The document discusses the geological concept of isostasy. There are two primary models for isostasy - the Pratt model and Airy model. The Airy model proposes that different topographic heights have the same rock density but different crustal thicknesses. The Pratt model proposes that different topographic heights have the same crustal thickness but different rock densities. The Himalayas have a thicker crust because, according to the Airy model, areas with higher elevation are supported by thicker crust and deeper roots.