Igneous rocks are the product of consolidation (cooling and crystallizing) of hot molten material (magma; temperature ranges from 600 to 1300°C) i.e. the molten silicates which underlie the solid crust of the Earth. A very high temperature and a molten state are, therefore, two very important conditions for the original material from which the igneous rocks are believed to be formed.
Igneous rocks are divided into 3 sub- groups:
- Volcanic rocks: These are the igneous rocks formed on the surface of the Earth by cooling and crystallization of lava erupted from crystallization of erupted lava from volcanoes. Since the lava cools down at very fast rate (compared to magma), the grain size of the crystals formed in these rocks is very fine often microscopic.
Example – The Deccan Traps of India, Panjal Traps of Kashmir Himalayas.
- Plutonic rocks: The magma forming at depth remain arrested to their site of formation i.e. near the magma chambers and because of this fact such chambers undergoes veru slow cooling resulting in the formation of very course grained rock, known as plutonic rock.
Example- Granite, Gabbro, Syenite etc.
- Hypabyssal rocks: Some magma during its ascent starts solidifying within the fractures or layers of rocks at certain depths little below the surface, generally upto 2 kms below the surface of the earth. The rate of cooling of such magma bodies, which are not exposed to the surface is considerably sluggish. These rocks exhibit mixed characteristics of volcanic and plutonic rocks. The Hypabyssal rock, which crosscut the regularly layered bodies are called dykes whereas the rocks which occur parallel to some layering in rock are called sills.