As old mountains wear down, the tremendous pressure of the molten mantle deep below pushes the peaks back up--but not as much as it ought to. The reason, according to a new study, is a bit of alchemy: The mountains' roots slowly metamorphose into denser minerals. If geologists can figure out the details, they'll better understand how Earth's surface evolves.
Earth's crust floats atop the even denser mantle like a skin of cream on milk. Mountains are roughly symmetrical swellings in the crust: Their skyward-thrusting slopes are counterbalanced by an equivalent, inverted mountain's worth of mass underneath. As old mountains erode away, they bob upward like icebergs with their tops melted off. Mountains' roots rise too--but only to a certain point. In many cases, a vestige of the roots remains even after an ancient mountain range has eroded completely away. Why the roots don't eventually level out with the rest of the crust is a mystery.
The new study offers an answer. In the 27 June issue of Nature, geologist Karen Fischer of Brown University in Providence, Rhode Island, presents her analysis of previously published gravity studies--which measure tiny variations in gravity's pull across a region--and seismic reflection experiments, which reveal the depth and shape of the crust-mantle boundary deep underfoot. Fischer combined these sets of data for young and old mountain chains around the globe. After estimating the densities of their crustal roots, she concludes that they remain sunken because a geochemical process called metamorphosis has converted them into denser minerals as they cooled over hundreds of millions of years.
"The explanation is certainly plausible," says geochemist John Eiler of the California Institute of Technology in Pasadena. He says the findings fit with other current thinking about the crust's nether regions and should contribute to our understanding of how the lower crust and the shape of the surface are related.