Metamorphic Rocks

The dense form of a sedimentary or igneous material is known as Metamorphic Rock. The metamorphic rocks develop when the pressure and heat alters the chemical and physical composition of the existing rock. The pressure under the deep Earth's crust causes the formation of these rocks. The pressure arises from the friction or collision of the tectonic plates or by the magma’s heat. The soil erosion and uplift causes the Metamorphic rock to rise above the surface. The study of metamorphic rock helps to understand the pattern of heat and pressure formed under the earth’s crust. Examples of Metamorphic rocks are Slate, Schist, Quartzite, Gneiss, Marble and many others.

The process in which the metamorphic rocks are formed is known as ‘metamorphism’, meaning “change in form”. The temperature required to undergo the process is above 150 degree Celsius under a pressure of about 1500 bars.

Types of Metamorphic Minerals

Similar to the metamorphic rock, the minerals are also formed under great pressure and temperature inside the earth’s crust. The metamorphic pressure develops many minerals like Kyanite, Garnet, Sillimanite, Andalusite and Staurolite. These specific minerals are also referred as index minerals.

Other than these minerals, some other minerals are formed on the metamorphic rocks due to the crystallization process. These minerals are Pyroxenes, Micas, Quartz, Amphiboles, Feldspars and Olivines. They remain unchanged during the metamorphic process, and remain chemically stable even if the pressure or temperature rises. But every mineral is stable only till a specific limit.

Metamorphism process

There are mainly 4 agents that carry on the metamorphism process to produce the Metamorphic rocks, namely heat, pressure, fluids and strain. Pressure and heat are directly related and hence work simultaneously in the formation of these rocks. As the depth of the crust increases, you can find the temperature and pressure to increase. The high pressure and temperature result in the breakdown of the minerals on the rock surface. The new minerals thus formed are found stable in the existing condition. The Fluids also play an important role in the metamorphism process. When the sediments convert into rocks, the water gets accumulated in the rocks, which turns into a liquid mineral form, consisting of metal compounds, silica, carbonates or sulfides. This water content interacts with the rocks while escaping to other places, which causes the rocks to change chemically. This process is called as metasomatism. Strain also changes the physical shape of the rock by producing force of stress. This often happens in the shallow rocks, where the sheer forces create stress, which crush and grind the rocks and the minerals. This causes the mineral to release cataclasite. Thus the different types of metamorphism processes produce different types of minerals. The crystallization process can destroy the rock texture and the fossils present in the rock, whereas the original composition of the rock gets completely changed by the metasomatism process.

Texture of Metamorphic rock

The various types of metamorphism processes form different types and textures of rocks. There are basically five types of textures found in the metamorphic rocks, namely, Slaty, Schistose, Gneissose, Granoblastic and Hornfelsic. The slaty texture is found on the Slate and Phyllite rocks, and their type of foliation is known as “slaty cleavage”. The texture found on the Schist rock is known as the schistose type, and it possesses schistosity foliation. The gneissose texture is found on the Gneiss type of rocks, and it has a gneissosity foliation. The granoblastic texture is seen on some of the Marbles, Granulite and Quartzite, and the hornfelsic type are founded on Skarn and Hornfels rocks.

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