The moving forces of plate tectonics

As mentioned above, mantle convection could, directly or indirectly, drive plate motion. In either case, the amount of heat energy carried by convection to the Earth's surface would probably be greater than that required by plate tectonics. The total observed heat flux released by the Earth from its surface per time unit is ~ 44 TW (Pollack et al., 1993). The total radioactive heat production of the BSE, calculated from K, U and Th in Table 17.1, is ~ 16 TW, of which the present-day continental crust with a mean U concentration of 1.3 ppm and the canonical ratios Th/U « 4 and K/U « 10000 (Table 26.3) produces - 6.5 TW. The mantle and D" thus produce — 9.5 TW of radioactive heat, and the source of the remainder could be the core. The Nusselt number for the mantle is — 20 (Section 17.5), which indicates that moving mantle matter, e.g. plumes and rising branches of convective cells, is by far the major carrier of this heat from the Earth's interior to the surface.

Hewitt et al. (1975) showed that ~ one-sixth of the convection-delivered mantle heat, ~ 6 TW, is converted into mechanical power that could drive plate tectonics.

By comparison, the observed amount of energy actually released in plate tectonics is quite small: ~ 0.03 TW is released in earthquakes, 0.08 TW by volcanic activity and 0.2 TW by orogenesis. This is in total ~ 0.3 TW, a factor 20 less than the total supply available. Hence, mantle convection carries more than enough power to enable plate tectonics (see Zharkov, 1983 and references therein; Tackley, 2000).

The processes directly responsible for the movement of plates are the gravitational sliding of segments away from an oceanic ridge, which pushes a plate, and the sinking of cool dense lithosphere at a subduction zone, which pulls it. The second factor appears to be by far the more important. For example, plates having very extensive subduction zones, e.g. Nazca, Cocos and Pacific, generally move faster than others (Forsyth and Uyeda, 1975). These mechanical forces have played a prime role in plate tectonics, at least during the second half of Earth's history. However, the power that plate movements consume appears to be of second order in importance compared with the total energy delivered to the crust and uppermost mantle by convection, which is dissipated as heat.

Continents, even though covering only a third of the Earth's surface, are especially important actors in the plate-tectonic movie. They are thick, (generally) cool and have high viscosity; only conductive heat transfer, which is slow, is possible through them. A domain of the convecting mantle underneath a large stagnant (super)continent gradually heats up under this insulating lid, and becomes less dense. A swell can thus develop beneath the continent on a time scale of ~ 300 Myr. A plume from the deeper mantle can add to this effect and impel continental rifting. The results of recent seismic studies support the magma-assisted rifting of initially thick and strong continental lithosphere (Kendall et al., 2005). Break-up and divergence of the two or more resulting subcontinents could follow, accompanied by the formation of oceanic plates behind them.

In this way a giant plume separated the supercontinent Gondwana into several pieces between 200 and 150 Myr ago. Ultimately, the collisions of continents and their welding together in orogenic belts will lead to the formation of a new supercontinent (Trubitsyn, 2000).

Summarizing, there is little doubt that heat delivered by mantle convection to the Earth's tectonosphere is the major factor ultimately driving plate tectonics: the Earth is a geologically living planet owing to this phenomenon. However, this does not mean that there are simple straightforward relationships between the configuration of convective cells in the mantle and the position of oceanic ridges or subduction zones: after the latter are born, they develop further on their own (e.g. McKenzie and Bickle, 1988).

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