Isostasy refers to the recognition that the floats on the denser mantle

Lithospheric Crust which is referred as SIAL.

Explanation:

The iso meaning the same and Stacy means an adjustment in the crust of the earth, as the density on the surface of earth varies so does the amount of continental and oceanic crust too charges there relative density and hence the crust rests upon the denser mantle rocks from below. The isostasy also represents an arrangement of the geomaterial from the earth mantle towards the surface of the earth and these materials are recycled geometrically. Also explained by the elastic rebound theory.