The increase of temperature with depth. It usually refers to depths below 200 m. In the continents, the gradient is usually between 20 and 40 °C/km, although it can well exceed this in volcanic regions. In the oceans, the depth of penetration of most core barrels is so short that the gradient can be determined over only a few metres and varies considerably. The average geothermal gradient at the surface of the Earth is about 24 °C/km, but it is assumed to decrease with depth as widespread mantle melting would otherwise occur. The observed gradients are therefore modified to result in an estimated temperature of about 1200 °C at the top of the seismic low-velocity zone in the upper mantle. Within the mantle, the increase of temperature with depth is considered to be less than 0.1 °C/km greater than the adiabatic increase of 0.33 °C/km. See also heat flow.