On the relative importance of the driving forces of plate motion

Abstract

In a very ingenious paper, Forsyth and Uyeda used observed plate shapes and speeds to compute the ratios of the eight rms torques which eight different types of plate force exert on 12 crustal plates. We describe one resolution of the conceptual problems arising in Forsyth and Uyeda's estimates for the values and uncertainties of those ratios. We confirm Forsyth and Uyeda's conclusion that equilibrium is mainly between the negative buoyancy of descending slabs and the viscous resistance to their descent; the other six kinds of torque are smaller by at least an order of magnitude. We find the combined rms fractional error in the data and theory to lie between 9 and 16 per cent, at the 1 per cent confidence level. This error prevents determination of the other six torque ratios; so the absolute magnitudes of the eight torques cannot be found by multiplying the magnitude of one of them by the torque ratios. Copyright © 1981, Wiley Blackwell. All rights reserved