Modern theories on the formation of the Earth and other planets are of course based on the Copernican theory.
The age of the Earth was a matter of speculation till very recent times. It was only about 200 years ago, that scientific enquiries were started by geologists. According to their deductions, based on the study of rocks, the age of the Earth is 4.6 billion years.
Our knowledge of the internal structure of the Earth is derived from studies of earthquakes. The shock waves sent out by an earthquake indicate the physical nature of the regions through which they pass. These studies show that the centre of the Earth is a solid core–the Inner Core. The density of this core is about 13 g to the cubic centimeter. The Inner Core is about 1,370 km thick and is surrounded by an Outer Core of around 2,080 km. The Outer Core appears to be molten.
The Outer Core is surrounded by the Mantle which has a thickness of around 2,900 km. The Mantle is topped by the crust of the Earth, which varies widely in thickness–from 12 to 60 km. At the centre or the Inner Core, that is at a depth of some 6,370 km, temperature goes upto some 4,000°C and pressure reaches nearly 4 million at mospheres.
The mantle is important in many ways. It accounts for nearly half the radius of the Earth (2,900 km), 83% of its volume and 67% of its mass. The dynamic processes which determine the movements of the crust plates are powered by the mantle.
Starting at an average depth of from 45 to 56 km below the top surface of the Earth, the mantle continues to a depth of 2,900 km where it joins the outer core. The mantle is a shell of red hot rock and separates the Earth’s metallic and partly melted core (both the inner and the outer cores) from the cooler rocks of the Earth’s crust.
It is composed of sllicate minerals rich in magnesium and Iron. The density of the mantle increases with depth from about 3.5 gram per cubic centimetre to around 5.5 gram, near the outer core.
The outer surface of the Earth is divided into 4 spheres:
- Lithosphere means the entire top crust of the Earth and includes not only the land surface but also the ocean floor.
- Hydrosphere is the water surface which includes the oceans, lakes and rivers.
- Atmosphere is the blanket of air that envelops the Earth. It covers both the land surface and the water surface.
- Biosphere is this sphere of life which spreads over all the three other spheres.