Seismic observations of the mantle, which include long-wavelength structure, a k-1 dependence of heterogeneity on harmonic k, and a heterogeneous upper boundary layer, and supercontinent kinematics may be explained by the dynamic interaction between a continent like raft and thermal convection. We have formulated finite element models of convection with rafts simulating continental plates in a cylindrical geometry. The azimuthal interconnectivity of this geometry is vital to resolve the two-way dynamics between rafts and convection. Computations show that (1) raft motion is periodic, (2) long-wavelength thermal structure is significant within both thermal boundary layers and the fluid interior, and (3) the large-scale thermal structure with a wavelength longer than the width of raft is responsible for raft motion. These three results, which are observed for a range of Rayleigh numbers, internal heating rates, and raft sizes, are a direct consequence of the dynamic interaction between the raft and convection. The physical processes for a model with a Rayleigh number of 105 are representative: when the raft is stationary, due to the less efficient heat transfer through the raft and instabilities from the bottom boundary layer, heat accumulates beneath the raft and results in long-wavelength thermal anomalies. The long-wavelength thermal anomalies enhance raft motion. Accompanying the enhanced raft movement, the long-wavelength thermal anomalies diminish and the raft velocity decreases or the raft comes to rest. Since convection models without rafts generate less long-wavelength heterogeneity compared to the models with rafts, or continental plates, we suspect that continental plates may play a crucial role in mantle dynamics. Interestingly, raft motion with a period of about 10 transit times is usually significant; 10 transit times is about 600 m.y. if scaled to the Earth. This is close to the observed 300--500 m.y. period of supercontinent aggregation and dispersal. ¿ American Geophysical Union 1993