Some nonconductive heat transport in the mantle wedge above the subducting lithosphere is necessary to explain the volcanic and thermal activities in subduction zones. The physical mechanisms that have been proposed to explain the observed thermal processes are classified into the following three categories: (1) secondary convection mechanically driven by the motion of the slab, (2) thermal convection due to a large horizontal temperature gradient associated with the presence of the cool slab, and (3) flow induced by ascending mantle diapirs containing low-density melt. The relative importance of these three mechanisms is evaluated, using a simple model in which the conduit of the convective flow is prescribed. The first mechanism of mechanically driven flow dominates the other two when the effective viscosity of the mantle wedge exceeds 1020 Pa s (1021P). Known estimates wedge viscosity suggest that either the second or third mechanism is more likely to be the one that actually governs the tectonic process. It is proposed that both the second and third mechanisms are realized in different stages of a single tectonic sequence and that arc volcanism and back arc spreading occur, corresponding to these different modes of convection.