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Earth science

How do your mountains grow?

The soaring, jagged peaks of the world's second highest mountain range, the Andes, could owe their stature not to the power of the Earth but to that of the sea. So propose Simon Lamb and Paul Davis in this issue (Nature 425, 792–797; 2003).

Most mountain ranges are created by the collision of two continental plates, such as the grinding action of India against Asia that thrust up the Himalayas. But the Andes are perched on a point where an oceanic plate slips down beneath a continental one. Great mountains aren't usually born of such meetings. Heavy, dense oceanic plates tend to slip underneath continental ones, causing major earthquakes but not world-class mountains. If the push of the mid-Atlantic ridge on tectonic plates were the only factor driving up the Andes, one calculation shows that they would be no more than two kilometres high — half their actual height.

Lamb and Davis argue that the extra push comes from the fact that the forces of the plate collision are focused on a small area — a stretch of the plate boundary where the friction in the trench between the Pacific and South America is particularly high. The cold water current that sweeps up the west coast of Peru and Chile from high southern latitudes encourages little water evaporation, and therefore little rain. That means there are no rivers to dump worn mountain sediment back into the ocean — sediment that could act as lubrication. Instead, the trench off the coast of the Andes is rough and dry. That extra friction, say the authors, helps to prop up the mountains.

There could even be a positive feedback loop in place, causing the mountains to bulk up more and more over time. The higher the mountains grow, the drier the coastline becomes — as any wet air coming in from the Atlantic is blocked by the towering peaks — which in turn further reduces erosion and props the mountains up still more.

Complex relationships between rock, air and sea have been found before, although it's usually mountains that are thought to affect climate, rather than the other way around. The Himalayas, for example, are believed to have changed the air flow enough to spark the formation of the Indian monsoons. And as all that rain weathered away the mountains' rocks, carbon dioxide was taken out of the air and sent down streams as carbonate, to be buried at the bottom of the sea; this extraction of greenhouse gas is thought to have cooled the global climate. But in the Andes, rather than the mountains making the climate, the climate might actually have made the mountains.

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Jones, N. How do your mountains grow?. Nature 425, 781 (2003). https://doi.org/10.1038/425781a

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