Igneous rocks Characteristics & Economic Importance

In this blog article we have discussed about the igneous rocks characteristics, along with types of igneous rocks, definition & Economic importance of igneous rocks…

Table of Content

  • Igneous Rocks
  • Types of Igneous Rocks
  • Characteristics of Igneous Rocks
  • Characteristics of Igneous Rocks
  • Economic Importance of Igneous Rocks
  • Igneous Rocks at a Glance





Igneous Rocks


After the volatile state when earth took its present solid state the igneous rocks were the first to be formed. Later other types of rocks were formed from these. It is why igneous rock are also known as primary rocks. The igneous rocks continue to be formed even today. The lava which oozes out during volcanic eruptions on the surface of the earth gets cooled or sometimes cools deep inside the earth in the form of magma and solidifies are known as Veinus igneous rocks. The word ‘igneous’ is derived from the Latin word ‘ignis’ meaning ‘fire’ (The Hindi translation of ‘igneous’ too is derived from fire). These types of rocks are thus formed with the help of ‘fire’.




Figure: Forms of Igneous Rocks


Types of Igneous Rocks

According to Location

(1) Extrusive Igneous Rocks: Sometimes during volcanic eruptions magma is poured out on to the surface in the form of lava. After cooling and solidification it forms the extrusive rock. These types of rocks are also called Volcanic rocks.

On the surface of the earth lava spreads out and cools quickly. Due to sudden cooling and solidification there appear very small crystals. Sometimes the crystals of these rocks are very thin. Then these rocks look like glass. Best example of extrusive rocks is Basalt.

In the north-western part of peninsular India basaltic rocks are found over an area of about five lakh square kilometers. This area is famous as Deccan Trap.

(2) Intrusive Igneous Rocks: At times the magma while rising from deep inside the earth does not reach the surface but settles somewhere below the surface. It then cools and solidifies. These intrusive rocks made deep inside the earth are of two types:


(i) Plutonic Rocks: These rocks are made in a slow process very Extrusive Igneous Rock: (Volcanic rock) – Rocks with small deep inside the earth. These types of rocks have, therefore, very large crystals and are known as Plutonic Rocks. Granite is the best example of this type of rock. Other examples are gabbro, peridorite and diori. Regional Distribution: These types of rocks are spread over parts of Deccan plateau of South India, Chhotanagpur plateau and in parts of Rajasthan and the Himalayas.


(ii) Hypabyssal Rocks: When the magma is unable to reach the surface, it then intrudes into cracks and lines of weakness of the layered sedimentary and other rocks and gets solidified after cooling. The rocks thus made are known as Hypabyssal rocks. The crystals of these rocks are of medium size. They consist of well-formed conspicuous crystals (Phenocrysts). Dolarite and mica-pegmatite are examples of these rocks. They are found in the form of sill and dyke.


Types of Igneous Rocks on the basis of their Composition

On this basis igneous rocks are of two types:

(1) Acid Igneous Rocks: In these rock silica content in rocks is more than 55 per cent. Igneous rocks have low density & is light colored. These are also known as ‘Silalic rocks’. Example: Granite and rhyolite

(2) Basic and Ultra-basic: In these types of rocks the silica content is less than 55 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 and ultra basic rocks also referred to as ‘Simalic’.


Igneous Rock Bodies

The magma cools and gets solidified deep inside the earth in different shapes and sizes. This process gives birth to varied igneous rock bodies. They are named with reference to their shape, size, condition and surrounding rocks. These rock bodies are shaped by intrusive igneous rocks. These types of rocks include-batholith, stock, sill, dyke.

(1) Batholith: It is the largest rock body formed by intrusive igneous rocks. These bodies are hundreds of kilometers long and 50 to 80 kilometers wide. Their thickness is almost unknown. Their bases are never visible. Only the upper surface worked by denudation or removed by cutting and blasting becomes visible. Their upper surface is irregular and dome-shaped. They are made of granite masses. The granite mass of the plateau of Ranchi in Chhattisgarh and Irinpura granite of Rajasthan are examples of Batholiths.

(2) Stock: A batholith of smaller size is called stock. Their upper surface is also dome-shaped. Their other features are like batholiths. A stock is often less than 100 km in area.

(3) Laccolith: This is also a huge mass of igneous rock. When the magma rises from deep inside the earth but does not reach the surface, it intrudes into folded layers of sedimentary rocks. It cools and solidifies there in dome-shaped form. Below it lies the dyke. Laccoliths are formed at a lower depth below the surface. It is regarded as a type of miscarried volcano when the magma failed to break through the surface. With the passage of time their upper layer is removed by the agents of denudation. They can, therefore, be seen from far away like a dome. One can find a number of laccoliths on the Henry Mountains of Uttah in the United States.

(4) Sill: Sometimes the magma while trying to reach the surface of the earth squeezes between two parallel rock layers inside the earth. It cools and solidifies into thin horizontal sheets, between the bedding planes. This form of intrusive igneous rock is known as sill. These types of Sills can be found in the coal-fields of Jharkhand and Chhattisgarh. The Great Whin Sill in Great Britain is another striking example of this type of igneous rock body.

(5) Dyke (Dike): When near vertical rock bodies are formed due to solidification of magma inside the earth, these ‘walls’ of rock are called dykes. They may range from a few metres to kilometers in length. They are very hard. They are not easily eroded. When the surrounding rocks are eroded or carried away, dykes stand out as great walls. Hundreds of parallel dykes can be seen on the surface of the earth in Scotland. One can also find these dykes in the Singhbhum district of Jharkhand.


Characteristics of Igneous Rocks

(i) These are crystalline rocks. The crystals are smaller when magma cools quickly and they are very large when it cools slowly.

(ii) Many mineral substances are found in these rocks. Among the chief minerals found are magnetic iron. nickle, copper, lead, zinc, chromite, manganese and precious minerals like diamond, gold and platinum.

(iii) They are not layered but fissures and cracks can be found.

(iv) They don’t have pores so they don’t allow water to seep through.

(v) They don’t contain the remains of living beings.

(vi) They are less affected by the agents of denudation.


Economic Importance of Igneous Rocks

(i) They are a reservoir of minerals.

(ii) Economically important minerals are found in these rocks-Magnetic iron, nickel, copper, lead, zinc, chromite, manganese, tin.

(iii) Rare minerals like gold, diamonds, platinum are also found in these rocks.

(iv) Basalt and granite are used for construction of buildings and roads.


Igneous Rocks at a Glance

(1) They are made by solidification of lava or magma.

(ii) They are of two types-Extrusive and Intrusive

(iii) Extrusive-Solidification of lava on the surface of the earth, Basalt.

(iv) Intrusive: Formed inside the earth. Two types: (i) Plutonic – Large crystals – Granite Hypabyssal Medium crystals -Dolerite.

(v) Based on composition-two types – Acid Igneous (Granite) Basic Igneous (Basalt).

(vi) Igneous rock bodies – Batholith, Laccolith, Stock, Sill, Dyke.

(vii) Characteristics – Crystalline, Granular, Containing mineral substances, no layers, no pores, Don’t contain remains of living things (without fossils), hard, very little effect of denudation.

(viii) Economic Importance – Reservoirs of minerals, majority are metallic minerals, precious metals – gold, platinum, Diamonds, road and building construction materials, fertile regur soils.



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