The globe of the earth is divided into three layers namely (a) The Earth’s crust, (b) Mantle and (c) Core.
The Earth’s Crust : The outermost layer of the earth is called as the earth’s crust. It is about 30 km thick. In the region of continents it is thicker but at the floors of oceans it is thinner. It contains mainly rocks, silica and aluminium. It is called ‘sial’. The lower part of the crust is called ‘sima’. Humans and animals live on this part of the earth.
Mantle : The layer next to the crust is about 2900 km in thickness is called mantle. The density of the mantle material is 3 to 4.7.
Core : Below the mantle is the earth’s core. It is about 3400 km. thick. Core has two parts—outer core and inner core. Outer core is in liquid state and the inner core is in solid state. Core contains iron, nickel and sulphur. The temperature of the central part of the earth may be about 5000OC.
Rocks
The earth crust or solid part is called rocks. Rocks may not always be hard. Minerals are obtained from rocks. Study of rocks is very important.
Rocks are mainly of three types namely—
(i) Igneous rocks, (ii) Sedimentary rocks, and (iii) Metamorphic rocks.
1. Igneous rocks : During volcanic eruptions hot lava comes out and cools down forming rocks. The molten earth materials known as magma, cool down beneath the earth’s crust, again forming rocks. The two types of rocks are known as igneous rocks.
When the earth’s surface, in the beginning, cooled down from its hot lava in liquid state, the original rocks of the earth’s crust were formed. They were the primary igneous rocks. They are not usually found today.
Igneous rocks are hard and granular. The grains in these rocks are of different sizes and colours. These rocks do not have layers. Fossils are not found in igneous rocks.
The formation of igneous rocks takes place beneath and above the surface of the earth. Rocks formed by the cooling of liquid matter beneath the earth’s surface are called intrusive igneous rocks. Granite and gabbro are the main examples of these rocks. Very often, rocks beneath the earth’s surface melt on account of the earth movements. This process of cooling down is slow. That is why big crystals of minerals are formed. The intrusive rocks are thus crystalline rocks.
The molten matter or magma pours out on the earth’s surface at the time of volcanic eruptions. The gases in the magma mix with the atmosphere. The remaining liquid matter is called ‘lava’. This lava cools down quickly. Hence, the process of crystallisation is not completed. Rocks formed from the lava are called extrusive igneous rocks. Basalt is an example of extrusive igneous rocks. A very large area of the Deccan Plateau consists of basalt rocks.
2. Sedimentary rocks : The atmosphere is constantly at work on land, on account of which the rocks break up and wear out. They get transformed into smaller pieces and finally into soil. These substances are carried away by rivers, wind and other agents. They get deposited in layers one above the other. Such layers give birth to become sedimentary rocks.
As layers over layers get deposited, the lower layers are under great pressure of the layers above. Over a period of time, unified sedimentary rocks are formed on account of the tremendous pressure exerted by the layers above.
Lime, earth, clay and similar materials get deposited on the surface of the earth form a cement like paste. This paste binds the pieces of rock, and give hardness to sedimentary rock. Sometimes, the remains of plants, dead animals etc. are found in the deposited material. Therefore, in the sedimentary rocks fossils are found.
Sedimentary rocks are of various colours. Their colour depends on the minerals in the original rock, chemical actions, etc. Limestone is white as well as black. Sandstone is dull white, pink, bright red or sometimes black.
Sandstone, limestone, shale are some examples of sedimentary rocks.
3. Metamorphic rocks : The nature of igneous and sedimentary rocks changes due to the effects of heat or pressure, and different types of new rocks are formed. These new and transformed rocks are called metamorphic rocks.
Minerals in the rocks get restructured on account of heat and pressure. The colour, the size of the grains and structure of the rock changes and metamorphic rocks are formed.
Metallic minerals are found in the metamorphic rocks. Some examples of metamorphic rocks formed from igneous and sedimentary rocks are marble, mica, granite etc.
Importance of rocks : Rocks are important for building houses and roads. Various minerals are obtained from rocks. Soil is formed from rocks which is very important for agriculture.
Land surface keeps on changing constantly by Wind, rivers, glaciers, sea-waves and other agents. This is called erosion. That is why, it is necessary to study the rocks in order to understand the work of these agents.
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