This paper tests two hypotheses for the influence of thick lithosphere (the tectosphere) upon plate motion. The first is that the tectosphere reaches down into high-viscosity regions of the asthenosphere, effectively slowing the motion of continental plates. The second is that the tectosphere reaches down below the slow-moving convective boundary layer into regions of rapid mantle convection and effectively speeds (or slows) the motion of continental plates. Our data imply that the tectosphere speeds plate motion rather than impedes plate motion and are thus most consistent with the second hypothesis. ¿ American Geophysical Union 1996