GEOGRAPGY
CLASS 7
INSIDE OUR EARTH
1. How are sedimentary rocks formed?
- Ans – 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 microorganisms that once lived on them.
2. How are igneous rocks formed?
Ans – When the molten magma cools, it becomes solid. Rocks thus formed are called igneous rocks. They are also called primary rocks.
There are two types of igneous rocks: intrusive rocks and extrusive rocks
- When this molten lava comes on the earth’s surface, it rapidly cools down and becomes solid. Rocks formed in such a way on the crust are called extrusive igneous rocks
- Sometimes the molten magma cools down deep inside the earth’s crust. Solid rocks so formed are called intrusive igneous rocks.
3. State the importance of rocks.
Ans – Rocks are very useful to us.
- The hard rocks are used for making roads.
- For making houses and buildings.
- You use stones in many games.
For example, seven stones (pitthoo), hopscotch (stapu/kit kit), five stones (gitti).
4. What are minerals? How are they important to mankind?
- Ans – Minerals are naturally occurring substances which have certain physical properties and definite chemical composition.
- Minerals are very important to humankind. Some are used as fuels. For example, coal, natural gas and petroleum.
- They are also used in industries – iron, aluminium, gold, uranium, etc, in medicine, in fertilisers, etc.
5. Name the uppermost layer of the earth. Write any four features of this layer.
Ans – 1. 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. c. The main mineral constituents of the continental mass are silica and alumina. It is thus called sial (si-silica and al-alumina).
- The oceanic crust mainly consists of silica and magnesium; it is therefore called sima (si-silica and ma -magnesium)
6. How are extrusive and intrusive rocks formed?
Ans – When the molten lava comes on the earth’s surface, it rapidly cools down an becomes solid. Rocks formed in such a way on the crust are called extrusive igneous rocks. For Example – Basalt.
Sometimes the molten magma cools down deep inside the earth’s crust. Solid rocks so formed are called intrusive igneous rocks.
For Example – Granite
7. What do you mean by a rock cycle?
Ans – The process of transformation of rocks from one type to another, due to changes in certain conditions in a cyclic manner, is called a rock cycle.
8. What are the uses of rocks?
Ans – Rocks are useful for various purposes:
- It helps in making roads
- It is used in the construction of houses and buildings
- Small stones are used by children in different types of games
- They are used in fertilisers.
9. What are metamorphic rocks?
Ans – The type of rocks formed when igneous and sedimentary rocks experience heat and pressure are called metamorphic rocks.
10. What are minerals? How are they important to mankind?
- Ans –Minerals are naturally occurring substances which have certain physical properties and definite chemical composition.
- Minerals are very important to humankind. Some are used as fuels. For example, coal, natural gas and petroleum.
- They are also used in industries – iron, aluminium, gold, uranium, etc, in medicine, in fertilisers, etc.
11. Write any three features of the innermost layer of the earth’s interior.
- Ans – The innermost layer is the core with a radius of about 3500 km.
- It is mainly made up of nickel and iron and is called nife (ni – nickel and fe – ferrous that is iron).
- The central core has very high temperature and pressure.
Five-mark Questions
12. Explain the rock cycle.
Ans- This process of transformation of the rock from one to another is known as the rock cycle.
- 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
13. Differentiate between extrusive and intrusive igneous rocks.
Ans- Differentiate between extrusive and intrusive igneous rocks is as follows –
| Extrusive Igneous Rocks | Intrusive Igneous Rocks. |
| When this molten lava comes on the earth’s surface, it rapidly cools down and becomes solid. Rocks formed in such a way on the crust are called extrusive igneous rocks. | Sometimes the molten magma cools down deep inside the earth’s crust. Solid rocks so formed are called intrusive igneous rocks. |
| They have a very fine grained structure | Since they cool down slowly, they form large grains |
| Example – Basalt. The Deccan plateau is made up of basalt rocks | Example – Granite |