It is shown that the upper mantle is probably unstable. The instability expresses itself in diapirism under the ridges and sinking of the lithosphre under the island arcs. Both processes exert a force on the plate that can move it against the viscous drag from the flow of the asthenosphere. Order of magnitude estimates of the forces and of the required energy demonstrate that the process is plausible and that the drift rate can be determined from the resistance the plate encounters at its edges, particularly below the island arcs where it plunges into the mesosphere. The proposed mechanism can be tested by studying, e.g., the deep structure of the ridges, the stress field in the plates, and the gravity field over ridges and trenches.