Metamorphic rocks – Types and Characteristics
Introduction: Metamorphic rocks are formed by the transformation of pre-existing rocks (igneous, sedimentary) to a new type of rock and then followed by the process called metamorphism.
The pre-existing rock is subjected to high temperatures (greater than 150 to 200 °C), and high pressure, which causes physical and chemical changes in the earth’s crust. Example of metamorphic rocks is- Gneiss, marble, slate, schist, etc.
Metamorphic rocks make up a large part of Earth’s crust and form 12% of the Earth’s land surface.
The Process of Metamorphism:
In this process, the rocks transformed into denser, more compact rocks referred to as metamorphic rocks. By rearrangement of components or by reaction with fluids in rocks the new mineral is created. The pressure and temperature can change previously metamorphosed rocks into new types of rocks. The metamorphic rock formed by metamorphosis is squished, smeared out, and folded.
These rocks may be classified as:
- Common Metamorphic Rocks: Includes phyllite, schist, gneiss, quartzite, and marble.
- Foliated: Granite and biotite schist, are strongly banded or foliated metamorphic rocks. (Repetitive layering in metamorphic rocks)
- Non-Foliated Metamorphic Rocks: These rocks do not consist of platy or sheet-like structure. Limestone is not in a flat or elongated shape. Example – marble, quartzite, greenstone, etc.
Types and examples of Metamorphic rocks:
|Type of Metamorphic Rocks||Example|
It is a very common and widely distributed metamorphic rock. The gneiss is formed due to high temp. and pressure, the result of high-grade regional metamorphism.
Ortho gneiss is gneiss derived from igneous rock. Para gneiss is gneiss are metamorphic rock derived from sedimentary rock.
Marble is formed by the metamorphosis of Limestone. It is produced under the condition of thermal metamorphism.
It is a cleaved Metamorphic rock having affinities with both slate and mica–schist.
Quartzite is an arenaceous rock. It is formed when Quartz rich sandstone or chert has been exposed to high temperature and pressure. In this condition, the quartz grains get fused and a hard, dense irregular rock formation takes place.
Schist is well foliated, with a higher degree of metamorphism, it is medium to coarse-grained metamorphic rock.
the slate is produced by the low-grade metamorphism of shale. It is a very fine-grained metamorphic rock. Its characteristics are excellent foliation called slaty cleavage.
Characteristics of Metamorphic Rocks:
- These rocks were once igneous or sedimentary rocks, these are changed or metamorphosed due to intense heat and pressure within the earth’s crust.
- They are crystalline and often possess foliated textures.
- After metamorphosis, some rocks become harder than their original forms, such as diamonds from carbon, Quartzite from sandstone, and marble from limestone.
- The metamorphic rocks don’t contain fossils because the fossils of original sedimentary rocks were destroyed during metamorphosis.
- These rocks are found in a wide range.
- The imperfectly foliated are coarse-grained metamorphic rocks. ex. gneisses
- The fine-grained metamorphic rocks are perfectly foliated. eg. Schist from Shale.
- Most gneissic metamorphic rocks consisting bands of granular quartz and feldspar.
- Slates are used in construction, especially as the roof shingle.
- Some quartzite is used as dimension stones, slabs for flooring, walls, stairs, and for road aggregate.
- Marble is also used in construction and for sculpture formation.
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