3.1.1. Igneous Rocks

The word igneous is derived from the Latin word ‘ignis’ meaning fire. These rocks are of thermal origin and are associated with volcanic eruptions. These rocks have been formed due to solidification of hot and molten material called magma.

It is believed that at the time of its birth the Earth was in a molten state. The igneous rocks were the first to be formed as a result of the solidification of the outer layer of the Earth. Thus, igneous rocks are also known as the primary rocks. They can be divided into two types— intrusive1 igneous rocks and extrusive igneous rocks.

Igneous rocks that cool below the surface of the Earth are called intrusive igneous rocks. The rate of cooling is slow inside the Earth. Thus the crystals formed on cooling are large. Two common examples of intrusive rocks are dolerite and granite.

Igneous rocks that cool on the surface of the Earth are called extrusive igneous rocks. These rocks are also known as volcanic rocks. Due to rapid cooling, the crystals are fine grained such as in basalt.

On the basis of their composition the igneous rocks are also classified as acidic Igneous Rocks and Basic igneous rocks. In Acidic Igneous rocks silica content in rocks is more than 65 per cent. These rocks are light colored and have less density. These are also known as ‘Silicic rocks’. Granite and rhyolite are examples of these rocks. In Basic Igneous rocks the silica content is less than 65 per cent. They are composed predominantly of ferromagnesian minerals (rich in iron and magnesium). They are dark coloured and dense. Gabbro and basalt are basic rocks.

1 Igneous rock bodies will be discussed in chapter on volcanoes.