The earth is made up of several concentric layers with one inside another. The uppermost layer is called the crust. It is the thinnest of all the layers. On the continental masses it is about 35km. And on the ocean floors it is only 5 km.
The main mineral constituents of the continental mass are silica and alumina. It is thus called sial. The oceanic crust mainly consists of silica and magnesium. It is called sima.
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. The central core has very high temperature and pressure.
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 the fiery red molten magma coming out from the interior of the earth. 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 structure like 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. Granite is an example of such a rock. Grinding stones used to prepare pate or powder of spices and grains are made of granite.
Rocks roll down, crack, and hit each other and are broken down into small fragments. These smaller particles are called sediments. 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 micro-organisms that once lived on them.
Igneous and sedimentary rocks can change into metamorphic rocks under great heat and pressure. For example, clay changes into slate and limestone into marble.
The process of transformation of the rock from one to another is known as the rock cycle.
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 head and pressure melt down to form molten magma. This molten magma again can cool down and solidify into igneous rocks.