A rapid rise for the Andes

New evidence points to a sudden and dramatic growth spurt for the South American mountain chain

South America’s Andes reached their staggering heights after a sudden growth spurt millions of years ago, not gradually as many studies have proposed, new evidence suggests.

QUICK-RISE ROCKS Geochemical analyses of 29-million-year-old sedimentary rocks in eastern Bolivia, now sitting at an elevation of 3,600 meters, were initially deposited at an elevation below 500 meters. Science

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 two tectonic plates — the Nazca Plate, made of dense oceanic crust, and the South American plate of lighter, continental crust — crumpled the crust and gradually lifted the Andes. But new analyses of South American sediments cast doubt on that steady-growth scenario, says John Eiler, a geochemist at the California Institute of Technology in Pasadena.

Eiler and his colleagues looked at the mixture and distribution of rare chemical isotopes in sediments deposited in geological basins found in the Altiplano, a high-altitude region of Bolivia and Peru that lies between parallel chains of peaks in the central Andes. When sediments are deposited under low-temperature conditions, atoms of some rare isotopes are, on average, more likely to end up near each other in the resulting crystal structure, Eiler explains. The higher the temperature of the environment is, the more random the distribution of those atoms becomes. Using the results of such mineralogical analyses, plus measurements of the ratio of oxygen isotopes found in the rocks – which can be affected by both temperature and elevation — the researchers could estimate the elevation at which the sediments were deposited.

The presence of marine sediments in the Altiplano indicate 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 blink of a geological eye — 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 Teresa Jordan, a geologist at CornellUniversity. “For the Andes, that’s opened the door to surprises.”

Other evidence points to a rapid rise in the Andes during this period, Eiler says. For one thing, changes through time in the size, shape and variety of fossil leaves found in the sediments chronicle the change in climate that resulted from the Andean uplift. Patterns of erosion in the surrounding mountains also reflect the change, he adds.

Why the sudden change in elevation? Eiler and his 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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