What is Radiogenic heat production?

The radiogenic heat production of a rock is the heat released inside it by the decay of its radioactive atoms, per unit volume and time, in microwatts per cubic metre ($\text{µ}\mathrm{W}/\mathrm{m}^{3}$). Almost all of it comes from four isotopes: uranium-238, uranium-235, thorium-232 and potassium-40.

The radiogenic heat production $A$ of a rock is the heat released per unit volume and per unit time by the decay of the radioactive nuclei it contains, in $\mathrm{W}\,\mathrm{m}^{-3}$, in practice in $\text{µ}\mathrm{W}\,\mathrm{m}^{-3}$. A rock of density $\rho$ containing mass fractions $C_i$ of the heat-producing elements, each releasing $H_i$ watts per kilogram of element, has