The quest to identify habitable planets has raised interest in the surface dynamics of terrestrial bodies. In this context super-Earths (a new class of exoplanets) have become of special interest in the past decade. Scalings to super-Earth sizes, when compared to the Earth, suggest changes to convective stresses and mantle temperatures which can cause either an increase in surface mobility or in plate resistance. Mantle viscosity, which depends on temperature, stress and pressure, plays a critical role in both cases. New mineralogical assumptions suggest that the viscosity in super-Earths acts differently than in the Earth, and what had been assumed for super-Earths. In planets larger than the Earth, pressure will become so high that pressure-weakening and a decrease of viscosity in the lowermost mantle results. We present a numerical convection study featuring this viscosity decrease and find that this leads to a reduction in surface mobility.