Isostasy (also spelled Isotacy) is a geophysical phenomenon describing the force of gravity acting on crustal materials of various densities (mass per unit volume) that affects the relative floatation of crustal plates. Isostasy specifically describes the naturally occurring balance of mass in Earth's crust.

Continental crust and oceanic crust exist on lithospheric plates buoyant upon a molten, highly viscous aethenosphere. Within Earth's crustal layers, balancing processes take place to account for differing densities and mass in crustal plates. For example, under mountain ranges, the crust slumps or bows deeper into the upper mantle than where the land mass is thinner across continental plains. Somewhat akin to how icebergs float in seawater, with more of the mass of larger icebergs below the water than smaller ones, this bowing results in a balance of buoyant forces termed isostasy.

Isostasy is not a process or a force. It is simply a...