Distinct rigidly moving oceanic and continental plates of finite thickness are incorporated into a two-dimensional numerical model of mantle convection. We investigate upper mantle convection in models having aspect ratios as great as 24 and compare our findings with the results of earlier studies which were limited to aspect ratio 4 models. In addition, we implement models of whole mantle flow by specifying high Rayleigh number convection and thinner nondimensional plates. We are thus able to compare the results of continental collision models which include similarly sized continents in the cases of upper and whole mantle convection. For each case considered we model a pair of identical continents being carried toward a site of plate convergence by underlying counterrotating mantle convection cells. Upon collision, the continents form a motionless, rigid, model supercontinent, while oceanic plate material continues to recycle through the mantle. Following the continental collision, our models of upper mantle convection exhibit a reorganization of the convective planform below the model supercontinent into a smaller wavelength mode which is unable to generate the net stress needed to break apart the continent; alternating compressive and tensile deviatoric stress associated with the small scale flow results in a low integrated stress. In contrast the large scale of whole mantle convection enables flow reversals to produce shear stresses acting in a common direction over extensive areas of the base of a continent, the integrated effect of which is capable of causing continental rifting. The conventional view of the role of thermal blanketing in continental rifting does not apply in the whole mantle convection scenario. ¿ American Geophysical Union 1996