Class 7, Our Environment
Chapter 2, Inside Our earth
NCERT Solutions of Geography (Social Science) Chapter Exercise
Question.1:
a)
What are the three layers of the earth?
b)
What is a rock?
c)
Name three types of rock.
d)
How are extrusive and intrusive rocks formed?
e)
What do you mean by a rock cycle?
f)
What are the uses of rocks?
g)
What are metamorphic rocks?
Solution.1:
a)
The
three layers of earth are - Crust, Mantle and Core.
b)
Natural
aggregate of minerals that makes up the earth is called ‘rock’.
c)
The
three types of rocks are - (1) Igneous rock, (2) Sedimentary rock, and (3)
Metamorphic rock.
d)
Due
to extreme heat in the interior of the earth, rocks are found in the form of
molten material known as ‘magma’. Because of any volcanic activity or some
other reasons when magma comes on the surface of the earth, it cools down and
turns into solid rocks. Such rocks are termed as ‘extrusive rocks’ e.g. Basalt.
When the magma cools down within the interior of the earth, it becomes solid
known as ‘intrusive rocks’ e.g. Granite, Gabro.
e)
One
rock type changes into another rock type under certain natural conditions in a
cyclic manner. This process of transformation of the rock from one to another
type is called ‘Rock Cycle’. When the
molten magma cools down, it solidifies to become igneous rocks. These igneous rocks are broken down into small
particles. By the actions of various natural agencies like - water, wind,
glacier, human activities etc. they are transported and deposited to form sedimentary rocks. When these igneous
and sedimentary rocks are subjected to heat and pressure beneath the earth,
they change into metamorphic rocks.
The metamorphic rocks, which are still under great heat and pressure, melt down
to form molten magma.
f)
Rocks
have numerous uses. Hard rocks are used for various construction purposes such
as buildings, roads, dams, embankments etc. From rocks, we get different types
of minerals after their beneficiation. Some rocks are used as gemstones.
g)
When
the igneous and sedimentary rocks are subjected to heat and pressure beneath
the earth, they change into metamorphic rocks for example,
1.
Granite changes into Gneiss.
2.
Coal into Slate.
3.
Slate into Schist.
4.
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 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
Solution.2:
(i) a (ii) b (iii) c (iv) a (v) a
Question.4: Give
reasons:
(i) We cannot go to the
center of the earth.
(ii) Sedimentary rocks are
formed from sediments.
(iii)
Limestone is changed into marble.
Solution.4:
(i) We can not go to the center of the
earth because of so many reasons -
a)
The
distance of centre of the earth from its surface is 6371 km (radius of the
earth).
b)
With
the increasing depth temperature increases @ 1OC per 32 km.
c)
Because
of the extreme heat and also pressure due to overlying rocks everything is in
molten state.
d)
Absence
of oxygen.
(ii)
It is true that sedimentary rocks are formed from sediments. Igneous and
metamorphic rocks decompose, disintegrate, and wear down due to weathering.
These materials are carried away by various natural agencies such as rivers,
oceans, wind, glacier etc and are deposited as sediments in low lying areas.
When sediments are solidified into layers due to pressure from overlying
sediments they are called sedimentary rocks.
(iii) Limestone is changed into marble
because -
a)
The
overlying rocks put pressure on the underlying rocks.
b)
With
the increasing depth beneath the surface of the earth, temperature and pressure
go on increasing.
c)
Because
of the combined actions of this high temperature and pressure the limestone
converts into marble.
Solutions of CBSE Class 7, Geography Additional Questions
Question.1: Explain the
process of Rock Cycle.
Answer: Kindly refer to the answer of Q.1 (e)
above.
Question.2: What are
minerals? Write down their different uses.
Answer: Minerals
are naturally occurring substances, which have certain physical properties and
definite chemical composition.
Minerals are very important to us. Some
of their uses are -
* used as fuels e.g. coal, natural
gas, petroleum.
* used in different industries as raw
materials e.g. iron, copper, aluminium, uranium etc.
* used in medicine, fertilizers
etc.
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