Class 07 || S.St. (Geography) || Ch. 02 Inside Our Earth

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

★ Geologist 

The scientist who studies rocks is called a geologist.

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INTERIOR OF THE EARTH

The earth is made up of several concentric layers with one inside another, just like an onion. 

There are three layers of the earth 

1. Crust (uppermost layer)

2. Mantle (middle layer)

3. Core (central layer)

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.

1. Crust

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. 

Sial

The crust of the earth is also called sial because its is made up of silica (si-silica) and alumina (al-alumina) thus It is called sial.

Sail means silica and alumina.

2. Mantle

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

3. Core

Below the crust is the innermost layer called core with a radius of about 3500 km. The central core has very high temperature and pressure.

It is mainly made up of nickel and iron (ferrous) and is called nife. 

Nife 

The core of the earth is also called nife because its is made up of ni (ni-nickel) and iron (fe-ferrous i.e. iron) thus It is called nife.

Sail means ni nickel and fe ferrous i.e. iron.

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ROCKS AND MINERALS

Rock

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

Rocks are made up of different minerals. Rocks can be of different colour, size and texture. 

Types of rocks

There are three types of rocks

(i) Igneous Rocks 

(ii) Sedimentary Rocks 

(iii) Metamorphic Rocks

(i) Igneous Rocks 

The solid Rocks formed by the cooling down of magma or Lava of volcanoes is called igneous rocks.

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

Thse are also called firey rocks. They are also called primary rocks.

★ Igneous: Latin word Ignis meaning fire.

(ii) Sedimentary Rocks 

The Rocks formed by the sediments of the water of the river, lakes and sea so that these rocks are called sedimentary rocks.

★ Sedimentary: Latin word sedimentum meaning settle down.

(iii) Metamorphic Rocks

The rocks like that igneous or sedimentary rocks changed into another kind of rocks. This types of the rocks are called metamorphic rocks.

★ Metamorphic: Greek word metamorphose meaning change of form.

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★ Minerals

Elements or compounds which occurs naturally in the Earth's crust is called minerals. Minerals are naturally occurring substances which have certain physical properties and definite chemical composition. 

★ Magma

The hot or melted rocks is called magma.

★ Lava

The fiery red molten magma coming out from the interior of the earth on the Earth's surface is called Lava.

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★ The deepest mine in the world, is in South Africa. It is about 4 km. deep. In search for oil engineers have dug a hole about 6 km. deep.

★ To reach to the centre of the earth (which is not possible!) you will have to dig a hole 6000 km. deep on the ocean floor.

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There are two types of igneous rocks: 

(a) Extrusive igneous rocks and (b) Intrusive  igneous rocks.

(a) Extrusive igneous rocks

When the molten lava comes on the earth's surface, and rapidly cools down and becomes solid and form Rocks.  The 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. 

(B) Intrusive  igneous rocks.

Sometimes the molten magma cools down deep inside the earth's crust and becomes solid and form Rocks. 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.

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Sediments

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 micro- organisms that once lived on them.

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

Sedimentary rock turned into a Metamorphic rock

Igneous and sedimentary rocks can change into metamorphic rocks under great heat and pressure. For example, clay changes into slate and limestone into marble.

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Rocks are very useful to us. The hard rocks are used for making roads, houses and buildings. You use stones int many games. For example, seven stones (pitthoo), hop- scotch (stapu/kit kit), five stones (gitti). 

Find out some more such games by asking your grand parents, parents, meighbours, etc.

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Rock Cycle

One type of rock can change into another types of rock under certain conditions in a cyclic manner. This process of transformation of the rock from one form to another is known as the rock cycle. 

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We know that 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.

Molten Magma –> Igneous rocks –> broken down into sediment –> sedimentary rocks –> metamorphic rocks –> Molten Magma 

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Minerals are very important to humankind. Some are used as fuels. For example, coal, natural gas and petroleum. They are also used in industries - irorı, aluminium, gold, uranium, etc, in medicine, in fertilisers, etc.

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                             Exercises

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1. Answer the following questions.

(1) What are the three layers of the earth?

Ans. The earth is made up of several concentric layers with one inside another, just like an onion. 

There are three layers of the earth 

1. Crust (uppermost layer)

2. Mantle (middle layer)

3. Core (central layer)

(ii) What is a rock?

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

Rocks are made up of different minerals. Rocks can be of different colour, size and texture. 

(iii) Name three types of rocks.

Ans. There are three types of rocks

(i) Igneous Rocks 

(ii) Sedimentary Rocks 

(iii) Metamorphic Rocks

(iv) How are extrusive and intrusive rocks formed?

Ans.  There are two types of igneous rocks: 

(a) Extrusive igneous rocks and (b) Intrusive  igneous rocks.

(a) Extrusive igneous rocks

When the molten lava comes on the earth's surface, and rapidly cools down and becomes solid and form Rocks.  The Rocks formed in such a way on the crust are called extrusive igneous rocks. 

(B) Intrusive  igneous rocks.

Sometimes the molten magma cools down deep inside the earth's crust and becomes solid and form Rocks. Solid rocks so formed are called intrusive igneous rocks. 

(v) What do you mean by a rock cycle?

Ans. Rock Cycle : One type of rock can change into another types of rock under certain conditions in a cyclic manner. This process of transformation of the rock from one form to another is known as the rock cycle. 

(vi) What are the uses of rocks?

Ans. Uses of Rocks

★ Hard Rocks (stones and slates) are used in building bridges, embankments.

★ 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).

      • Hopscotch (stapu, kit).

      • Five stones (gitti).

(vii) What are metamorphic rocks?

Ans. The rocks like that igneous or sedimentary rocks changed into another kind of rocks. This types of the rocks are called metamorphic rocks.

★ Metamorphic: Greek word metamorphose meaning change of form.

2. Tick the correct answer.

(1) The rock which is made up of molten magma is

(a) Igneous   (b) Sedimentary  (c) Metamorphic

(ii) The innermost layer of the earth is

(a) Crust           (b) Core           (c) Mantle

(iii) Gold, petroleum and coal are examples of 

(a) Rocks        (b) Minerals        (c) Fossils

(iv) Rocks which contain fossils are

(a) Sedimentary rocks

(b) Metamorphic rocks

(c) Igneous rocks

(v) The thinnest layer of the earth is

(a) Crust             (b) Mantle           (c) Core

Ans.

(i)—> (a), (ii)—> (b), (iii)—> (b), (iv)—> (a), (v)—> (a).

3. Match the following.

(i) Core            (a) Earth's surface.    

(ii) Minerals    (b) Used for roads and buildings 

(ii) Rocks         (c) Made of silicon and alumina. 

(iv) Clay            (d) Has definite chemical composition 

(v) Sial             (e) Innermost layer. 

                          (f) Changes into slate

                          (g) Process of transformation of the rock

Ans. (i)->(e), (ii)->(d), (iii)->(b), (iv)->(f), (v)->(c)

Give reasons.

(1) We cannot go to the centre of the earth.

Ans. We cannot go to the centre of the earth because of the following reasons:

The thickness from crust to the centre of the core is 6371 km. (Radius of the earth).

Temperature increases with depth @ 1°C per 32 metre.

There is extreme heat and pressure of overlying rocks, everything is in a molten state.

There is no oxygen to survive.

(II) Sedimentary rocks are formed from sediments.

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

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. 

(iii) Limestone is changed into marble.

Ans. Limestone is changed into marble in the interior of the earth. Due to high pressure and increasing of heat, the original limestone changes into marble.

5. For fun.

(i) What are the minerals most commonly used in the following objects?

Ans.

 Object    Minerals that are frequently used in them

Karhai –> Iron, copper, stainless steel etc.

Ornaments –> Gold, silver, copper etc.

Lamp –> Brass, silver, gold etc.

Hammer –> Iron, steel, brass etc.

Bell –> Brass, silver , gold etc.

Pan/tawa –> Iron, steel, aluminium etc.

(ii) Identify some more objects made up of different minerals.

Ans: Graphite is used to make pencil lead. Copper wiring is used in wire, telephones. Sulphur and chromite are found in our carpets. Quartz and Pentlandite are used to make clocks.

Some other important questions 

Very Short Answer Type Questions

Q. 1. What is called the uppermost layer of the earth? 
Ans. It is called the crust.

Q. 2. What is the special feature of this layer? 
Ans. It is the thinnest of all the layers.

Q. 3. Name the main mineral constituent of the continental mass.

Ans. Silica and alumina

Q. 4. Name the constituents of the oceanic crust.
Ans. Silica and magnesium.

Q. 5. What is the radius of the core?
Ans. The radius of the core is about 3500 km.

Q. 6. What are the main constituents of the core?
Ans. The main constituents of the core are nickel and iron. It is usually known as nife.

Q. 7. What is the earth’s crust made of?     
Ans. It is made up of different types of rocks.

Q. 8. What are called igneous rocks?

Ans. When the molten magma cools, it becomes solid. Rocks formed in this way are called igneous rocks.

Q. 9. What are fossils?
Ans. The remains of the dead plant and animals trapped in the layers of rocks are called fossils.

Q. 10. What happens when igneous and sedimentary rocks go under great heat and pressure?
Ans. They change into metamorphic rocks.