Isostasy is an equilibrium regarding a planet's crust and mantle such that rigid sections of the crust are supported by buoyancy, i.e., if a relatively-solid portion of the crust is heavier than surrounding crust or has a weight loading it down (e.g., a volcano), its underside displaces some of the mantle, which given enough time, functions as a very-highly-viscous fluid. Isostasy is analogous to hydrostatic equilibrium, but accommodates the solid sections found in geological formations. The crust is treated having portions that are to some degree solid, plastic, or flexible, and the mantle as a fluid that is denser than the crust. Isostasy is an ideal regarding the surface of a planet, i.e., an approximation. The concept of such buoyancy explains some land-forms but others don't fit the picture, presumably due to sufficient time not having passed or the presence of mantle material less inclined to function as a fluid.