Metamorphic rocks and their types


Metamorphic Rocks

Metamorphic rocks are originated from already existed Igneous, and Sedimentary Rocks. The term metamorphic is derived from a Greek word meaning change. Thus the metamorphic rocks are those that have been changed by heat and pressure. Thus actually these are those igneous or sedimentary rocks, which have changed through metamorphism. Metamorphism is the process, which transforms the existing rocks to metamorphism rocks through heat and pressure. According to F.J. Monkhouse,

“Igneous and sedimentary rocks may undergo changes, both physical and chemical, which produce either new minerals or new structure within the rocks. These changing may be brought about by Earth’s movements subjecting the rocks to both heat (thermal) and pressure (dynamic).”

Similarly, Goh Cheng Leong explains the formation of metamorphic rocks in these words,

“All rocks whether igneous or sedimentary may become metamorphic or changing rocks under great heat and pressure. Their original character and appearance may be greatly altered by such forces, particularly during Earth movements. In this manner, clay may be a metamorphosed into slate, limestone into marble, sandstone into quartzite, granite into gneiss, shale into schist and coal into graphite,

Process of Metamorphism:

Metamorphism may be defined as the process of transformation of igneous or sedimentary rocks to metamorphic rocks ( changed rocks ) through the Earth’s movements ( producing heat and pressure). The metamorphism can be thermal when the intrusion of hot mass of igneous rock raises temperature of surrounding rocks. Similarly these can be a contact metamorphism when flowing when action by magma effects rocks in the zone near the batholiths or dike. 

Metamorphism produces changes in the minerals. Mostly the change reshapes the rocks particles along parallel planes which are different from the bedding. It is called flow cleavage and its best example is slate. Moreover, through the act of metamorphism fine-grained sediments such as shale are turned into Slates and Schists, coarse grained or crystalline rocks form Quartzite, Gneisses and Granulites and most of the rocks show foliation.

Types of Metamorphism:

On the basis of the agents which bring about the transformation in the rocks, the metamorphism can be divided into two types:

  • Contact Metamorphism
  • Regional Metamorphism

(1). Contact Metamorphism:

This type of metamorphism takes place due to the movement of hot molten magma (Lava). When the hot lava intrudes into the cavities, hollows or cleavage planes of rocks, it exerts pressure and hears up the surrounding rocks. High temperature brings about many changes. For example, it bakes limestone into marble and coal is turned into graphite.

(2). Regional Metamorphism:

It takes place due to the internal mountains building movements which exert great pressure on the rocks. Thus the entire shape of the rocks is changed. For example, shale becomes slate which then changes into schist. Due to mountain building movements, the submerged rocks are totally changed and become crystalline. Similarly, these submerged also bring back to the surface of Earth the deep rocks. Rocks such as graphite and quartzite are formed in this manner. Similarly, shale or muddy caly is turned into slates which have fine grains, Sandstones are changed into quartzite which are white colored but very hard in the texture.

Types of Metamorphic Rocks:

The calssification of metamorphic rocks is made upon the following basis:

  • Types on the Basis of Quality of Grains (Texture)
  • Types on the Basis of Fancies ( Features)
  • Types on the Basis of parentage
  • Types on the Basis of Mineralogical Assemblages
  • Types on the Basis of Arrangements of Minerals Grains

Types on Basis of Quality of Grains:

On the basis of quality of grains, the rocks are divided into following two categories:

  • Gneisses:

If the grains are coarse and the foliation is not fully developed then the metamorphic rocks are called gneisses. These are derived from granite.

  • Schist:

If the grains are fine and the rocks exhibits wavy hand but breaks along parallel planes. The schist is so altered that it is not possible to determine its previous form. The foliation is fully developed.

Types on Basis of Parentage:

If the metamorphic rocks have originated from the igneous rocks then these are called meta-igneous or ortho-metamorphic rocks. However, if the metamorphism rocks are formed from sedimentary rocks then these are given the name of Para-metamorphic or meta-sedimentary rocks. Thus the two categories are as under:

  • Ortho-metamorphic rocks
  • Para-metamorphic

Types on Basis of Mineralogical Assemblages:

The metamorphic rocks can also be categorized on the basis of their mineral assemblages. After the completion of the phenomenon of metamorphism, the assemblage of minerals is in the proper ratio with one another. Thus the degree of metamorphism also provides a solid ground for distinction.

Types on Basis of Arrangement of Grains:

The metamorphic rocks are divided into two major types on the basis of foliation:

(1). Non-foliated metamorphic rocks:
These rocks lack orientation and originate as a result of contact (Thermal) metamorphism. For example, Hornfels, quartzite, and marble.

(2). Foliated metamorphic rocks:
These rocks exhibit foliation in the form of bands. They are marked by the parallel arrangement of salty minerals. For example, schists, slates and gneisses.

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