Extraction of partial melt to form oceanic crust reduces the density of upwelling mantle beneath spreading centers. This compositional buoyancy can significantly affect both the rate of upwelling and the pattern of mantle flow. The relative importance of buoyant upwelling and upwelling due to plate spreading depends on the mantle viscosity. For a low enough mantle viscosity, buoyant upwelling can significantly enhance the thickness of the crust relative to that which would be produced by plate spreading alone. For a mantle viscosity of 1022 Pa s, upwelling due to plate spreading is dominant, and crustal thickness is predicted to be a strong function of spreading rate. However, for a mantle viscosity of 1019 Pa s, buoyancy dominates mantle upwelling, and the calculated crustal thickness, like the oceanic crust, is nearly independent of spreading rate. Thermal and compositional density variations result in opposing buoyancy forces that can cause time dependent upwelling. For a mantle viscosity of 1020 Pa s and a low 1 cm/yr spreading rate this behavior takes the form of undamped crustal thickness oscillations with a single, well defined period. For even slightly lower mantle viscosity, the frequency increases, and crustal thickness oscillations become more complex with two or more frequencies present. While numerical experiments have thus far been carried out only for viscosities significantly greater than that likely to characterize the mantle beneach spreading centers, we speculate that these crustal thickness oscillations may explain important aspect of seafloor structure. ¿ American Geophysical Union 1989