Assume that a region of continental crust has a typical surface heat flux of 65 mW/m², crustal density of 2750 kg/m³, and typical crustal abundances of radioactive nuclides (as given in Table 20.6). If 60% of the surface heat flux comes from radioactivity in the upper crust, to what depth does this typical crustal composition extend?

Table 20.6

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Subsequent decay chain Average Initial decay Half-life (y) Nuclide Crustal abundance (C) energy 238 U → 206 Ph + 8 a + 47.7 (51.6) MeV 234Th 231 Th 228 Ra B- "Ca (89%) 4.2 MeV 238U 235 U 232Th 40K 2.7 ppmm 19 ppbm 9.6 ppmm 2.4 ppmm 4.46 x 10° 4.4 McV 232 Th- 208Pp + 6 a + 40.5 (42.6) MeV 4.0 MeV 0.56 MeV EC 40Ar (11%) 1.46 MeV 14.0 x 10 1.25 x 10