Earth's heat: How energy resides
Why Earth's heat leaks out so slowly
Why Earth's heat leaks out so slowly What this tab covers The physical form in which the energy is held or carried, where it comes from, the chain of transformations that put it there, and how concentrated it is.Geothermal energy is sensible heat stored in the rock and fluids of the Earth's interior, left over from the planet's formation and topped up by the radioactive decay of uranium, thorium and potassium. It leaks out at the surface very slowly, so it is captured where hot water or steam is concentrated near the surface.
Form Sensible heat in rock, water and steam Origin Nuclear (radioactive decay) and primordial heat from planetary formationHow it got there
Gravitational collapse and accretion of the early Earth Radioactive decay of uranium, thorium and potassium in the mantle and crust Heat conducted and convected towards the surface Hot water and steam in permeable rock near volcanoes and rifts Wells bring fluid to the surfaceHow concentrated it is
| Measure | Value | Note |
|---|---|---|
| Average heat flow through the surface | 91.6 mW per m23 | Continental crust 70.9 mW/m2, oceanic crust 105.4 mW/m2 |
| Temperature gradient | about 25-30 deg C per km of depth4 | Normal crust; much steeper near volcanoes and rifts |
| Share of the Earth's surface energy budget | about 0.03%3 | Compared with about 173,000 TW of incoming sunlight |
The Earth's interior is hot, and that heat moves outwards by conduction through rock and by the slow convection of the mantle. Roughly half of the outflow is produced today by radioactive decay; the rest is primordial heat still escaping from the planet's formation.
The flow through an average square metre of ground is under a tenth of a watt, far too weak to harvest directly. Useful geothermal energy therefore depends on places where geology concentrates the heat: magma close to the surface, or permeable rock where circulating water collects heat over large volumes and thousands of years. Elsewhere, wells must go several kilometres deep to reach useful temperatures, which is why most of the vast stored heat remains out of reach.
Sources
- Earth's internal heat budget Wikipedia, 2025
- World Energy Resources 2013: Geothermal World Energy Council, 2013