You just can't wear them down.

You just can't wear them down

Erosion has leveled the tallest mountains, gouged deep ravines into the hardest rock and even obliterated most meteor craters on earth. But George Boyajian and David Rowley of the University of Chicago believe they have found a match for this ever-grating phenomenon. Using a series of simple calculations, they have found that craters larger than 20 kilometers in diameter are theoretically impervious to erosion.

According to the researchers, large craters escape erosion because the earth's crust floats on a denser, more fluid mantle. The process of erosion is delicately balanced by the forces of buoyancy that keep the crust afloat. Erosion, therefore, can only shave off the top 4 km of the continental crust before the mean height of the crust falls below sea level, at which point erosion stops. Craters larger than 20 km are usually deeper than 4 km, however, which implies that even if the entire continent is worn down, these structures will remain.

This kind of analysis could help those who study the insides of other planets and moons, say the researchers. The ratio of large to small craters may indicate whether a similar buoyancy exists, giving clues about what lies beneath a planet's crust.