The Earth’s interior heat is due to follow sources: - From the remnants of heat from impacts with planetesimals early in Earth's history. Impacts with large bodies accumulate the thermal energy of the collision in the surrounding rock of the planet, and may have been enough in certain circumstances to completely melt the early Earth.
- A remnant of an early Earth event known as the Iron Catastrophe. With much of early Earth still molten, denser metals, particularly iron and nickel, migrated to the center of the planet. Tremendous amounts of frictional heat was created, enough to completely melt the planet once again.
- From compression due to gravity.
- From the decay of radioactive elements. This source of heat is gradually declining due the decreasing amounts of radioactive isotopes, the decrease being caused by the decay. Heat flow continually regenerated by the decay of radioactive elements that occur in all rocks.
- Heat flow may be created by electromagnetic effects of the magnetic fields involved in Earth's magnetic field, as suggested by some contemporary folk theories.
- Heat flow may be generated by tidal force on the Earth as it rotates; since land cannot flow like water it compresses and distorts, generating heat.

Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth's interior. Away from tectonic plate boundaries, it is 25–30°C per km of depth in most of the world. The geothermal gradient varies with location and is typically measured by determining the bottom open-hole temperature after borehole drilling. To achieve accuracy the drilling fluid needs time to reach the ambient temperature.
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 60 m below the surface, the temperature is constant at about 11°C. In a zone between the near surface and about 120 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 with both tectonic setting and the thermal properties of the rock.


High gradients (up to 200°C/km) are observed along the oceanic spreading centers (for example, the Mid-Atlantic Rift) and along island arcs (for example, the Aleutian chain). The high rates are due to molten volcanic rock (magma) rising to the surface. Low gradients are observed in tectonic subduction zones because of thrusting of cold, water-filled sediments beneath an existing crust. The tectonically stable shield areas and sedimentary basins have average gradients that typically vary from 15–30°C/km.
The geothermal gradient is important for the oil, gas, and geothermal energy industries:
- Downhole logging tools must be hardened if they are to function in deep oil and gas wells in areas of high gradient.
- Calculation of geothermal gradients in the geological past is a critical part of modeling the generation of hydrocarbons in sedimentary basins.
- In Iceland, geothermal energy, the main source of energy, is extracted from those areas with geothermal gradients ≥40°C/km.