By analyzing convection characteristics and surface and basal heat flux we investigate the influence on mantle convection of finite thickness plates with dynamically determined velocities and mobile boundaries. Our 6 × 6 × 1 Cartesian geometry numerical model features periodic sidewall boundary conditions, a lower mantle that increases in viscosity and a Rayleigh number of 5 × 106 (based on the uniform viscosity upper mantle). A calculation with 9 plates and fixed plate boundaries is compared with a second calculation incorporating evolving plate boundaries. The number of plates in the latter case fluctuates between 6 and 9. We find that mean heat flux output decreases during periods characterized by the presence of larger plates and that evolving plate boundaries cause surface (and basal) heat flux output to vary much more than the heat flux observed with either fixed plate boundaries or a free-slip surface.