Extrusive and Intrusive Igneous Rocks
Extrusive Igneous Rock:
When magma gushes out on the earth’s surface, it comes in contact with air and cools and solidifies very fast to form Extrusive Igneous Rocks. They are also called Volcanic Rocks as they come out on the earth’s surface through either volcanic or fissure eruptions. As these rocks cool rapidly they become fine grained and non crystalline, e.g., basalt, rhyolite, andesite. The Deccan Trap in Gujarat and Maharashtra and Rajmahal hills of Jharkhand are made of basalt rocks.
Extrusive rocks are further subdivided into:
- Lava Rock: Either through volcanic or fissure eruptions, in the earth’s crust lava comes in contact with air and cools as well as solidifies rapidly. Most of the extrusive rocks are of this type, e.g., basalt.
- Pyroclastic Rock: Greek word ‘Pyro’ means ‘fire ‘and `klastos’ means ‘broken pieces’. Solidification of lava, along with the fragments ejected during previous eruption— like ash, cinder and other materials of fiery origin that are blown off from the vent and the crater of the volcano, forms rocks which are called Pyroclastic Rocks, e.g., tuff.
Intrusive Igneous Rocks:
Often magma solidifies at some depth in the earth’s crust to form Intrusive Igneous Rocks.
Intrusive Igneous rocks are further subdivided into:
- Plutonic Rocks: These rocks are named after ‘Pluto’, the ancient Roman God of the underworld. These rocks are formed by the solidification of magma at a great depth in the earth’s crust where in the absence of air cooling is very slow. Therefore, large crystals are formed. Hence, Plutonic rocks are also called crystalline rocks. e.g., granite, gabbro. Plenty of granites are found in Chotanagpur plateau of Jharkhand and in Purulia district of West Bengal.
- Hypabyssal Rocks: When magma solidifies at shallow depth in the earth’s crust, Hypabyssal Rocks are formed. In the presence of little air, cooling is moderate. So, medium size crystals are formed. Hence, these rocks are also called semi crystalline rocks e.g., dolerite, porphyry. Dolerite is found in the Singbhum district of Jharkhand and in some parts of South India.
– Rahul Chopra.