Understanding the mechanism of plate tectonics is one of the most important problems in the geosciences. Are the tectonic plates pulled and pushed from the side, as the Orowan-Elsasser model suggests, or does mantle convection play an active role in driving the plates? This question can be addressed by studying the deformation of deep continental roots. The application to North America shown here indicates that the deeper mantle moves at a higher velocity than the North American plate and that the mantle plays an important role in driving the plates. I suggest that this finding (1) provides a natural explanation for why the motion of North America slowed down dramatically during the past 100 m.y. and (2) predicts that North American motion will eventually come to a halt.