Studies, using a variety of techniques, of the subduction beneath western South America and the southwest Honshu-Kyushu region of Japan indicate that volcano-free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere. The subduction is initiated at normal dip angles, but the plate deforms at some depth (100 km under central Peru) to travel horizontally immediately beneath continental lithosphere. The most plausible reason for this geometry is that the subducted plate is buoyant. A model is developed constrained by age of the plate, bathymetry, and heat flow. Estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts. If the high-density transformation of crustal basalt to eclogite is retarded by low temperatures, the plate can remain buoyant to considerable depth for long periods of time. Heat flow data from western South America are consistent with this model.