Inside Our Earth

"Our earth is an oblate spheroid in shape. Its surface is solid, and is made up of rocks. After a study of various sources, geologists concluded that the earth is made up of three main layers in the form of concentric shells."

In class $6^{\text{th }}$ we have already learnt about are earth and its position in the Solar System. Let us now see how earth looks from inside.

1.0Introduction

The earth, our homeland is a dynamic planet. It is constantly undergoing changes inside and outside.

2.0Interior of the Earth

• Just like an onion, the earth is made up of several concentric layers with one inside another.
• The uppermost layer over the earth's surface is called the crust.
• It is the thinnest of all the layers. It is about $35\mathbf{km}$. on the continental masses and only 5 km . on the ocean floors.
• 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 (sisilica and ma-magnesium).
• Just beneath the crust is the mantle which extends up to a depth of $2900\mathbf{km}$. below the crust.
• The innermost layer is the core with a radius of about $3500\mathbf{km}$.
• It is mainly made up of nickel and iron and is called nife (ni - nickel and fe - ferrous i.e. iron).
  
  Continental Crust and Oceanic Crust
• The central core has very high temperature and pressure.
• The crust forms only 1 per cent of the volume of the earth, 84 per cent consists of the mantle and 15 per cent makes the core.
• The radius of the earth is 6371 km .

3.0Rocks and Minerals

• The earth's crust is made up of various types of rocks.
• Any natural mass of mineral matter that makes up the earth's crust is called a rock.
• Rocks can be of different colour, size and texture.
• 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.
• Lava is actually fiery red molten magma coming out from the interior of the earth on its surface.
• When this molten lava 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.
• Since they cool down slowly they form large grains.
• Granite is an example of such a rock. Grinding stones used to prepare paste/powder of spices and grains are made of granite.
• 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. For example, sandstone is made from grains of sand.
• These rocks may also contain fossils of plants, animals and other microorganisms that once lived on them.
• Igneous and sedimentary rocks can change into metamorphic rocks under great heat and pressure.
• For example, clay changes into slate and limestone into marble.
• Rocks are very useful to us.
• The hard rocks are used for making roads, houses and buildings.
• You use stones in many games.
• For example, seven stones (pitthoo), hopscotch (stapu/kit kit), five stones (gitti).

Soil is the thin layer of material covering the earth's surface and is formed from the weathering of rocks. It is made up mainly of mineral particles, organic materials, air, water and living organisms-all of which interact slowly yet constantly. A layer of soil measuring few centimetres can take up to thousands of years to form.

• You will be surprised to know that one type of rock changes to another type under certain
• conditions in a cyclic manner.
• This process of transformation of the rock from one to another is known as the Rock cycle.
• You have already learnt when the molten magma cools; it solidifies to become igneous rock.
• These igneous rocks are broken down into small particles that are transported and deposited to form sedimentary rocks.
• When the igneous and sedimentary rocks are subjected to heat and pressure they change into metamorphic rocks.
• The metamorphic rocks which are still under great heat and pressure melt down to form molten magma.
  
• This molten magma again can cool down and solidify into igneous rocks.
• Rocks are made up of different minerals.
• Minerals are naturally occurring substances which have certain physical properties and definite chemical composition.
• Minerals are very important to humankind.
• Some are used as fuels. For example, coal, natural gas and petroleum.
• They are also used in industries - iron, aluminium, gold, uranium, etc, in medicine, in fertilisers, etc.

4.0MIND MAP