Geothermal Heat Flow Calculator

Heat flow is the conductive flux of Earth's internal heat through a horizontal surface, obtained from Fourier's law as the product of thermal conductivity and vertical temperature gradient. With conductivity in W/m/K and the gradient expressed in degrees C per kilometre the product falls out directly in mW/m2, so 3.0 W/m/K acting over 30 degrees C/km gives 90 mW/m2. Obtaining it requires two independent measurements rather than one: a borehole temperature log, ideally recorded after months of thermal recovery from drilling, and a laboratory conductivity determination on core from the same interval, made with a divided-bar apparatus or a needle probe. The global mean is roughly 65 mW/m2 on continents and near 100 mW/m2 on oceanic crust, where young lithosphere close to spreading ridges is still cooling; the planetary total is around 44 to 47 TW. Values below 40 mW/m2 mark stable cratons, 40 to 70 normal continental crust, 70 to 100 thinned or extended crust, and figures above 150 characterize active rift and volcanic zones. Interpretation carries real caveats. Shallow holes are contaminated by groundwater advection, topographic refraction of the heat field, and the surface temperature history of the last glaciation, so continental determinations generally need depths beyond 200 to 300 m. Conductivity is anisotropic in bedded rock and varies with saturation and temperature. Resource estimates built on a heat flow number also depend heavily on an assumed recovery factor, which is commonly only about 2 percent of the heat in place.

Fourier's law of conduction, published in 1822, made a heat flow number computable in principle, but the first genuine determinations came more than a century later. Edward Bullard measured heat flow in deep South African gold mines in 1939, and A.E. Benfield reported values from British boreholes the same year. Francis Birch developed the divided-bar method for measuring rock conductivity around 1950, giving the second half of the measurement the precision it needed to be meaningful. The oceans were opened up by Bullard's probe design, which drove a thermistor-bearing lance into seafloor sediment. Roger Revelle and Arthur Maxwell published Pacific seafloor measurements in 1952, and the finding that oceanic heat flow was comparable to continental values, despite ocean crust containing far less radiogenic material, became an important argument for mantle convection and later for seafloor spreading. The International Heat Flow Commission was established in 1963 to standardize and collect these data, and its global database now holds tens of thousands of entries. Henry Pollack, Suzanne Hurter and Jeffrey Johnson's 1993 analysis in Reviews of Geophysics produced the widely cited global figure of roughly 44 TW, and heat flow compilations remain the foundation for lithospheric thermal models and geothermal resource assessment.