Inside Our Earth



--> 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.

--> The deepest mine in the world, is in South Africa. It is about 4 km.

--> To reach to the centre of the earth (which is not possible!) you will have to dig a hole 6000 km. deep on the ocean floor.

--> 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).Just beneath the crust is the mantle which extends up to a depth of 2900 km. below the crust.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 i.e. iron).

--> The crust forms only 1 per cent of the volume of the earth, 84 per cent consists of the mantle and 15 per cent makes the core.

--> The radius of the earth is 6371 km.

--> Any natural mass of mineral matter that makes up the earth’s crust is called a rock. Rocks can be of different colour, size and texture.

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

--> Lava is actually fiery red molten magma coming out from the interior of the earth on its surface. 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. They have a very fine grained structure. For example, basalt. The Deccan plateau is made up of basalt rocks. Sometimes the molten magma cools down deep inside the earth’s crust. Solid rocks so formed are called intrusive igneous rocks.

--> Igneous: Latin word Ignis meaning fire.

--> Sedimentary: Latin word sedimentum meaning settle down.

--> Metamorphic: Greek word metamorphose meaning change of form.

--> Fossils: The remains of the dead plants and animals trapped in the layers of rocks are called fossils.

--> 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.

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

--> 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.

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

Match the following.

(i) Core              (a) Earth’s surface

(ii) Minerals              (b) Used for roads and buildings

(iii) Rocks               (c) Made of silicon and alumina

(iv) Clay               (d) Has definite chemical composition

(v) Sial               (e) Innermost layer

                         (f) Changes into slate
                        (g) Process of transformation of the rock