Relative velocities of crustal plates are used for the first time to test the conventional assumption that such plates are rigid. Spreading rates derived from magnetic anomaly data, fracture zone trends, and earthquake mechanisms at the Pacific-North America and Pacific-Cocos plate boundaries are used to predict the direction of thrusting of the Cocos plate under North America to be 031°. As this direction is in near accord with previously determined earthquake mechanisms at that plate boundary, the assumption that the three plates are rigid appears to be largely justified. The calculated rate of underthrusting at the Cocos-North America plate boundary is about 8 cm/yr.