NCERT Solutions For Class 7 Geography Social Science Chapter 2 Inside Our Earth

Question 1.
Answer the following questions briefly.

What are the three layers of the earth?
What is a rock?
Name three types of rocks.
How are extrusive and intrusive rocks formed?
What do you mean by a rock cycle?
What are the uses of rocks?
What are metamorphic rocks?

Answer:

Three Layers of the Earth
- Crust
- Mantle
- Core.
Rock
- A mass of mineral matter that makes up the crust of the earth is called rock.
  Or
- Aggregates of minerals are termed as rocks. Examples: See part
Three Types of Rocks
- Igneous Rocks
- Sedimentary Rocks
- Metamorphic Rocks.
Formation of Extrusive and Intrusive Rocks
- Due to extreme heat in the interior of the earth, rocks are found in the form of molten material called magma.
- When magma comes out on the surface of the earth, it cools down and turns into solid rocks. Such rocks are termed as extrusive rocks.
  Examples:
  ★ Basalt.
  ★ Deccan Trap is built of basalt.
- When the molten magma cools down within the interior of the earth, it becomes solid to form intrusive rocks.
  Examples:
  ★ Granite.
  ★ Gabro.
Rock Cycle
Igneous rocks change into sedimentary rocks, igneous and sedimentary rocks under heat and pressure change into metamorphic rocks, metamorphic rocks into igneous rocks or sedimentary rocks due to melting or wearing down. This process is called the rock cycle.
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).
- Hopscotch (stapu, kit).
- Five stones (gitti).
- Rocks (stones and slates) are used in building bridges, embankments.
Metamorphic Rocks
- When under heat and pressure igneous rocks and sedimentary rocks change their form and more precious rocks are formed to be known as metamorphic rocks.
  Examples:
  ★ Granite into granite gneiss.
  ★ Coal into slate.
  ★ Slate into schist.
  ★ Limestone into marble.

Question 2.
Tick the correct answer.
(i) 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 example of
(a) Rocks
(b) Minerals
(c) Fossils.

(iv) Rocks which contain fossils are
(a) Sedimentary rocks
(b) Metamorphic rocks
(c) Igneons rocks.

(v) The thinnest layer of the earth is
(a) Crust
(b) Mantle
(c) Core.
Answer:
(i)—(a), (ii)—(b), (iii)—(b), (iv)—(a), (v)—(a).

Question 4.
Give reasons:

We cannot go to the centre of the earth.
Sedimentary rocks are formed from sediments.
Limestone is changed into marble.

Answer:

We cannot go to the centre of the earth because of the following reasons:
- The thickness (from crust to 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.
Sedimentary rocks are formed from sediments because of the following reasons:
- Igneous rocks and metamorphic rocks, decompose, disintegrate and wear down due to weathering.
- This material is carried away by running water (rivers), wind, glacier.
- The material is then deposited in low lying areas and is called sediments.
- When sediments are solidified into layers due to pressure from overlying sediments they are called sedimentary rocks.
Limestone is changed into marble due to the following reasons:
- The overlying layers of rocks put pressure on the underlying rocks.
- From the surface to the interior of the earth, temperature and heat go on increasing.
- Due to pressure and heat, the original limestone changes into marble.

Very Short Answer Type Questions

Question 1.
What is called the uppermost layer of the earth? [Imp.]
Answer:
It is called the crust.

Question 2.
What is the special feature of this layer? {Imp.]
Answer.
It is the thinnest of all the layers

Question 3.
Name the main mineral constituent of the continental mass.
Answer:
Silica and alumina

Question 4.
Name the constituents of the oceanic crust. [V. Imp.]
Answer:
Silica and magnesium.

Question 5.
What is the radius of the core?
Answer:
The radius of the core is about 3500 km.

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

Question 7.
What is the earth’s crust made of? [V.Imp.]
Answer:
It is made up of different types of rocks.

Question 8.
What are called igneous rocks?
Answer:
When the molten magma cools, it becomes solid. Rocks formed in this way are called igneous rocks.

Question 9.
What are fossils?
Answer:
The remains of the dead plant and animals trapped in the layers of rocks are called fossils.

Question 10.
What happens when igneous and sedimentary rocks go under great heat and pressure?
Answer:
They change into metamorphic rocks.

Short Answer Type Questions

Question 1.
What are minerals? How are they useful for mankind? [V. Imp.]
Answer:
Minerals are naturally occurring substances which have certain physical properties and definite chemical composition.
Minerals are very useful for mankind. Some minerals like coal, natural gas and petroleum are used as fuels. They are also used in industries. Iron, aluminium, gold, uranium etc. are used in medicine, in Fertilizers, etc.

Long Answer Type Questions

Question 1.
Mention various types of rocks IV. [Imp.]
Answer:
Rocks are of the three types:

Igneous rocks
- Sedimentary rocks
- Metamorphic rocks
- Igneous rocks. When the molten magma cools, it becomes solid. Rocks formed in this way are called igneous rocks.
- They are also called primary rocks. They are of two types—intrusive rocks and extrusive rocks.
Extrusive rocks.When molten lava comes on the earth’s surface, it rapidly cools down and becomes solid. Rocks formed in this way on the crust Eire called extrusive igneous rocks. For example, basalt.
Intrusive 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. For example, granite.
Sedimentary rocks. Small fragments of rocks are called sediments. These sediments are transported and deposited by wind, water, etc.These loose sediments are compressed and hardened to form sedimentary rocks. For example, sandstone is made from grains of sand.
Metamorphic rocks. When igneous and sedimentary rocks Eire subjected to great heat and pressure they chsmge into metamorphic rocks. For example, clay changes into slate and limestone into marble.

Question 2.
What do you know about the interior of the earth? [V. Imp.]
Answer:
Our earth is made up of several concentric layers with one inside another. These layers are three in number—crust, mantle smd core.

Crust. It is the uppermost layer over the earth’s surface. It is the thinnest of all the layers. It is about 35 km on the continential masses and only 5 km on the ocean floors.

The continental masses are made up of silica and alumina. It is thus called sial (si-silica and al-alumina). The oceanic crust mainly consists of silica and magnesium. It is thus called sima (si-silica and ma-magnesium).

Mantle:It is just beneath the crust. It extends up to a depth of 2900 km below the crust

Core: It is the innermost layer. Its radius is about 3500 km. It is mainly made up of nickel and iron and is known as nife (ni-nickel and fe-ferrous, Le., iron). The central core has a very high temperature and pressure.