The heat transport by a viscous fluid with temperature dependent viscosity has been studied numerically. As opposed to previous models, the top surface of the fluid clearly defines a tectonic plate with horizontally uniform velocity and subduction. Past studies failed to incorporate plates resulting in inefficient heat transport. With tectonic plates, the heat transport is as efficient as Rayleigh-Benard convection with constant viscosity; there is a strong buffering between internal temperature and heat loss. Past studies of parameterized convection which incorporated parameters indicative of a strong buffering between internal temperature and total heat output still provide the most physically plausible representation of the Earth's thermal evolution. ¿ American Geophysical Union 1989