspars can also be found sometimes in the metamorphic rocks. But they are not necessarily the result of the metamorphism process.

Recrystallization and metamorphism:
During the metamorphism process, the change in the rocks particle size is called the recrystallization. As an example, through the process of recrystallization, the limestone crystals change into marbles which are made of larger crystals. The pressure and high temperature, both contribute to the recrystallization process. The high temperatures allow the ions and atoms in the solid crystals to migrate and therefore to reorganize the crystals.

Foliation:
The process through which the metamorphic rocks are formed is known as Foliation. The process occurs when the rock get shortened during the process of recrystallization. This results in elongated or platy crystals of the minerals like the chlorite and mica.

Different textures can be found in the foliated and non-foliated categories of metamorphic stones. Sometimes cleavages can be found in the foliated rock. On the other hand, the non-foliated rocks do not have any planar pattern of the strain.
Another important aspect of the metamorphism process is the chemical reactions which take place between the minerals of the rocks without melting them. In this procedure, the atoms of the minerals get exchanged and new minerals get formed. So, the process of metamorphism results in the drastic change in the chemical composition of the metamorphic rocks.

Reference:
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