The geothermal gradient is the rate of change of temperature (ΔT) with depth (ΔZ), in the earth. Units of measurement are °F/100 ft or °C/km. In the geosciences, the measurement of T is strongly associated with heat flow, Q, by the simple relation: Q=KΔT/ΔZ, where K is the thermal conductivity of the rock.

Temperatures at the surface of the earth are controlled by the Sun and the atmosphere, except for areas such as hot springs and lava flows. From shallow depths to about 200 ft (61 m) below the surface, the temperature is constant at about 55°F (11°C). In a zone between the near surface and about 400 ft (122 m), the gradient is variable because it is affected by atmospheric changes and circulating ground water. Below that zone, temperature almost always increases with depth. However, the rate of increase with depth (geothermal gradient) varies considerably...