Cartoon illustrating the major tectonic and magmatic processes operating in the thin (mobile)-lid regime (>0.7 Ga) and in the thick (stagnant)-lid regime (~0.5 Ga to present). The boxed inset shows thermal lithosphere thickness (km) versus age calculated from a thermal model. In the thin-lid era, lithospheric recycling was associated with mantle downwellings whose surface expression included ridge belts. In the thick-lid era, lithospheric extension over a heating mantle produces corona chains and casmata, which mark broad diffusive upwellings; the topographic basins of the plains reflect mantle downwelling (Phillips and Hansen, 1994, Annu. Rev. Earth Planet. Sci., 22, 597).