Explanation: The isostatic equilibrium occurs when enormous tectonic plates composed of crustal and upper mantle material are able to float on top of the astenosphere, which in turn is composed of denser rock material. In order for such a thing to be achieved the lithosphere and the astenosphere have to have balanced densities between each other. In other words, the density of the material on top, has to be in balance with the density of the material below it, thus to not have any uplift or sinking occurring between them.
