NCERT Class 7 Geography Chapter 2 - Inside Our Earth

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INTERIOR OF THE EARTH 

The uppermost layer over the earth’s surface is called the crust. 

The main mineral constituents of the continental mass are silica and alumina. It is thus called sial (si-silica and al-alumina). 

The oceanic crust mainly consists of silica and magnesium; it is therefore called sima (si-silica and ma-magnesium).

Just beneath the crust is the mantle which extends up to a depth of 2900 km below the crust. 

The innermost layer is the core with a radius of about 3500 km. 

It is mainly made up of nickel and iron and is called nife (ni – nickel and fe – ferrous i.e. iron). 

The central core has very high temperature and pressure. 

ROCKS AND MINERALS 

Any natural mass of mineral matter that makes up the earth’s crust is called a rock. 

There are three major types of rocks: igneous rocks, sedimentary rocks and metamorphic rocks. 

When the molten magma cools, it becomes solid. Rocks thus formed are called igneous rocks. They are also called primary rocks. 

There are two types of igneous rocks: intrusive rocks and extrusive rocks. 

When molten lava, a fiery red molten magma coming out from the interior of the earth on its surface, comes on the earth’s surface, it rapidly cools down and becomes solid. Rocks formed in such a way on the crust are called extrusive igneous rocks. 

They have a very fine grained structure. For example, basalt. The Deccan plateau is made up of basalt rocks. 

Sometimes the molten magma cools down deep inside the earth’s crust. Solid rocks so formed are called intrusive igneous rocks. 

Rocks roll down, crack, and hit each other and are broken down into small fragments. These smaller particles are called sediments. 

These sediments are transported and deposited by wind, water, etc. These loose sediments are compressed and hardened to form layers of rocks. These types of rocks are called sedimentary rocks. 

Igneous and sedimentary rocks can change into metamorphic rocks under great heat and pressure. 

The process of transformation of the rock from one to another is known as the rock cycle. 

Minerals are naturally occurring substances which have certain physical properties and definite chemical composition.