20 - The surface heat flux

Summary

Preamble

As has been known for at least 200 years, there is a steady flux of heat from the crust into the atmosphere and oceans. The crustal temperature gradient, measured in the top kilometre or so in stable continental areas, is typically 0.025 K m^{− 1} (25 K/km). With a conductivity of 2.5 W m^{− 1} K^{− 1} this corresponds to a conducted heat flux of 62.5 m W m^{− 2}. The heat flux can be much higher in continental tectonic and thermal areas, but they cover a sufficiently small fraction of the total area of the Earth to have little influence on the global average. There are now more than 10,000 measurements of the heat flux from continents, with a wide range of values but sufficient data to be confident of the average, 65 m W m^{− 2} (Pollack et al., 1993).

The observation of the continental temperature gradient has had a central role in the history of geophysics. Extrapolated downwards, it suggested that rock would be at its melting point at a depth of 50 km or so. This observation prompted Kelvin's cooling Earth calculation and the age of the Earth debate in the 1800s (Section 4.2). When radioactivity was discovered and found to be concentrated in crustal rocks, especially granite, it invited the conclusion that crustal radiogenic heat explained all of the observed heat flux and that the deep Earth was thermally passive.