By LEE BOWMAN
Scripps Howard News Service
January 28, 2006
Each of the studies - one appearing Friday in the journal Science, the other next week in Earth and Planetary Science Letters - used new and different methods for measuring ancient mountain elevations, but the results are in close agreement.
"These results really change the paradigm of understanding how mountain belts grow," said Carmala Garzione, assistant professor of earth and environmental sciences at Rochester and co-author of both studies.
"We've always assumed that the folding and faulting in the upper crust produced high-elevation mountains. Now, we have data on ancient mountain elevation that shows something else is responsible for the mountains' uplift."
Even as mountains lift up, weather is constantly eroding them, making it difficult to estimate how high a mountain was at any point in the past, or how rapidly it was formed.
Most work in this area, called paleoaltimitry, has until now relied mostly on dating the fossils of leaves found in rocks from the mountain to calculate how high those plants might have lived, or by dating when certain minerals began moving to the surface. Neither approach has produced finely calibrated dating for mountains, since plant characteristics and erosion patterns can change drastically over time.
Garzione has focused instead on the stories that might be told in sediment that eroded from the tops of mountains and wound up in streambeds at the base of mountain ranges. Materials on mountains experience different types of weather, and the researcher has been looking for ways to retrieve the record of those changes from minerals that grow near the surface at different altitudes on a mountainside.
One method measured oxygen concentrations in sediment taken from Bolivia's Altipano basin, where sedimentary rock built up at a relatively high elevation between 12 million and 5 million years ago. The type of oxygen isotopes left in the sediments from ancient rainwater shows how high up the sediments were when they were washed away.
The second method used the same sediment, but focused on the temperature present when the carbonate minerals formed. Garzione worked with Prosenjit Ghosh and John Eiler of the California Institute of Technology using a method developed at CalTech that looks at how one oxygen isotope and a one form of carbon are bonded together.
Bonds formed at warmer temperatures that are found at low elevations tend to break more easily; those formed at higher elevations are less brittle.
Both studies came up with the same conclusion: the Andes shot up between 10 million and 7 million years ago, from a modest 2,500 feet to more than 2 miles high.
"When I first showed this data to others, they had a hard time believing that mountains could pop up so quickly," said Garzione. "But with the supporting data from the new paleotemperature data, we have more confidence in the uplift history."
Specifically, the findings give support to a controversial tectonic theory, called "deblobbing," which proposes that a dense root of Earth's crust is forced downward into the liquid mantle beneath a point where two tectonic plates collide. As the crust buckles upward trying to form mountains, the blob acts like an anchor, much as a fishing weight on a small bobber holds the bobber low in the water.
In the case of the Andes, the researchers propose that they rose about a half-mile before the blob beneath them broke off and began to sink, and just like cutting away the fishing weight, the mountains suddenly bobbed high above the surrounding crust to reach a height of roughly 2-1/2 miles in less than 3 million years.
Some geologists think similar blobs anchor the Sierra Nevada Mountains of California, based on patterns observed as seismic waves from earthquakes pass beneath them. Most experts think those mountains might be several million years away from a growth spurt like that experienced by the Andes, however.
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