concept

Hypsometry

How a planet's surface elevations are distributed — which sounds like dull statistics until you see that it is one of the sharpest tectonic tests available. Earth has two distinct peaks: continents cluster around 840 m above sea level, ocean floor around 3,700 m below it. Two peaks mean two kinds of crust with different densities. Venus and Mars have a single peak, because they have a single kind of crust. One elevation curve rules out a great many hypotheses.