The rise in temperature with increase in depth is observed in mines and deep wells.
The rate of increase of temperature is not uniform from the surface towards the earth’s center.
It is faster at some places than at others.
In the beginning, this increase is at an average rate of 10°C for every 32 meters increase in depth.
At such a constant rate of increase in temperature, at 10 km depth, the temperature will be approximately 300°C and at 40 km depth, it will be 1200°C.
At this rate, the earth’s interior should be in a molten state.
While in the upper 100 km, the increase in temperature is at the rate of 12°C per km, in the next 300 km it is 20°C per km but is only 10°C per km below it.
Thus the rate of increase of temperature beneath the surface decreases towards the center.
The temperature at the center is estimated to lie somewhere between 3000°C and 5000°C.
The pressure also increases from the surface towards the center of the earth.
In deeper portions the pressure is high.
The pressure near the center is considered to be 3 to 4 million times the pressure of the atmosphere at sea level.
At high temperature, the material beneath will melt towards the central part of the earth.
Due to increase in pressure and presence of heavier materials towards the earth’s centers, the density of earth’s layers also goes on increasing.
Obviously the materials of the innermost part of the earth are very dense as already stated.