Inside Our Earth Chapter Notes | Social Studies (SST) Class 7

What is Earth?

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

• The earth comprises three layers: crust, mantle, and core.
• Constant changes take place inside as well as outside the earth.  
  Interior of the Earth

• 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 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 (si-silica and ma-magnesium).
• Mantle is just beneath the crust, 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 a very high temperature and pressure.

Continental crust and Oceanic crust

Rocks 

Any natural mass of mineral matter that makes up the earth’s crust is called a rock. It can be of different colour, size and texture.

The earth’s crust is made of various types of rocks.

Types of Rocks

There are three major types of rocks:

(i) Igneous or Primary Rocks

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

They are of two types, extrusive igneous rocks and intrusive igneous rocks.  

• Extrusive igneous rocks: When 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. For example, basalt. The Deccan plateau is made up of basalt rocks.

• Intrusive igneous rocks: Sometimes the molten magma cools down deep inside the earth’s crust. Solid rocks so formed are called intrusive igneous rocks. Granite is an example of such a rock. Grinding stones used to prepare paste/powder of spices and grains are made of granite.

(ii) Sedimentary Rocks

• Rocks roll down and break into small fragments and these smaller particles are called sediments. These sediments are transported, 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 micro-organisms that once lived on them.

(iii) Metamorphic 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.
• For example: Clay changes into slate and limestone into marble.

Rock Cycle  

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

• The molten magma cools down and 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.
• The molten magma again can cool down and solidify into igneous rocks.  

Minerals

Rocks are made 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. 

Frequently Asked Questions

Q.1. What is lava?
Ans: Lava is the hot red magma coming out from the interior of the earth to the surface. It cools down and become solid.

Lava

Q.2. What are the uses of rocks?
Ans: Uses of Rocks:

• Hard rocks are used in making buildings and barrages.
• Houses and buildings are built of rocks (stones, slates, granite, marble).
• Stones are used in numerous games: Seven stones (phitthoo), Hop scotch (stapu, kit kit), Five stones (gitti).
• Rocks (stones and slates) are used in building bridges, embankments.

Q.3. What are fossils?

Ans: The remains of the dead plants and animals trapped in the layers of rocks are called fossils.