The thermal gradient is often defined as the rate of increase in temperature with respect to the increase in the subsurface depth.This accounts for more than 50% of the total heat energy of the earth.The geothermal gradient is approximately 25° C/km beneath the earth's surface. This heat results from the decay of the radioactive elements that are at great depths. In addition to this, the latent heat is also produced due to the crystallization of the core. Due to this heat energy, that is transmitted to the upper mantle, the plates are able to move over the layer of the Asthenosphere.
This heat energy obtained from the interior of the earth is known as geothermal energy.
