Earth's interior is hot, as is evident from geothermal heat flow, the existence of volcanoes, and the mobility of tectonic plates. But just how hot is it? The temperature increases rapidly with depth through the rigid lithosphere in order to conduct geothermal heat flow, but this cannot continue downward indefinitely without reaching the melting point of rocks. Yet there is no global molten layer below the lithosphere. The propagation of shear waves through the upper mantle shows that it is solid. Rather, the conductive heat flow gives way at some depth to the transport of heat by solid-state convection in the ductile asthenosphere. Convecting systems evolve to an adiabatic temperature profile that can be characterized by a single reference, the “potential temperature.” Determining the average and range of variability of the potential temperature of the mantle below the lithosphere allows geoscientists to link observations of phenomena such as postglacial rebound and seismic wave speeds, through laboratory data on viscous and elastic properties, to the composition of Earth. On page 942 of this issue, Sarafian et al. (1) report new experimental observations of the melting of mantle rocks with the appropriate amount of water and infer a higher value of the potential temperature than previous estimates. Author: Paul D. Asimow
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