We have performed numerical mantle convection experiments in an annulus with plates whose geometries evolve with time. A thermally activated rheology results in a high-viscosity surface boundary layer, which is mobilized by weak zones (small regions of low viscosity). These weak zones are advected with the flow, leading to evolving plates. Surface plates organize the mantle flow system, resulting in large-scale structures, and a thermal angular power spectrum dominated by low angular degrees. Plate size and plate velocity distributions are used to characterize the system of surface plates. These statistics are sensitive to the radial flow structure and provide a new way of evaluating convection models using reconstructions of Earth's platetectonic record. A direct comparison of the results from our two-dimensional (2D) experiments to the plate-tectonic record for the past 120 Ma indicates that models with a 30-fold viscosity increase in the lower mantle are most consistent with these observational constraints, while calculations with a strong endothermic phase change that leads to intermittently layered convection are least consistent with the plate-tectonic record. ¿ American Geophysical Union 1995