The influence of near-surface radioactivity on continental surface heat flow has been considered in Section 4–8. From Equation (4–29) and the data given in Figure 4–11 we found that the heat flow beneath the near-surface layer of heat-producing elements is about 37 mW m−2. We assume that heat production beneath the near-surface radioactive layer can be neglected and that the thermal structure of the continental lithosphere has reached a steady state. Therefore, it is appropriate to assume the heat flow through the continental lithosphere beneath the near-surface heat-producing layer, qm, is constant. The resulting geotherm in the continental lithosphere is given in Figure 4–56 for qm = 37 mW m−2 and k = 3.35 W m−1 K−1. The thickness of the continental lithosphere is about 200 km.
Our discussion so far has centered on the thermal state of the shallow upper mantle; the geotherms in Figure 4–56 extend only to a depth of 400 km. If the entire mantle were homogeneous and strongly convecting, the adiabatic temperature gradient given by Equation (4–254) would be a good approximation of the slope of the temperature profile throughout the mantle. We have noted, however, that the distribution of density with depth has significant discontinuities near depths of 410 and 660 km (see Figure 4–55).
The density discontinuity at 410 km is associated with the transformation of olivine to a spinel structure. The phase change from olivine to spinel is exothermic with a heat of reaction L = 90 kJ kg−1.