A rock is an aggregation of different mineral constituents which form the earth’s crust. Different types of rocks possess different engineering properties that make them suitable to be used in various construction works. The classification of rocks based on different factors with examples is discussed in this article by Pritish Kumar Halder. 

Classification of Rocks

Rocks are classified based on three major factors as follows :

  • Geological classification
  • Physical classification
  • Chemical classification

classification of rocks

Geological classification

Rocks are classified into three types based on their geological formation and they are :

  • Sedimentary rocks
  • Igneous rocks
  • Metamorphic rocks

Sedimentary rocks

Sedimentary rocks are formed by the deposition of sediments obtained by the weathering of pre-existing rocks and these sediments are transported by various agents such as water, wind, frost, gravity, etc. These transported sediments form layered structures and give rise to the sedimentary deposits.

If the sediments remain at the place of origin then the formed deposits are known as residual deposits. Some sediments formed by various chemical reactions such as decomposition, precipitation, evaporation, etc. give rise to the formation of chemical deposits. Similarly, the sediments formed by the action of various organisms such as plants and animals are known as Organic deposits.

Examples: Sandstone, limestone, lignite, etc.

Igneous rocks

Igneous rocks are formed by the solidification of magma below the earth’s surface. When the magma is unable to erupt through the earth surface during its upward journey, it is held up below the earth’s surface and unable to descend. This magma cools down gradually and solidifies into igneous rocks.

The structure of igneous rocks varies according to the depth at which magma solidified. If the magma hardens at a significant depth from the earth surface, then the rocks possess coarsely grained crystalline structure and these rocks are known as plutonic rocks. Granite is the best example of plutonic rock.

Similarly, if magma hardens at shallow depth from the earth’s surface, the finely grained crystalline structure of rock will be obtained. These rocks are called as hypabyssal rocks. Dolerite is an example of a hypabyssal rock.

If the solidification of the rock occurs near to the earth surface, then the rocks obtained are known as volcanic rocks. These rocks possess extremely fine-grained structure. Basalt is an example of volcanic rock.

Metamorphic rocks

Metamorphic rocks are formed by the metamorphism process. Metamorphism is the process of changing the characteristics of the pre-existing rocks under the influence of heat and pressure. The pre-existing rocks may be of the sedimentary or igneous type of rocks.

Examples: Slate, Gneiss, Schist, marble, soapstone etc.

Physical classification

Rocks are classified physically into three types as follows :

  • Stratified rocks
  • Unstratified rocks
  • Foliated rocks

Stratified rocks

Stratified rocks consist of different layers in its structure and these layers are separated by planes of stratification. These planes are also called cleavage planes or bedding planes. These rocks can easily split up along these bedding planes. Most of the sedimentary rocks such as sandstone, limestone, shale, etc. are the best examples of stratified rocks.

Unstratified rocks

The structure of unstratified rocks is crystalline or compact granular. They possess a similar kind of structure throughout their whole body. Most of the igneous rocks and some sedimentary rocks come under unstratified rocks.  Granite, marble, trap are few examples of Unstratified rocks.

Foliated rocks

Foliated rocks possess a layered or banded structure which is obtained by exposure of pressure and heat. Unlike the stratified rocks, these rocks can split up in a certain direction only. Most of the metamorphic rocks formed by metamorphism come under foliated rocks. Some examples are gneiss, schist, slate etc.

Chemical classification

Rocks are classified into three types based on their chemical composition and they are as follows :

  • Argillaceous rocks
  • Calcareous rocks
  • Siliceous rocks

Argillaceous rocks

The word Argil means clay. Hence, the rocks in which clay content is predominant are called argillaceous rocks. These rocks are soft in nature and with the presence of water they can be crumbled easily. In the dry state, these rocks can be crushed easily because of their brittleness. Shale, slate, laterite, etc. are some of the argillaceous rocks.

Calcareous rocks

The rocks in which calcium carbonate is the major ingredient are known as calcareous rocks. These are generally hard but their durability is dependent on surrounding constituents which may react with calcium and affect the durability of rock. Marble, limestone, dolomite, etc. are some of the calcium predominant rocks.

Siliceous rocks

The rocks which contain silica in predominant amount are called as siliceous rocks. Presence of a large amount of free silica makes them harder and durable. It also provides strong resistance to weathering. Granite, chert, quartzite, etc. are examples of siliceous rocks.