How are extrusive and intrusive rocks formed?

When the molten lava comes on the earth’s surface, it rapidly cools down and becomes solid. Rocks formed in this way on the crust are extrusive igneous rocks.

Sometimes the molten magma cools down deep inside the earth’s crust. Solid rocks so formed are the intrusive igneous rocks.

Intrusive Igneous Rocks

Intrusive igneous rocks cool underground. Deep in the crust, magma cools slowly. Slow cooling gives crystals a chance to grow. Intrusive igneous rocks have relatively large crystals that are easy to see. Intrusive igneous rocks are also called plutonic. A pluton is an igneous rock body that forms within the crust.

Granite is the most common intrusive igneous rock. Pictured below are four types of intrusive rocks.

Extrusive Igneous Rocks

Extrusive igneous rocks form above the surface. The lava cools quickly as it pours out onto the surface (Figure below). Extrusive igneous rocks cool much more rapidly than intrusive rocks. The rapid cooling time does not allow time for large crystals to form. So igneous extrusive rocks have smaller crystals than igneous intrusive rocks. Extrusive igneous rocks are also called volcanic rocks.

Intrusive Rocks

Igneous rocks which form by the crystallization of magma at a depth within the Earth are called intrusive rocks. Intrusive rocks are characterized by large crystal sizes, i.e., their visual appearance shows individual crystals interlocked together to form the rock mass. The cooling of magma deep in the Earth is typically much slower than the cooling process at the surface, so larger crystals can grow. Rocks with visible crystals of roughly the same size are said to have a phaneritic texture.

A body of intrusive igneous rock that crystallizes from cooling magmas beneath the Earth’s surface is called a “pluton”. If the pluton is large, it may be called a batholith or a stock. Minor plutons include dikes and sills. If a penetrating intrusion cuts across the geological layers it is called a dike. If it runs parallel to the layers, it is called a sill. If an intrusion causes the rocks above to rise and form a dome, it is called a laccolith.

Extrusive Rocks

Igneous rocks which form by the crystallization of magma at the surface of the Earth are called extrusive rocks. They are characterized by fine-grained textures because their rapid cooling at or near the surface did not provide enough time for large crystals to grow. Rocks with this fine-grained texture are called aphanitic rocks. The most common extrusive rock is basalt.