Classification of Igneous Rocks

The igneous rocks are also known as primary rocks as they form the basis of the formation of other types of rocks. Igneous rocks are formed because of the cooling, solidification, and crystallization of molten material known as magma. Classification of Igneous Rocks is discussed below

Classification of Igneous Rocks

Igneous rocks may be classified into extrusive igneous rocks and intrusive igneous rocks.

Extrusive Igneous Rocks

  • Extrusive igneous rocks are formed by cooling molten magma on the surface of the Earth. The molten magma comes on to the Earth’s surface through cracks, fissures and volcanic eruption.
  • They are also known as volcanic rocks. Because these rocks solidify at a faster pace, they are smooth, crystalline and fine grained.
  • Basalt is a common example of extrusive igneous rock.

Intrusive Igneous Rocks

  • Intrusive igneous rocks are formed when magma cools and solidifies below the surface of the Earth.
  • Because these rocks cool down slowly, they have coarse texture with large crystals. Dolomite and granite are some examples of intrusive igneous rocks.

Types of Intrusive Igneous Rocks

There are different types of intrusive igneous rocks such as batholiths, laccoliths, dykes, and sills.

Batholiths: Batholiths are formed because of the cooling and solidification of magma below the surface of the Earth. They are generally dome-shaped. They have been exposed to the surface of the Earth because of the action of the agents of erosion.

Laccoliths: When magma is not fully able to reach the crust of the Earth, it solidifies just below the crust. The upper layers of laccoliths are dome-shaped, but the bottom is almost flat.

Sills: When magma is forced to rise, it pushes itself between two layers of rocks and solidifies. The sills are horizontal rocks.

Dykes: When the magma is forced in the upward direction, it fills the cracks and fissures in the existing rocks. It solidifies in these cracks forming dykes. Dykes are often vertical or slanting in position and are often an offshoot of rock.

Necks: When the passage of an extinct volcano is filled with magma, it solidifies. This is known as the volcanic neck or plug.


Classification on the basis of Chemical Composition

Acid Igneous Rocks: These rocks have between 65% and 85% of silica but generally lack in iron and magnesium and so are light. Example: Granite

Basic Igneous Rocks: These rocks are generally heavy and dark. Example: Basalt


Also, Read Classification of Sedimentary Rocks