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Andes rose rather rapidly: analyses of sediments suggest new scenario explaining South American mountains' height.

South America's Andes reached their staggering heights after a sudden growth spurt millions of years ago, new evidence suggests.

The central part of the Andes, one of the world's longest and tallest mountain chains, is home to some of the Earth's thickest crust: In spots, the crust extends to depths of 70 kilometers (SN: 1/15/05, p. 45). Previous studies have suggested that the slow, steady collision between the Nazca Plate, made of dense oceanic crust, and the South American plate, of lighter, continental crust, gradually lifted the Andes. But new analyses of South American sediments cast doubt on that steady-growth scenario, says John Eiler, a geochemist at Caltech in Pasadena.

Eiler and his colleagues looked at the mix of rare chemical isotopes in sediments found in the Altiplano, a high-altitude region of Bolivia and Peru that lies between parallel chains of Andean peaks. When sediments are deposited at low temperatures, atoms of some rare isotopes are more likely to end up near each other in the resulting crystal structure, Eiler explains. The higher the environmental temperature, the more random the distribution of those atoms. Using such mineralogical analyses, plus measurements of oxygen isotope ratios affected both by temperature and elevation, the researchers could estimate the elevation at which the sediments were deposited.

The presence of marine sediments in the Altiplano indicates that the region, which now averages about 3,800 meters (12,500 feet) in elevation, sat just below sea level about 65 million years ago. Between 29 million and 25 million years ago, sediments now lying high in the Altiplano were being deposited at elevations below 500 meters, the new analyses suggest. Growth of the Andes was slow between 25 million and 10 million years ago, but then between 10 million and 6 million years ago the landscape rose about 2.5 kilometers, the researchers report in the June 6 Science.


"These techniques allow us to measure a major attribute of Earth's history- its elevation--rather than infer it," says Cornell University geologist Teresa Jordan. "For the Andes, that's opened the door to surprises."

Why the sudden change in elevation? Eiler and colleagues suggest that a large mass of dense rock that often forms at the base of Earth's crust--a type of rock called eclogite--detached beneath the Andes and then sank into the mantle. Relieved of that weight, the overlying, relatively light continental crust bobbed upward like a cork, thereby raising the mountains.
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Title Annotation:Earth
Author:Perkins, Sid
Publication:Science News
Geographic Code:1USA
Date:Jul 5, 2008
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