How is Metamorphic Rock Formed

Are you wondering how a metamorphic rock is formed? There is a simple answer. Before that, the surface of the earth is covered with mainly three types of rocks namely igneous, sedimentary, and metamorphic rocks. Metamorphic rocks are so called because they are created from the metamorphism of pre-existing rocks. However, many people find it hard to understand and visualize the formation of these very important rocks that are found in all parts of the world. This article attempts to explain how a metamorphic rock is formed in easy to understand ways.

Metamorphic Rock Facts

Metamorphic rocks are formed from rocks that are pre-existing

Metamorphic rocks are pre-existing rocks on the surface of the earth that change in composition and density over a long period under the influence of pressure and heat. They could be either igneous or sedimentary rocks. They could even be formed from pre-existing metamorphic rocks. Different rocks have different compositions as they are, usually, made up of one or more minerals. Rocks that get morphed into another type are called metamorphic rocks. Some metamorphic rocks may have the same composition as other sedimentary or igneous rocks, but rocks are mainly classified on the basis of the process of their formation.

How is metamorphic rock formed

Sedimentary and igneous rocks lying deep within the surface of the earth are subjected to extreme heat and pressure.

Processes involved in the formation of metamorphic rocks

There are two processes of formation of metamorphic rocks that are as follows.

Contact metamorphism

This is a process that starts when hot magma finds its way inside an existing rock. Heat of the molten magma bakes the surrounding rocks and they develop fissures that allow passage of this hot liquid inside them. Change takes place in the composition of these rocks and they become metamorphic rocks. This is a change that is local in nature and the extent of changes taking place is also very small. This is why contact metamorphism is referred to as low-grade metamorphism. Marble is a fine example of contact metamorphism as it changes from limestone when subjected to extreme heat by magma.

Regional metamorphism

These are rocks that are formed over a long period in pre-existing large masses of rocks. This is a high-grade metamorphism that is associated with the process of mountain building. The rocks that are at the bottom are subjected to extreme pressure from the weight of the overlying rocks. Their composition changes and they become compact and denser than before. Pressure causes a change in the crystals of the rocks and they get arranged into layers. Slate is a great example of a metamorphic rock formed with the help of regional metamorphism. It is used to make roof tiles as its layers can be easily separated to make tile sheets. Friction and movement of tectonic plates of the earth also leads to the creation of metamorphic rocks.

Some examples of metamorphic rocks

• Granulate is a metamorphic rock that results when metamorphism of basalt (igneous rock) takes place.

• Mudstone is a sedimentary rock that is metamorphosed into slate.

• Sandstone is a sedimentary rock that gives way to metamorphic rock quartzite. 


Image Courtesy:

  1. Gneiss image by Siim Sepp (CC BY-SA 3.0)

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