Tectonic plates are formed from the convecting mantle’s upper thermal boundary layer. Convective vigour, expressed quantitatively by the system Rayleigh number and observationally by the boundary layer thickness, is impacted by the mantle’s activation energy, surface temperature and reference viscosity. Viscosity magnitude and gradient profoundly influence the mean temperature of the mantle, which in turn affects the yield strength of the lithosphere. Multiple parameters determining mantle viscosity and plate strength result in a feedback loop that ultimately determines plate thickness. We present results from a systematic investigation of the feedback between Rayleigh number and yield stress on plate thickness in a global mantle convection model. Mantle convection is modeled in a spherical annulus solving the dimensionless equations for mass, momentum, and energy conservation in a bimodally heated infinite Prandtl number Boussinesq fluid. Previous studies of mantle convection models often specify a total viscosity contrast across the convecting system in order to obtain a numerical solution. However, such studies typically neglect the influence of absolute surface temperature on viscosity. Here we focus on the impact of an activation energy inspired by a pyrolitic composition and Arrhenius viscosity law, and an Earth-like absolute surface temperature, on the thickness of the thermal boundary layer that forms in calculations spanning reference viscosity (affecting Rayleigh number) and lithospheric yield stress space. Our results are compared to a model utilizing a commonly employed equation for nondimensional viscosity. The findings have implications for generating mantle convection models that reproduce Earth-like structure at seafloor spreading boundaries