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Describe the nature and mode of origin of the chief types of rock at the earth’s crust


Describe the nature and mode of origin of the chief types of rock at the earth’s crust. How will you distinguish them?


Earth's crust is the top layer of earth in solid form. It is made up of three types of rocks.

  • Igneous rocks
  • Sedimentary rocks
  • Metamorphic rocks

Igneous rocks:

Mode of origin of Igneous rocks:

It is primary rock and made up of the cooling of molten magma. 

Nature of Igneous rocks:

  • It is very hard and not get weathered easily.
  • Igneous rocks are granular and they do not have layered like sedimentary rocks.
  • They do not contain any fossil remains.
  • For example, Granite, gabbro, basalt, etc.

Sedimentary rocks.

Sedimentary rocks are secondary rocks.

Mode of origin of sedimentary rocks:

Sedimentary rocks are formed by the lithification process of sediments.

Nature of sediments rocks:

  • It is not as hard as Igneous rocks.
  • It gets weathered easily.
  • It has a layered structure and contains fossils of plants and animals.
  • For example, sandstone, limestone, coal, potash, etc.

Metamorphic rocks:

Metamorphic rocks are also secondary rocks as it is derived from Igneous rocks and sedimentary rocks.

Metamorphic means "change of form".

Mode of origin of metamorphic rocks:

Under the action of PVT( pressure, volume, and temperature) and changes of PVT, a new form of rocks emerged which has different property than igneous and sedimentary rocks are called metamorphic rocks.

Nature of metamorphic rocks:

  • Due to PVT, the rock gets recrystallized and minerals or grains get arranged in layered or lines; such arrangement is called foliation or lineation.
  • Foliation or alienation, and banding are important features of metamorphic rocks.
  • For example, granite, slate, marble, etc.
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