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Temperature and Pressure Inside the Earth. The evidence of volcanic eruptions and hot springs indicates that high temperatures prevail in the interior of the earth. A progressive rise in temperature with increasing depth is recorded in mines and deep wells all over the world, the average rate of increase being 1 degree C for every 32 meters of descent The rate of increase in temperature is now considered to be variable and there is no uniform increase from the surface to the centre of the earth. The rate of increase in the overlying pressure makes the melting point higher but only to a certain degree. In the upper 100 km, the increase is estimated at 12oC per km. It is 2oC per km in the next 300 km and 1oC per km below it. By this calculation, the temperature is actually 2,000oC at the core of the earth.

The heat or rise in temperature is the result of internal forces, automatic disintegration of radioactive substances, chemical reaction and other sources keeping the interior hot. It indicates the liquid or perhaps gaseous conditions prevailing at greater depths. But at the same time there is a tremendous increase m the pressure of overlying layers on earth's interior. Thus, even under extremely high temperature towards the central part of the earth, the liquid nature of its core has acquired the properties of a solid and is probably in a plastic state. The earth is rigid and behaves mostly as a solid down to a depth of 2,900 km because of such pressure conditions. The surface on the ocean beds run faster than on the surface of the continents, indicating that the surface of the earth is lighter than the surface of ocean. It proves the presence of liquid material in the core of the earth surface.