Earth's mantle is cooled by the introduction of cold slabs. This plate-tectonic mode of cooling is somewhat unusual, as mantle convection in the other large terrestrial bodies in our solar system operates differently, as explained in § 31.1. Mantle convection acts to convey heat toward the surface at a rate greater than can be achieved by conduction alone. This rate is parameterized in § 31.3. This chapter concludes in § 31.4 with a simple model of the mantle thermal structure and a discussion in § 31.4.1 of slab divergence.

Modes of Mantle Convection

All planetary bodies in our solar system (and likely elsewhere) formed by accretion relatively quickly (≈ 10 million years or possibly less) and the associated gravitational energy released by this process led to a set of hot young planets. Ever since, these planets have been doing their best to cast this heat off to outer space as fast as they can. Convection of molten silicate is an efficient mode of cooling for a terrestrial planet; upwelling brings hot molten silicate material to the surface, where its heat can readily be radiated to outer space. Young planets with this mode of convection are in the magma-ocean phase. This state is fairly short-lived (several million years), as the surface temperature soon falls below the solidus temperature, and the mode of convection that ensues depend whether there is significant amounts of liquid water present or not. If little or no water is present, the planet transitions through three cooling modes having progressively weaker convection, weaker heat flow and lower temperatures. Fortuitously, since the rate of transitioning varying inversely with the size of the planet, we have good examples of each of these three modes within our own solar system: