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This applet illustrates the geotherms for two different models of heat production distribution which contribute the same total crustal heat flow. The blue line shows a heat production distribution that decreases exponentially with depth, with a characteristic length-scale of hr (i.e., at depth hr the heat production is 1/e of the surface value). The red line shows a heat source distribution which is uniform in an upper crustal layer of thickness hr, and zero below this. Note that the exponential model produces rather hotter deep crustal temperatures than the homogeneous model.

Why is this so?

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Symbol Parameter
Default Value

crustal contribution to heat flow

30 mW m-2
qm  mantle heat flow
30 mW m-2 (1.0)
hr length-scale for heat production
11.7 km (0.33)
Hi upper crustal heat production)
2.6 µ W m-3
k crustal conductivity  
3 W m-1 K-1
zm depth to Moho  
35 km