concept

Isostasy

The principle behind why a mountain range stands high at all: light crust floats on denser mantle exactly as an iceberg floats on water, and how far it rises depends on how deep it reaches. The formula is startlingly simple — the root equals the height times crustal density divided by the density difference between mantle and crust — but the number is not: a 5 km peak needs a root of some 28 km. Most of a mountain is underground. And that is why measuring a range's gravity tells you immediately whether it floats or is merely being propped up.