The growth of thickened crustal units both depletes the mantle of heat producing elements, and thermally insulates it. These effects either cool or heat the mantle, respectively, and play an important role in subsequent crustal formation. We employ mantle convection models including thickened crustal units of variable heat production to show that for increasing crustal extent, mantle temperatures and melt production can either increase of decrease, depending on the degree of enrichment of the crust with respect to the mantle, the total heat production, and the Rayleigh number. The formation of the continents on Earth efficiently cooled the upper mantle, resulting in lower subsequent rates of melt production and continental formation. In contrast, the growth of the Martian highlands would have raised the temperatures of the Martian mantle, increasing rates of melt production, and leading to runaway crustal growth. This would have continued as long as the lithosphere of Mars was mobile.