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Metamorphic Rocks

The name of metamorphic rocks is from the Greek word ‘Meta’ which literally means change and ‘morphs’ which means form. A metamorphic rock changes whenever it is exposed to a physical condition which wears it down gradually. These changes occur when these rocks are exposed to an environment which is newer to the one these are originally from.

Metamorphic rocks form when rocks changes over a period of time due to lot of physical factors like heat, pressure and chemical activity. The minerals and rest of rock features go through a lot of changes when these rocks undergo changes due to heat, pressure and the chemical reaction. 

Metamorphic Rocks

Metamorphism or metamorphic rocks processes and mineral transformations in rocks happen at a very high temperature and pressure which are fundamentally combined with the chemical reactions. The most dramatic form of changes that occur from sedimentary form of rocks into metamorphic rocks is by addition of heat during the formation of mountain by huge volume of magma in the crust. 

 

Definition

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Metamorphic rock definition allows understanding the process of metamorphic rock formation in brief. The metamorphic rocks form when sedimentary form or igneous rocks go through certain physical process like heat changes, pressure exposure and tectonic plate movement at plate edges. In one word, metamorphic rocks are rocks which has gone through changes or going through changes before they arrive at the present form. 

How are Metamorphic Rocks Formed?

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Most of the minerals in any rock forms expand when these are heated and cause the atoms inside to expand and weaken the bond holding the molecules together. Beyond a particular temperature the minerals melt as the bond holding these breaks. Meanwhile in metamorphosis process some of the atoms break while some retain the bond which result only a few of these melting and forming new molecules or metamorphic minerals.

This result in changing both the chemicals composed as well as rock structure. 
Pressure effect on minerals is quite opposite to that of heat. Pressure forces the minerals to combine and close together. The stress these rocks go through enables them to break apart or rearrange into a very compact form. The pressure is due to the deep burial or due to movements in Earth’s crust. Unlike heat, the pressure deep inside Earth is same all across and that helps in uniform changes in rock forms.

The chemical activity in rock also helps in changing its form and hence is a major cause of metamorphosis. The dissolved ions in rock minerals move through the molten matter and form newer molecules. Once the magma cools down the molten matter in surrounding rocks also forms new structure and composition. In some cases the water also helps in forming new minerals as in case of olivine which reacts with water to form talc.

Types of Metamorphic Rocks

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• Metamorphic rocks could be classified into two main categories although there are hundreds of different types of metamorphic rocks. The most abundant ones are foliated and un-foliated rocks. These are on the basis of either presence or absence of foliation. 

• Metamorphic rocks could be classified into seven classes which are arranged according to the complexity of chemical molecules. 

• These are ultramafic rocks, carbonate rocks, Pelites or shale, Marls, Mafic rocks, Quartzo and Feldspathic rocks, and finally the bulk compositions. Out of these forms the politic rocks are the most common type of sedimentary forms and their metamorphic equivalents are called Metapelites. 

Foliated Metamorphic Rocks

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• These rocks are more common and could be the product of regional metamorphism and is one of the more important characteristics of dynamic thermal metamorphism where all the stress directed tend to develop various types of planar elements.

• Schistosity develops in intense metamorphic rock form where the mica and chlorite minerals grow into visible crystals and develop the foliation. In origin these minerals grow in a metamorphic zone where it attains a fine grained texture and grow foliation cleavage known as slaty cleavage. 

• Gneissic foliation due to high grade metamorphosis which develops altering layers of light and dark minerals that get re arranged. 

• Non foliated homogeneous rocks are of 2 types which are basically thermal metamorphic rocks while the second type develops only if the newly deformed metamorphic minerals are qui dimensional with no basic orientation. Example: limestone, sandstone and dunite. 

How are Metamorphic Rocks Classified?

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The rocks could be classified into three genetic groups, and although there is definitely a gradation among them.

1) Thermal or contact metamorphic rocks found at the margins of the intrusive form of igneous rock such as batholiths. 

2) Dynamic metamorphic rocks resulted from the breaking and grinding of rocks.

3) Regional metamorphic rocks have large occupying areas and is deep inside the crust but come up only during tectonic movement and get eroded to get exposed.

Metamorphic Rock characteristics

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1) A few characteristics of metamorphic rock are as follows: the Cataclastic metamorphism occurs only along the tectonic plate faults where the rocks brush against each other. This is the only type of metamorphism which results in the grain size reduction. 

2) The transformations of metamorphic rocks are classified as one which is low grade and could form non foliated rocks. 

3) The rock circulation results in huge number of mineral water chemical reactions which results in all kinds of precious stones and metals.

Sedimentary, Metamorphic & Igneous Rocks

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All of these forms of rocks conceive the rock cycle and are interrelated where each of these forms of rocks precede or succeed each of the other. These forms are either pushed down into the crust along the fault ridges. While the rocks come up in form of magma and then solidify again to form the different rock types. The igneous rocks leads to metamorphic rocks which further leads into sedimentary rock forms during these rock cycle.

Metamorphic Rock Examples

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• The metamorphic rock examples are as follows: marble, quartzite, slate, phyllite, schist, and gneiss.

• Marble is originated from either dolomite or limestone.

• Quartzite is derived from sandstone and has coarse grainy surface.

• Slate is basically a metamorphic rock which is finely grained.

• Phyllite is a finely grained metamorphic rock.

• Schist is a metamorphic rock but has coarse grained surface.

• Gneiss is a metamorphic rock type which has medium to coarse grain.
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