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Radioactive decay of U, Th, and K generates ~10–50 mW/m³ in continental crust, contributing substantially to surface heat flow. Heat production decreases with depth and is concentrated in granitoid rocks; accounting for radiogenic heat is essential in thermal models.
From your understanding of heat flow measurement, you know that geothermal heat flow quantifies how much thermal energy escapes through Earth's surface per unit area. But where does that heat come from? Part of it is primordial — left over from Earth's formation and core crystallization. The rest is generated continuously within the crust and mantle by radioactive decay, and understanding this internal heat source is essential for building accurate thermal models of the lithosphere.
Three radioactive isotope systems dominate terrestrial heat production: uranium-238 (and U-235), thorium-232, and potassium-40. Each decays through a chain of alpha and beta emissions, and the kinetic energy of those particles is absorbed by surrounding rock and converted to heat. Uranium produces the most heat per kilogram, thorium somewhat less, and potassium-40 the least — but potassium is far more abundant in crustal rocks, so its total contribution is significant. Together, these three elements account for essentially all radiogenic heat in the crust.
The critical observation is that these elements are incompatible — they preferentially concentrate in silica-rich, low-density minerals during partial melting and magmatic differentiation. This means they are strongly enriched in the upper continental crust, particularly in granitic rocks, and depleted in mafic lower crust and mantle peridotite. Typical heat production values range from 1–5 μW/m³ in granites down to 0.01–0.02 μW/m³ in mantle rocks — a difference of two orders of magnitude. The practical consequence is that radiogenic heat production decreases sharply with depth in the continental crust, often following an exponential decay with a characteristic length scale of about 10 km.
This depth dependence has a direct, measurable consequence known as the linear heat flow–heat production relationship: regions with more radioactive upper crust (measured from surface rock samples) have proportionally higher surface heat flow. The intercept of this linear relationship gives the reduced heat flow — the heat contribution from the mantle and deep crust — while the slope reflects the thickness of the enriched layer. For thermal modeling, ignoring radiogenic heat production leads to large errors. A continental geotherm constructed without it would predict temperatures far too low in the upper crust and too high a proportion of mantle heat flow. Getting the radiogenic contribution right is therefore a prerequisite for realistic models of crustal temperature, rheology, and tectonic behavior.