Floating on the mantle

Isostasy is the idea that the crust floats on the denser mantle below, just like a block of wood floats on water. A thicker or lighter block rides higher; a thinner or denser block sits lower.

This is why continents stand above sea level and ocean floors sit far below: thick, light continental crust floats high, while thin, dense oceanic crust floats low. A tall mountain range must also have a deep "root" of crust pushing down into the mantle — like the hidden bulk of an iceberg.