inside earth
The engine that moves plates, builds mountains and melts rock runs on two fuels, not one: heat inherited from the planet’s formation, and heat still being made by radioactive decay. Separating the two took a particle detector.
About: Earth · Radiogenic heating · Secular cooling · Mantle convection
Heat leaves the Earth for space at a total rate of 44.2 terawatts, with an uncertainty of about one terawatt.[1]
It comes from two main sources: the decay of radioactive isotopes — chiefly uranium, thorium and potassium — and residual heat left over from the planet’s formation by the falling together and compression of cosmic debris.[1,2,3]
Which fuel supplies more was, for a long time, genuinely unresolved. The answer came from geoneutrinos — antineutrinos emitted by radioactive decay that pass through the whole planet virtually unaffected, and can therefore be counted at the surface.[1]
Combining measurements from two liquid-scintillator detectors, radioactive decay was found to supply about half of Earth’s total heat flux. The corollary matters as much as the number: the planet’s primordial heat supply has not yet been exhausted.[1]
Not all of it is measurable this way. Neutrinos from potassium-40 decay fall below what these detectors can see; the four terawatts potassium contributes had to be supplied from other knowledge.[1,2]
Convection carries that heat outward: hotter material rises, spreads, cools and sinks to be reheated. It cannot happen without a heat source, and the two sources above are it.[1]
What convection does in detail is much less settled. How and why the escape of interior heat becomes concentrated into particular regions remains unresolved, and the number, origin and structure of the mantle’s convection cells are open questions.[1,2]
Often said
“Earth’s internal heat is left over from its formation.”
What the evidence supports
That is half the story. Residual heat from accretion and radiogenic heat from decaying uranium, thorium and potassium each supply roughly half the total, and the primordial component is not yet spent.
Supporting records: earth-two-heat-sources · radiogenic-share
Bracketed references link to the structured entity records whose claims support each paragraph. Last updated 2026-09-06.