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Classify the igneous rocks and give a detailed description of them.

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Igneous rocks are the first to be formed, thus, they are also called Primary Rocks. The lack of fossils is found in these rocks. On the basis of the composition of minerals, texture, formation of particles, shape and formation place of the igneous rocks, they are classified as follows:

(I) Classification of Igneous Rocks on the basis of formation – place: 

(i) Intrusive Igneous Rocks: Rocks which are formed due to the accumulation of magma beneath the earth’s surface are called Intrusive Igneous Rocks. 

These rocks are further classified into two parts:

. Plutonic Rocks 

2. Hypabyssal

1. Plutonic Rocks: These rocks are formed when the magma cooled down and solidified at too much depth beneath the earth’s surface. Granules of big size are found in these rocks. The main example is Granite. 

2. Hypabyssal Rocks: Rocks formed when the magma is solidified in the cracks at some depth beneath the earth’s surface are called Hypabyssal Rocks. These rocks are presented with the help of the following fig.

(ii) Extrusive Igneous Rocks: These type of rocks are formed when the lava erupted above the earth’s surface cooled down and solidified. 

The size of granules is small in these rocks.

(II) Classification of Igneous Rocks on the basis of chemical composition :

1. Acidic Rock: Rocks in which the amount of silica is more than 65% are included in the category of Acidic Rocks. 

2. Basic Rock: Rocks in which the amount of silica is in between 45% to 55% are included in the category of Basic Rocks. 

3. Intermediate Rock: Rocks in which the amount of silica is in between Acidic and Basic Rocks. 

4. Ultra – Basic Rock: The amount of silica is less than 45% in these rocks.

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