Swift lift

Natural History, Feb, 2004 by Caitlin E. Cox

Why are the central Andes so high, and how did they get that way so quickly? Geologists Simon Lamb of the University of Oxford and Paul Davis of the University of California, Los Angeles, think they have the explanation. It's the lack of lubrication, caused by an arid climate.

About 14 million years ago, when ice sheets around Antarctica expanded drastically, the west coast of South America between ten degrees and thirty-three degrees south latitude--the belt that today comprises the most dramatic peaks and valleys--became much cooler and drier. In the relative absence of rain, the erosion of rock and soil decreased drastically. As a consequence, little sediment was washed into the mammoth trench in the Pacific seafloor where the Nazca tectonic plate has been sliding under the South American plate and pushing up the Andes.

Ordinarily, sediment acts like oil, lubricating the boundary between colliding tectonic plates. When there's little or no sediment, though, the friction between the two plates can exert a lot of force. In the central Andes that force is so strong and focused that it helps support the mountains, much as strong, deep foundations hold up a skyscraper. Lamb and Davis estimate that, had the climate not become "hyperarid," the region's mountains would be no higher than 6,500 feet. ("Cenozoic climate change as a possible cause for the rise of the Andes," Nature 425:792-97, October 23, 2003)