Andes Mountains Jumped Like a Cork
for National Geographic News
|June 5, 2008|
After millions of years of slowly being squeezed, warped, and folded higher, the central Andes suddenly shot toward the stars in a geological blink of the eye, according to a new study.
The growth spurt came after chunks of dense, deep rock that anchored the South American mountain range suddenly plopped deeper into the mantle, freeing the buoyant crust to pop up like a cork unleashed from a lead weight.
"This will cause the rapid elevation change that we see over the short period of time," said study co-author Gregory Hoke, a geologist at the University of Rochester in New York.
In a span of as little as a million years, the central Andes grew at least 5,000 feet (1,500 meters)—more than a third of their average height—Hoke and colleagues report in tomorrow's issue of the journal Science.
The spurt occurred between ten and six million years ago, concludes the team, which was led by Carmala Garzione, a geologist at the University of Rochester.
Scientists generally think most mountains rise in a relatively steady, lockstep manner as pieces of Earth's crust crumple and buckle together in the process of plate tectonics.
Garzione and her colleagues used a newly developed technique that interprets chemical signatures from isotopes of oxygen, carbon, and other elements in ancient soils to determine the history and rate of elevation rise.
The ratio of isotopes varies depending on what elevation ancient precipitation fell onto the mountains. Other data allow the researchers to determine when the rain or snow fell.
This so-called paleo-elevation record suggests the mountains rose slowly for tens of millions of years and then suddenly lifted at a rate that is "faster than the commonly accepted tectonic process," Hoke said.
The best explanation for the rapid rise, he added, is delamination—the loss of dense rock at the base of the crust.
The rapid uplift, he noted, helps reconcile the timing of a major reorganization of the landscape on both sides of the Andes.
Assuming Too Much?
Jason Barnes is a geologist at the University of Michigan in Ann Arbor who was not involved in this research but recently completed his doctoral thesis on the deformation and erosion history of the central Andes.
He said the Science study is based on solid data but paints a different picture than the one suggested by his own research and preliminary modeling studies by his colleagues.
The difference, Barnes explained, has to do with interpreting isotope data.
Isotope variations deposited in soil depend largely on regional weather patterns. The major assumption of the Science study is that the modern weather pattern is representative of the climate pattern millions of years ago.
(Related: "Andes' Height Due to Climate, Study Says" [October 22, 2003].)
But "the climate might have actually been significantly different from today," Barnes said.
His colleagues' preliminary modeling studies of the ancient climate suggest that precipitation at times reached the central Andes from the Pacific Ocean side of South America, not the Atlantic side as it does today.
Precipitation from the Pacific, Barnes noted, provides a different isotopic signature. When this difference is factored in, it could suggest "the rise of the Andes was slow and steady rather than rapid and fast."
While he doesn't dismiss Garzione's work, he said "the jury is still out."
Maria Teresa Ramírez-Herrera is a geologist and visiting scholar at the University of California, Berkeley, who is studying the recent rise of bedrock in a section of the southern Andes in Argentina.
(Her research is funded by a grant from the National Geographic Society, which owns National Geographic News.)
She said her data "is suggesting rapid uplift" but cautioned that the results are preliminary and need to be confirmed.
If correct, she added, then globs of dense rock at the base of this section of the Andes could be dropping deeper into the mantle now, allowing the mountains to spring up in the same manner suggested by Garzione.
Other mountain-building processes such as crust thickening, she said, are also likely at play.
Barnes, of the University of Michigan, noted that the delamination theory has gained traction among geologists and is believed to also be occurring in the southern section of California's Sierra Nevada range.
The trick now is to further develop techniques that directly measure mountain elevations at various points in time, he said.
"It's adding a really cool component to our efforts to reconstruct the birth, life span, and death of a mountain range."
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