Sunday, June 8, 2008

Andes Rose More Rapidly than Earlier Thought

Sediment analysis shows that part of the Andes rose relatively suddenly from below sea level to over 12,000 feet about 10 million years ago.

South America’s Andes – one of the world’s tallest mountain chains – soared to their height much more suddenly than scientists have previously thought. Earlier studies contended that the slow collision between the Nazca Plate and the South American plate gradually lifted the Andes skyward.

But new analyses of South American sediments deposited in a high-altitude region of Bolivia and Peru now points to a much more abrupt growth of the towering range between 10 and six million years ago.

In examining the sediments, John Eiler, a geochemist at the California Institute of Technology, and his team of researchers suggest that eclogite – a dense rock often forming at the base of the earth’s crust – broke loose beneath the Andes and sank into the earth’s mantle. The resulting lightened continental crust then shot upward, raising the mountains in far fewer millennia than scientists had believed.

The presence of marine sediments in the region indicates that it sat just below sea level about 65 million years ago. Growth of the Andes was slow between 25 and 10 million years ago, but then the region rose some 2.5 kilometers to its present elevation of 12,500 feet.

Other evidence supporting the team’s contention includes the changing size, shape and variety of fossil leaves found in the sentiment, chronicling climate changes that resulted from the Andean uplift.

Here is the article in Science News.

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