HOW TO CLASSIFY ROCKS BY ORIGIN OR GENESIS?

Rocks are classified into three major groups on the basis of origin or mode of origin.

  1. igneous rocks
  2. sedimentary rocks
  3. metamorphic rocks
igneous rocks
igneous rocks

1. igneous rocks

These types of rocks are formed by the solidification of molten magma in the Earth’s interior.

When a rock is formed by the cooling and solidification of magma beneath the Earth’s surface, it is called plutonic igneous rocks. Due to the relatively slow rate of solidification, plutonic igneous rocks have a coarse-grained structure.

If rock is formed by the solidification of lava on the Earth’s surface, the rate of cooling is rapid and rapid solidification occurs. This type of rock is known as volcanic igneous rocks. Volcanic igneous rocks have a fine-grained structure.


Examples of igneous rocks are granite, dolerite, basalt, rhyolite, etc.

sedimentary rocks
sedimentary rocks

2. sedimentary rocks

These types of rocks are formed due to the weathering and decomposition of the earth’s crust or from any type of rock.

When a rock on the earth’s crust is weathered or disintegrated and transported and re-deposited, and later partially or completely consolidated and cemented, the new product is known as sedimentary rocks.

Weathering agents act on a pre-existing rock surface which may be igneous, sedimentary or metamorphic. These weathering products later become components of new sedimentary rocks. The weathering agents involved are rain, frost, wind, temperature, river, sea etc.

The most important sedimentary rocks from an engineering point of view are sandstone, shale and limestone.

metamorphic rocks
metamorphic rocks

3. metamorphic rock

Due to high pressure, high temperature as well as high shear stress on existing igneous or sedimentary rock masses, beneath the earth’s crust, the rocks recrystallize and the resulting masses are known as metamorphic rocks.

The principle agents of high temperature and pressure involved in recrystallization may be the following:

  • Earth’s speed and pressure
  • liquids and gases, mainly water
  • Heat

Due to the effect of the above factors, new minerals are formed. As if

  • limestone turns into marble
  • sandstone turns into quartzite
  • Shell turns to slate

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.