Intrusive Igneous Rocks

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

Intrusive Igneous Rocks

  • They 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.


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