Plate tectonics and convection in the Earth's mantle: toward a numerical simulation

Plate tectonics is a kinematic description of Earth that treats the outer shell of its mantle as a number of plates or rigid spherical caps that move with respect to each other (see the “Plate tectonics” sidebar). The mantle is the outer, solid 3,000-km-thick shell that overlies Earth’s fluid outer core. An enormous amount of geological and geophysical data has gone into determining the motion of the plates,1 and within the last few years direct GPS measurements have corroborated the geological constraints on the motions of plates. A fundamental question in geology has been, what drives the plates? This question has largely been solved—the plates are part of a system of large-scale thermal convection—and geodynamicists have moved on to more difficult questions, such as what are the details of the coupling between surface motions and deeper mantle flow? and why do we have plate tectonics as opposed to some other mode of tectonics or thermal convection? Answering these more subtle questions is complicated by the fact that the primary effect of plate motion is to consume the old ocean floor and recycle it into the mantle. The primary evidence of plate history is therefore limited to the past 100 to 200 million years or so (less than 10% of the overall history of plate tectonics). We therefore rely heavily on evidence drawn from theoretical and computational models and from the continents that are not consumed wholesale by plate motions. Ideally these two sources of evidence go hand in hand to reinforce each other.