NCERT/CBSE Textbook Exercise (Important Questions only)
(i) What are the three layers of the earth?
(ii) What is a rock?
(iii) Name three types of rock.
(iv) How are extrusive and intrusive rocks formed?
(v) What do you mean by a rock cycle?
(vi) What are the uses of rocks?
(vii) What are metamorphic rocks?
(i) The three layers of earth are – Crust, Mantle and Core
(ii) Natural aggregate of minerals that makes up the earth is called ‘rock’.
(iii) The three types of rocks are – (1) Igneous rock, (2) Sedimentary rock, and (3) Metamorphic rock.
(iv) 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.
(v) 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.
(vi) 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.
(vii) When the igneous and sedimentary rocks are subjected to heat and pressure beneath the earth, they change into metamorphic rocks for example,
>> Granite changes into Gneiss.
>> Coal into Slate.
>> Slate into Schist.
>> Limestone into Marble.
Q.2: Tick the correct answer:
(i) The rock which is made up of molten magma is
(ii) The innermost layer of the earth is
(iii) Gold, petroleum and coal are examples of
(iv) Rocks which contain fossils are
a. Sedimentary rocks
b. Metamorphic rocks
c. Igneous rocks
(v) The thinnest layer of the earth is
(i) a (ii) b (iii) c (iv) a (v) a
Q.4: Give reasons:
(i) We can not go to the center of the earth.
(ii) Sedimentary rocks are formed from sediments.
(iii) Limestone is changed into marble.
(i) We can not go to the center of the earth because of so many reasons;
>> The distance of centre of the earth from its surface is 6371 km (radius of the earth).
>> With the increasing depth temperature increases @ 1OC per 32 km.
>> Because of the extreme heat and also pressure due to overlying rocks everything is in molten state.
>> 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. The reasons being as follow –
>> The overlying rocks put pressure on the underlying rocks.
>> With the increasing depth beneath the surface of the earth, temperature and pressure go on increasing.
>> Because of the combined actions of this high temperature and pressure the limestone converts into marble.
HTTP://CBSEKEY.COM(Additional Important Questions)
Q.1: Explain the process of Rock Cycle.
Ans: Refer to the answer of Q.1 (v) above.
Q.2: What are minerals? Write down their different uses.
Ans: 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 listed below:
* 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 and fertilizers etc.
Q.3: Draw a diagram of the interior of the Earth.
Ans: Refer to the NCERT text book.