Inside Our Earth

Q21. What are igneous rocks?

Ans. When the molten magma cools, it becomes solid. Rocks thus formed are called igneous rocks.

Q22. What are minerals?

Ans. Minerals are naturally occurring substances which have certain physical properties and definite chemical composition.

Q23. Limestone is changed into marble. Give reason?

Ans. Marble is a metamorphic rock that forms when limestone is subjected to great heat and pressure.

Q24. Write about the deepest mine in the world.

Ans. The deepest mine in the world, is in South Africa. It is about 4 km. deep. In search for oil engineers have dug a hole about 6 km. deep.

Q25. Why we cannot go to the centre of the earth?

Ans. To reach to the centre of the earth we will have to dig a hole 6000 km. deep on the ocean floor. This is not possible, so we cannot go to the centre of the earth.

Q26. What do you mean by a rock cycle?

Ans. One type of rock changes to another type under certain conditions in a cyclic manner. This process of transformation of the rock from one to another is known as the rock cycle.

Q27. What happens to igneous and sedimentary rocks when they are subjected to great heat and pressure?

Ans. Igneous and sedimentary rocks change into metamorphic rocks under great heat and pressure.

Q28. How are minerals useful to mankind?

Ans. Minerals are useful to humankind in the following ways:

                       i.        Some are used as fuels. For example, coal, natural gas and petroleum.

                      ii.        They are also used in industries – iron, aluminium, gold, uranium, etc, in medicine, in fertilisers, etc.

Q29. What are the uses of rocks?

Ans. Uses of rocks

                        i.        The hard rocks are used for making roads, houses and buildings.

                       ii.        Stones are used in many games. For example, seven stones (pitthoo), hopscotch (stapu/kit kit), five stones (gitti).

Q30. How are extrusive and intrusive rocks formed?

Ans. Extrusive rock- When 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. They have a very fine grained structure.

Intrusive rocks - Sometimes the molten magma cools down deep inside the earth’s crust. Solid rocks so formed are called intrusive igneous.

Q31. Differentiate between sial and sima.

Ans. Difference between sial and sima

Q32. Sedimentary rocks are formed from sediments. Give reason.

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.

Q33. What do you know about earth's interior?

Ans. The earth is made up of several concentric layers with one inside another.

Crust - 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.

Mantle - Just beneath the crust is the mantle which extends up to a depth of 2900 km. below the crust.

Core - 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. The central core has very high temperature and pressure.

Q34. Write a note on various types of rocks?

Ans. There are three major types of rocks: igneous rocks, sedimentary rocks and metamorphic rocks.

     1.   Igneous rocks - 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.

                     i.        Extrusive igneous rocks - When 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. For example, basalt.

                    ii.        Intrusive igneous rocks - Sometimes the molten magma cools down deep inside the earth’s crust. Solid rocks so formed are called intrusive igneous rocks. Granite is an example of such a rock.

      2.   Sedimentary rocks - 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.

      3.   Metamorphic rocks - Igneous and sedimentary rocks can change into metamorphic rocks under great heat and pressure. For example, clay changes into slate and limestone into marble.