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[Perspective] A measure of mantle melting

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…

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