Numerical models of upper mantle convection incorporating both plates and a substantial low viscosity layer show that long-wavelength topography is only moderately reduced compared with a model without the low viscosity layer. In particular, topography due to hot upwelling from a putative hot boundary layer at 670 km depth is still large. In the case of a migrating spreading center, the symmetry and square-root-of-age dependence of seafloor topography would be destroyed. These results strengthen the case against mantle models in which heat transport across the 670 km seismic discontinuity is predominantly by conduction. A substantial mass flux through 670 km is implied. ¿ American Geophysical Union 1989