The Earth's thermal gradient

Vast amounts of heat are stored in the Earth, often breaking dramatically through the surface at volcanoes and hot springs such as that shown in figure 1. Just how this heat is distributed within the Earth has concerned geophysicists for years. Recently their theories of the temperature distribution have been influenced in several ways by the revolutionary plate‐tectonics model of the Earth. This model depicts the relative motions and mutual interactions of several large surface plates, driven by material welling up in some parts of the globe (ridges) and descending in others (trenches), as illustrated in figure 2 (a map of the worldwide plate boundaries is shown in the accompanying article by Carl Kisslinger). Such a model has forced the abandonment of a cherished geophysical boundary condition—spherical symmetry in the temperature profile. Overwhelming evidence has established that temperature distribution beneath the ridges is very different from that at the convergent plate boundaries; the temperature profile in the upper portion of the globe depends very much on whether it is below a zone of spreading plates, a zone of colliding plates or a region of plate interiors.