In it's simplist form, the rate of heat flowing out of the Earth is calculated by \(Q = -\lambda \frac{\delta T}{\delta z} \), where \(\lambda\) is thermal conductivity, an intrinsic physical property of a given rock type, and \(\delta T\) the temperature gradient over a given depth interval \(\delta z\). In this equation \(Q\) has units \(W m^{-2}\), however, when referring to geothermal heat flow, it is more typical to report values in \(mW m^{-2}\) as estimates are more clearly represented in this manner outside of active volcanic zones. The negative sign out front is by convention as heat is determined to flow in the positive direction with decreasing temperature.

For continental heat flow estimates, it is often necessary to take into account the effect of radiogenic heat production due to the presence of elements \(K\), \(Th\) and \(U\) throughout the crust. Our initial heat flow equation can be supplemented to account for this and becomes \(Q = -\lambda \frac{\delta T}{\delta z} + A\), where \(A\) is the contribution to heat flow from the summation of heat generating elements throughout the entire depth interval. When estimating surface heat flow, it is necessary to produce an estimate of heat generation throughout the entire crustal column, a difficult task considering the vertical distribution of heat producing elements throughout the crustal column is relatively unknown and cannot be directly measured. Heat generation can therefore be one of the largest sources of uncertainty when estimating surface heat flow.

The calculation of heat flow therefore requires the measurement of three properties, temperature, \(^\circ K\), thermal conductivity, \(\lambda\), and heat generation, \(A\), which all happen to be stored in the ThermoGlobe database! Obviously there is a lot more to it and books can and have been written on the subject. For anyone interested in learning more about heat flow, we recommend the book
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Crustal Heat Flow : A Guide to Measurement and Modelling
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, by Beardsmore and Cull (2001).