ABSTRACT

A persistent problem in planetary science is how and when plate tectonics can begin in planetary evolution. On Earth, plate tectonics is thought to be facilitated by the low-viscosity asthenosphere, which obtains its low viscosity partly through low pressure, and partly through a water content on the order of hundreds of parts per million, likely trapped in the crystal structure of nominally anhydrous silicate minerals. Subduction zones introduce water contents of that magnitude to the mantle that circulates above the sinking oceanic plate, and subduction zones are sometimes cited as the process that hydrates an originally dry planetary interior. Thus there is a chicken-and-egg problem: If a damp asthenosphere is needed for plate tectonics, but plate tectonics itself creates the damp asthenosphere, how does the process initiate? I will present models for the interior water content of the Earth following accretion, and hypothesize about dynamic processes that may have sped the development of plate tectonics, including solid-state overturn following magma ocean solidification, and early tidal heating. These results suggest that larger planets are more likely to have early conditions suitable for plate tectonics.