The earth is a dynamic planet because it is constantly undergoing changes inside and outside. It is made up of rocks. The earth is made up of several concentric layers with one inside another. There are three types of layer- crust, mantle and core. The Crust is the uppermost layer over the earth’s surface which is about 35 km on the continental masses and only 5 km on the ocean floors. Continental mass is constituents of silica and alumina constituents, thus it is called sial. The oceanic crust mainly consists of silica and magnesium; it is therefore called sima (si-silica and ma-magnesium). Mantle is just beneath the crust which is 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 earth’s crust is made up of various types of rocks that can be of different colour, size and texture. There are three major types of rocks: igneous rocks, sedimentary rocks and metamorphic rocks which are discussed below:
1. Igneous rocks: They are formed, when the molten magma cools and becomes solid. There are two types of igneous rocks: intrusive rocks and extrusive rocks. An extrusive igneous rock is formed from molten magma coming from the interior of the earth on its surface, rapidly cools down and becomes solid. Example- Basalt An intrusive igneous rock is formed when the molten magma cools down deep inside the earth’s crust. Example- Granite
2. Sedimentary rocks: It is formed by transported and deposited small fragmented rocks by wind, water etc and these loose sediments are compressed and hardened to form layers of rocks. These rocks contain fossils of dead plants, animals and other microorganisms. For example, sandstone is made from grains of sand.
3. Metamorphic rocks: These rocks are formed due to great heat and pressure over igneous and sedimentary rocks. 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 called the rock cycle. When molten magma cools, it solidifies to become igneous rock. When igneous rocks, 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.
They are naturally occurring substance with certain physical properties and definite chemical composition. These are used in industries – iron, aluminium, gold, uranium, etc, in medicine, in fertilizers, etc. For example, coal, natural gas and petroleum.
Earth is the third planet from the Sun and only celestial body that have life. It was formed by volcanic activity and outgassing that included water vapour. The origin of the world's oceans was condensation augmented by water and ice delivered by asteroids, protoplanets, and comets. A crust formed when the molten outer layer of Earth cooled to form a solid.
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