Arid ascension. Dry conditions may have pushed the Andes to remarkable heights.

Drought Pushed the Andes Higher

Many dramatic events in Earth's history are attributed to climate change, from mass extinctions to the drying up of inland seas. Now, two geologists suggest that climate change may also fiddle with events deep inside Earth. They propose that over the last 40 million years, bouts of cool and dry climate along the west coast of South America may have leveraged the towering Andes to much greater heights than other, similar ranges.

The Andes rose to 5 kilometers above sea level as oceanic crust slid beneath the lighter South American continent. That's a typical way to raise mountains, yet the towering peaks are a good 1 to 2 kilometers higher on average than mountains formed in similar situations around the Pacific rim. What caused the extra tectonic stress needed to bulk up the Andes is unknown.

Faced with this conundrum, University of Oxford geologist Simon Lamb and geologist Paul Davis at the University of California, Los Angeles, calculated how much stress would be needed to support the height and weight of various mountains along the length of the Andes. Lamb and Davis then made what they think is a key observation: The central Andes, where the peaks are highest, faces an extreme lack of rain and rivers. That means little sediment is carried offshore to settle in the oceanic trench where the ocean plate dives beneath the continent. In the 23 October issue of Nature, the pair suggests that less sediment equated to less lubrication. As the ungreased oceanic plate scraped beneath the continent, it provided the greater stress needed to build the mountains higher.

To see if dry conditions were also prevalent when the Andes were on their way up, Lamb and Davis checked records of deep ocean temperatures. These suggested that periodic pulses of cold water from the Antarctic along the South American coast could indeed have kept rainfall low, and these cold periods also seem to match times of very rapid uplift in Andean height.

The idea has merit, says geologist Richard Allmendinger of Cornell University in Ithaca, New York, and is certainly worth investigating further. But he cautions that it's hard to get accurate chronologies for climate and mountain uplift and the correlations may not hold up as more data come in.

,pRelated sites
More information about mountain formation
Links to various Andes geology research projects