Partial melting to generate the crust of a planet creates compositionally buoyant residual mantle. In the absence of mantle flow associated with plate tectonics, this buoyant, refractory layer may collect at the top of the mantle with important implications for evolution of the interior and surface. In this study models of the thermal and chemical evolution of a planetary interior demonstrate the possible consequences of a chemically buoyant depleted mantle layer. As the depleted layer thickens the melting temperature at the top of the underlying convecting mantle also increases and the degree of partial melting of mantle added to the depleted layer decreases. As less depleted mantle with less positive compositional buoyancy is added, negative thermal buoyancy of the layer eventually exceeds its positive compositional buoyancy. The depleted layer then sinks into and mixes with the convecting interior. The top of the convecting mantle then moves to a shallower depth, larger degrees of melting resume, and a new depleted layer accumulates. This accumulation and instability of the depleted layer occurs repeatedly over a substantial portion of the planet's evolution with a period of 300--500 Myr. On Venus the population of impacts craters is indistinguishable from a random distribution over the surface and give a surface age of about 500 Myr. We speculate that the mechanism described above may explain this episodic global resurfacing of Venus. ¿ American Geophysical Union 1992