Science: South America's Towering Andes Range Formed Abruptly

Sedimentary rocks of the Salla Formation, Eastern Cordiller, Bolivia. These rocks were deposited between approximately 29 and 25 million years ago. Paleoelevation constraints from these rocks suggest that they were deposited at less than 500 meters, whereas now, they sit at approximately 3600 meter elevations.

[Image © Science]

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The majestic Andes Mountains, the second-largest mountain belt in the world, formed rather abruptly—between 10 and 6 million years ago—much faster than previously assumed, according to new research appearing in the 5 June issue of Science.

Carmala Garzione of the University of Rochester, with colleagues from Michigan, Florida, California, India, and Germany, reviewed in-depth studies of sediment, magma, and the movements of the Earth's crust. Her findings suggest that the abrupt formation of the Andes Mountains most likely was triggered when dense material at the base of the Earth's crust shifted further downward, into the planet's mantle. In her Science review article, Garzione concludes that the transformation of plateaus into mountain ranges can consist of long periods when mountains do not grow much for tens of millions of years, but then rapidly begin to rise by 1 kilometer or more within 1 to 4 million years.

Our planet's core is surrounded by a layer of Earth known as the mantle, beneath the outermost layer, called the crust. The Earth's mantle and crust are constantly shifting around beneath our feet, and over long periods of time, those underground movements can actually shape the landscape around us, forming mountains and rivers and islands.

Carmala Garzione high in the Andes, where she studies paleoelevation; the science of how mountains rise.

[Credit: University of Rochester]

Traditionally, mountain formation has been described as a gradual folding and faulting of the Earth's upper crust. Now, researchers are realizing that the formation of gigantic mountains may occur far more quickly than we once believed. Garzione and her colleagues now suggest that there are times during the formation of mountains when they stay the same height for tens of millions of years, and then rapidly grow taller over several million years.

The Andes Mountains play a key role in the South American climate, so understanding their formation may help reveal future environmental changes. Garzione's findings of rapid mountain uplift in the Andes "means the current theory of plate tectonics will have to be modified to include a process called `delamination,'" according to the National Science Foundation (NSF), which funded Garzione's research. Delamination was proposed decades ago, the NSF notes, but "it has been controversial because mechanical models of mountain building have a hard time reproducing it, and, until the new findings, there has been a lack of reliable paleoelevation measurements."

As oceanic and continental plates collide, delamination theory proposes that heavy, high-density "roots" clinging to sections of buckled continental crust can suddenly break free, launching mountain ranges upward.

The Science article, "Rise of the Andes" (6 June 2008, vol. 320, pages 1304-1307) was prepared by Garzione and Gregory D. Hoke of the University of Rochester; Julie C. Libarkin and Saunia Withers of Michigan State University; Bruce MacFadden of the Florida Museum of Natural History at the University of Florida; John Eiler of the California Institute of Technology; Prosenjit Ghosh of the Indian Institute of Science in Bangalore; and Andreas Mulch of the Universität Hannnover in Germany.

Brandon Bryn and Ginger Pinholster

5 June 2008