7 Ch. 3

 A. Tick the correct option.

1. Living organisms do not survive inside the earth due to

Answer: high temperature

2. Earthquakes and volcanoes are caused by

Answer: internal disturbance

3. Respective layers of the earth from top to bottom are

Answer: crust, mantle, core

4. The full form of SIMA is

Answer: silica and magnesium

5. The original rock from which Gneiss formed is

Answer: granite

B. Fill in the blanks.

1. Earth was formed about ___________ million years ago.

2. The behaviour of ___________ waves tell us about the earth’s interior.

3. The ___________ of crust varies from place to place.

4. Rocks are classified into three groups on the basis of their ___________.

5. The ___________ of the earth is rich in iron and nickel.

Answer: (1) 4600 (2) seismic (3) thickness (4) formation (5) core

C. Give a single term for each of the following statements.

1. It is the hardest substance.

2. The molten rock material present inside the earth.

3. Rocks which are formed due to cooling and solidification of magma inside the earth.

4. Shaking and trembling of the earth’s surface.

5. A naturally occurring solid substance made up of a mixture of minerals.

Answer: (1) Diamond (2) Magma (3) Intrusive Igneous Rocks (4) Earthquake (5) Rock

D. Answer the following questions in brief.

1. What is the special feature of sedimentary rocks?

Answer: Sedimentary rocks are layered.

2. How are metamorphic rocks formed?

Answer: Rocks which originally are igneous or sediment in nature get changed in the metamorphic rocks due to the heat and pressure.

3. What is lava?

Answer: The fine red molten magma coming out from the interior of the earth on its surface is called lava.

4. Why are igneous rocks called the primary rocks?

Answer: Igneous rocks are called the primary rocks as they were formed first over the earth surface.

5. What are sediments?

Answer: The weathering process breaks the rocks into small pieces. These are then carried away by water and then deposited on the river beds or sea beds, these deposits are called sediments

 E. Answer the following questions:

1. Differentiate between the P-Wave and the S-Wave.

Answer: The P-Wave or primary wave which can travel through both solid and liquid mediums and is faster in speed. Whereas, the S-Wave or secondary wave can pass through only a solid medium at a slower speed.

2.Write two characteristics of each of the following:

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

Answer: two characteristics of Crust, Mantle and Core are:

(a) Crust

Crust is the uppermost layer over the earth’s surface.

It is the thinnest of all the layers.

(b) Mantle

Mantle is a very thick layer of rocks.

It lies just beneath the crust.

(c) Core

Core is the innermost layer of the earth.

It is mainly made up of nickel and iron.

3. What are fossil fuels? Name any two fossil fuels and specify their importance.

Answer: The remains of animals and plants get buried and trapped under the layers, and over a period of time get converted into coal, oil and natural gas. They are called fossil fuels.

Oil and natural gas are the two fossil fuels. They have great economic importance all over the world as they are a major source of energy.

4. Why is the knowledge of the earth’s interior based on indirect observation?

Answer: The intensive heat neither allows us to dig deep nor is there any possibility of living organisms deep inside the earth. It is therefore, we have little knowledge about the earth’s interior. Whatever knowledge we have, is based on indirect observation.

5. What are minerals? On what basis can the minerals be identified? Give examples.

Answer: A naturally occurring substance that have a definite chemical composition is called a mineral.

They are identified on the basis of their colour, hardness and lustre. Diamond, gold and silver are some of the examples of minerals.