Geological Classification of Rocks

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Stones are obtained from the rocks and hence the knowledge of rocks is very essential in the study of stones. The rocks can be classified on the basis of its formation geologically. It can also be classified on the basis of its physical character, and chemical composition. In this article, we will talk about the geological classification of rocks.

Geologically, the rocks are classified as follows:

1.     Igneous rocks

2.     Sedimentary rocks

3.     Metamorphic rocks

Igneous rocks

The temperature inside the earth's surface is very high which results in the fusion of materials at ordinary pressure. The materials inside the earth's surface remain in the molten state at this temperature and pressure and known as magma. They try to come out from cracks or volcanoes. When this magma cools, forms igneous rock.

It is further classified as follows:

  • Plutonic rocks
  • Hypabyssal rocks
  • Volcanic rocks

Plutonic Rocks

These rocks are formed by the cooling of magma at a considerable depth from the earth's surface. The cooling at this depth is slow and hence, coarsely grained crystalline structure forms. This type of igneous rocks is commonly used in the building industries. Granite is one of the examples of plutonic rock.

Hypabyssal rocks

These rocks are formed by the cooling of magma at shallow depth from the earth's surface. Quick cooling takes place at this depth and hence, fine-grained crystalline structure forms. Dolerite is one of the examples of a hypabyssal 

Volcanic rocks

These rocks are formed by the cooling of magma at the earth's surface. very rapid cooling takes place at the earth's surface and hence, extremely fine-grained structure forms. These rocks contain a frequent amount of glass which is non-crystalline in nature. Basalt is one of the examples of volcanic rocks. 

Sedimentary rocks

Rocks formed by the deposition of products of weathering on pre-existing rocks. These products are carried away by frost, rain, wind, etc.

These types of rocks are generally classified into four categories:

  • Residual deposits 
  • Sedimentary deposits
  • Chemical deposits
  • Organic deposits 

Residual deposits

This is the product of the weathering of the rocks that remain at the site of origin.

Sedimentary deposits

The insoluble products of weathering which are carried away in suspension get deposited to form sedimentary deposits.

Chemical deposits

Some material, that is carried away in solution may get deposited by some physio-chemical processes such as evaporation, precipitation, etc, and give rise to chemical deposits.

Organic deposits 

This portion of weathering products gets deposited through the agency of organisms.

Metamorphic Rocks

The change in the character of pre-existing rocks with the temperature and pressure is known as metamorphism. The rocks thus formed are known as metamorphic rocks.

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