Metamorphic Rocks  

Rocks are the solid masses that occur naturally. They have a unique combination of minerals, chemicals, textures, shapes, grains, etc. These distinctive characteristics are vital for the categorization of the rocks into Igneous, Sedimentary, and Metamorphic rocks. Igneous rocks form by the solidification and cooling of lava or magma, with or without crystallization.

Basalt and Granite are examples of Igneous Rocks. Sedimentary rocks form by deposition of the sediments or debris on the Parent rock. Sandstone, limestone, and mudstone are examples of sedimentary rocks. The metamorphic rocks form due to temperature, pressure, and various chemical or physical changes. Marble is an example of metamorphic rocks.

What are Metamorphic Rocks?

Metamorphic refers to change to form or transform. In nature, various things go through metamorphism in order to transform. In Geology, metamorphic rock is the name given to those rocks that undergo a change or through the process of metamorphism. Metamorphism includes the change in mineralogy as well as the change in the fabric of the original or pre-existing rock.

The metamorphic rocks may form by igneous, sedimentary or other metamorphic rocks undergoing the process of metamorphism or physical changes due to factors like heat, pressure or chemical reactions. Usually, they are formed buried inside the Earth’s surface due to the pressure and temperature exercised on them by the rock layers above them.

The minerals of the original or pre-existing rock react with one another and thus produce a new mineral assemblage. This new mineral assemblage is thermodynamically stable under the new temperature and pressure conditions.

The metamorphic rocks are not pre-existing rocks like igneous and sedimentary rocks. These are igneous or sedimentary rocks that have gone through metamorphism and then transformed into metamorphic rocks. They have a crystalline nature, ribbon-like layers, and foliated texture usually. Though the metamorphic rocks do not crystallize like the igneous rocks, the high-temperature metamorphism may lead to partial melting of the parent rock.

Examples of metamorphic rocks are Marble, Slate, Granite gneiss, quartzite, and biotite schist.

Types of Metamorphic Rocks

The metamorphic rocks are on the basis of their textures are of the following two types:

1. Foliated metamorphic rocks

In Geology, foliation refers to a process of repetitive layering. The metamorphic rocks having layers as thin as sheets of paper or thick over a meter are known as foliated metamorphic rocks. These rocks form in the Earth’s interior when pressure and stress are in one particular direction. Due to foliation, the new minerals develop, and the minerals in the parent rock reshape. The foliated rocks are also known as banded rocks as the bands on them show the colours of the minerals that formed them. For the formation of foliated rocks, the pressure or stress needs to be differential but in one particular direction.

2. Non-foliated metamorphic rocks

Non-foliated metamorphic rocks form when the temperature is high but the pressure is relatively low. In these rocks, the minerals are crystallized and packed tightly together.

Types of Metamorphism

Metamorphism is of the following types:

1. Contact Metamorphism: Change due to the intrusion of the hot magma into cooler surrounding rocks is Contact Metamorphism.
2. Regional Metamorphism: The change due to large-scale tectonic movements of Earth’s lithospheric plates altering the pressure-temperature conditions of the rocks is Regional metamorphism.
3. Dynamic Metamorphism: Dynamic Metamorphism or cataclasis occurs mainly due to mechanical deformation and long-term temperature changes.
4. Hydrothermal Metamorphism: It occurs due to the extensive interface of rocks with high-temperature fluids. Thus, the chemical reactions take place due to the difference in the composition of the rocks and fluids. These high-temperature fluids may usually originate from Magma and circulate in the nearby crusts.
5. Burial Metamorphism: In burial metamorphism, rocks undergo uniform stress of lithostatic pressure but do not foliate. It usually occurs in the rocks that are buried deep in the sediments. Zeolite, a group of low-density silicate minerals, generally grows during the burial metamorphism.

Classification of Metamorphic rocks

The metamorphic rocks can be classified broadly as:

1. Schist: In these rocks, metamorphic minerals are easily visible by eye or hand and the mineral grains have a highly orientated fabric.
2. Slate: A slate is a very fine-grained rock. It usually forms from clay-rich sediments and exhibits perfect planer layering and slaty cleavage. It is rich in micas and chlorites.
3. Gneiss: It occurs due to intense metamorphism at high pressure and high temperature. The grain size in Gneiss is coarser than schists. Also, the layering is well developed and the mineral orientation is not so perfect as in schists.
4. Hornfels: These form due to the contact metamorphism and thus show little sign of directed pressure. These are fine-grained rocks and crystals display little orientation in them.
5. Marbles: Marbles form due to the metamorphism of carbonate sediments that contain calcite or dolomite. The grain size increases as a consequence of metamorphism.
6. Mylonites and cataclastic: These rocks exhibit only slight development of minerals. Their texture is due to the ductile shearing or mechanical shattering of grains.

FAQs about Metamorphic Rocks

Q.1. Where are metamorphic rocks found?

Answer. The metamorphic rocks are usually found in the regions where metamorphic processes are most intense and active. The region of the Pacific margin is an area where these are formed intensely due to the seismic and volcanic activity there. The margins of continents and regions of mountain building are also the places where these rocks are intensely found. Places, where sediments accumulate, are also places where metamorphic rocks can be found. Thus, they are distributed throughout the geologic column.

Q.2. What amount of heat and pressure is required for the formation of metamorphic rocks?

Answer. When the original rock gets exposed to temperatures greater then 150 degrees to 200 degrees Centigrade and a pressure of 100 megapascals or more, it causes physical and chemical changes in the original rock. Thus, the metamorphic rocks are formed.

Q.3. Name the minerals that can be found in the Metamorphic rocks.

Answer. Chlorite, Garnet, Epidote, Staurolite, Kyanite and Sillimanite are the minerals that occur in the metamorphic rocks.

Q.4. What are the uses of metamorphic rocks?

Answer. The most commonly used metamorphic rocks in our daily life are Quartzite and Marbles. Marbles can be used as decorative stones, for flooring, for carving statues, etc.