The answer is; Isostasy
Just like a boat displaces water in which it floats, so does the solid lithosphere on the fluid asthenosphere. The heavier regions of the lithosphere, such as those with high mountains above them penetrate deeper into the asthenosphere and displace a huge portion. Isostasy occurs when each block settles into equilibrium with the underlying mantle (buoyed with a force equal to the weight of the displaced fluid).
