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In this article we are discussing about the types of rocks.

Earth’s crust is made up of various kinds of rocks, which differ from one another in texture, size, colour, composition, permeability, mode of appearance and degree of resistance to denudation.

Petrology is the science of rocks. A petrologist studies rocks in all their variety.

On the basis of their origin & appearance the rocks can be categorised into three different groups:

1)   Igneous rocks :

a)      They are primary rocks. These rocks are first to originate on the Earth’s surface. Other rock types are direct or indirect derivatives of igneous rocks.

b)       Igneous rocks are formed when molten rock (magma) from beneath the Earth’s crust cools and solidifies.

c)       Normally, these rocks are crystalline in nature. They do not occur in layers.

d)      These rocks do not contain fossils.

e)      These rocks can be subdivided on the basis of their mineral composition. When they contain high proportion of silica, they are acidic. Acid igneous rocks such as granite are less dense and have lighter colour than basic igneous rocks.

f)       When the igneous rocks contain high proportion of basic oxides like iron, aluminium & magnesium, they are basic igneous rocks. They are denser and darker in colour.

g)      Igneous rocks can also be divided on the basis of their origin as explained below:In this article we are discussing about the types of rocks.

h) 
Basalt is a common type of volcanic rock. It forms lava flows, lava
sheets & lava plateaux. The Deccan plateau in India is an example of basaltic
rock.

i)
 Some types of basalt form long polygonal
columns while solidifying. In some types, molten lava may push itself to the
surface through cleft & passages, forming vertical dykes or horizontal
sills.

j) Igneous rocks are very hard & resistant.
So, they are usually quarried for road making and used for monuments.

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