Isostasy is a theory of geophysics to explain the anomalies in the gravitational field at the Earth's surface. It is actually a simple application of Archimedes' principle of isostatic equilibrium.
In 1851, the Astronomer Royal Georges Airy, known above all for his contributions to the theory of diffraction in wave optics, had tried to explain why the Himalayan mass did not attract a pendulum as much as it should have done. By introducing the idea of a light continental crust floating like an iceberg on a heavier mantle, he deduced that any large positive relief should have prolongations and the greater the altitude above sea level, the deeper the prolongation. A chain of mountains should therefore have "roots" descending into the mantle.
Thus the resulting gravitational field was not that of a simple relief placed on a crust above a homogeneous mantle, and the observations were quite well borne out by the calculations.
Other models, also based on Archimedes' principle, were introduced, including that of Pratt. In this one, here are no roots in the crust and it is the density of the crust that varies as a function of the height of the relief.
The term isostasy was proposed in 1889 by the American geologist Clarence Edward Dutton. It is derived from the Greek word isostasios, from iso (equal) and statikos (stable).
Illustration of isostasy with, from top to bottom, the Airy and Pratt models, and a model to explain the isostatic rebound due to glaciations.