Rochester Review
March–April 2010
Vol. 72, No. 4
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Mountain view: By analyzing the sedimentary strata in a high-elevation basin in the Andes, Garzione has found that mountains
in the South American range rose several times faster than previous research indicated. (Photo: Courtesy Carmala Garzione)ore than 12,000 feet above sea level in the Andes mountains of South America, Carmala Garzione finds herself at the center of a seismic shift in how she and other scientists understand the forces at work beneath one of the world’s longest continental mountain ranges.
An expert on the geological processes that can push the Earth’s upper crust skyward, Garzione, a professor of earth and environmental sciences at Rochester, is pioneering a new approach that she and colleagues say offers a more accurate picture of how such mountain ranges rose to where they are today.
Based on news methods of paleoaltimetry, the science of judging ancient mountain heights, that Garzione helped developed, her research indicates that the Andes rose to their current height in as little as 2 million years sometime between 6.4 million and 10 million years ago.
That’s a remarkable growth spurt for a mountain range that now features peaks between 5,000 and 7,000 meters (17,000 and 23,000 feet).
“That’s several times faster than geologists had estimated before,” Garzione says, noting that some previous work estimated that the Andes took as long as 50 million years to reach their current heights. “It means there is some unexpected process going on beneath the Earth’s crust that’s creating mountains like these.”
Investigating how that process works has earned wide recognition for Garzione. In 2007, she received the Young Scientist Award from the Geological Society of America, which cited her research as “groundbreaking.” Last year, the New York Academy of Science followed suit, honoring Garzione with its Blavatnik Award.
Her findings, which are based on detailed comparisons of the mineral composition of sediment that erodes from mountains over the life of their growth, are forcing geologists to rethink how mountains form and even how their growth contributes to global climate change.
In a process that geologists know as “shortening,” mountain ranges such as the Andes and the Himalayas are formed when vast sections of the Earth’s lithosphere, called tectonic plates, collide and push against each other. The plates buckle like a wrinkling rug, pushing up a long range of mountains. Exactly how quickly a range of mountains rises has long been shrouded in mystery because few scientists can measure how high a mountain may have been when it first started its ascent.
Garzione’s research, geologists estimated that uplift by examining the fossils left by vegetation or by dating when certain minerals from deep underground began moving to the surface. But plant characteristics can change radically over millions of years, and changes in climate can also vary the speed of erosion, throwing significant question marks into the equation.
Instead, Garzione theorized that by examining the mineral composition of sediment and comparing it to atmospheric conditions at different altitudes she would have a better picture of the time it took for a mountain to reach its height.
“I wrote my doctoral dissertation on the possibility of retrieving atmospheric information from ancient sediment in the Himalayan mountains, dating it, and forming a record of the Himalayas’ and Tibet’s uplift history,” says Garzione. “Based on my estimates, southern Tibet and the Himalayas appeared to have been high throughout their depositional history, so I was eager to put this technique to the test in a place that appears to have been at a lower elevation more recently.
We focused on the sediment that was deposited in the high Andes mountains because fossil estimates put them much lower just 10 million years ago,” she says. “However, trees cannot grow at the modern elevations in the Andes, so this fossil-based approach cannot tell us when and how fast the mountains rose. “As a mountain range rises, it experiences different atmospheric conditions due to its change in height. Those atmospheric changes, such as temperature and the amount and composition of rainfall, are recorded in minerals that form near the surface at different altitudes on the mountainside.
“The challenge was to see if I could get a clearer idea of the Andes’ growth than we’d ever had before.”
n the Bolivian Altiplano—a high-elevation basin in the Andes—Garzione took samples of sedimentary rock that had accumulated between 12 million and 5 million years ago. Garzione analyzed the mineral composition of sedimentary strata in the Altiplano, studying the ratio between the mineral carbonate, which is released from surface water during erosion, and the isotopes oxygen-16 and oxygen-18.
More than 99 percent of the oxygen in water is made up of oxygen-16 and less than 1 percent is oxygen-18, but as vapor rises to higher altitudes in the form of clouds, oxygen-18 is removed from the clouds. As rain falls, the clouds are slowly depleted of the isotope. Because the change is locked in the minerals that form on the mountains’ surfaces, Garzione was able to uncover a record of the altitude at which the minerals formed.
Garzione also used a second method to look at the Bolivian sediment that focused on the temperature at which the surface-forming carbonates were created. Since air temperature decreases with altitude, the rocks’ original altitude should be preserved in a temperature-based mineral snapshot. Garzione, along with Prosenjit Ghosh and John Eiler of the California Institute of Technology, employed a technique developed at CalTech to examine the abundance of oxygen-18 and carbon-13 isotopes that bonded together.
Using the CalTech method, Garzione and the CalTech team gauged the temperature at which the carbonates formed—from the hot Amazonian jungle climate to the freezing peaks of the Andes. Both studies pointed to the same conclusion: Between 10 million and 6.4 million years ago, the Andes lifted more than a mile. “When I first showed this data to others, they had a hard time believing that mountains could pop up so quickly,” says Garzione. “With supporting data from the new paleotemperature technique, we have more confidence in the uplift history and can determine the processes that caused the mountains to rise.”
How did the Andes rise so dramatically, geologically speaking?
Garzione says the answer may come in the not-so-scientific-sounding process known as “deblobbing.” That’s the colloquial term given to a process by which a dense root in the Earth’s mantle becomes detached from the Earth’s crust. As plates thicken during mountain building, the dense lower crust and upper mantle also thicken and are heated to higher temperatures in the Earth’s interior. At hotter temperatures, they become unstable and begin to flow downward under the force of their own mass into the Earth’s mantle, much like a more dense blob in a lava lamp flows downward.
When two tectonic plates collide, such as when the Nazca oceanic plate in the southeastern Pacific collides with the South American continental plate, the continental plate begins to buckle. Floating on a less dense and partially molten mantle, the plates press together and the buckling creates the first swell of a mountain range.
Below the crust, however, there’s another kind of buckling going on in the more elastic portions of the upper mantle. The dense mantle “root” clings to the underside of the crust, growing in step with the burgeoning mountains above. The root acts like an anchor, weighing down the whole range and preventing it from rising, much like a fishing weight on a small bobber holds the bobber low in the water. In the case of the Andes, the mountains swelled to a height of one to two kilometers before the mantle root disconnected and sunk into the deeper, partially molten mantle. The effect was like cutting the line to the fishing weight—the mountains suddenly “bobbed” high above the surrounding crust, and in less than 3 million years, the mountains had lifted from less than two kilometers to roughly four.
The process had been proposed since the early 1980s, but it has never stood up to scrutiny because the techniques to estimate surface elevation have only been recently developed.
“People have largely ignored the role of the upper mantle because it is difficult to look 50 to 200 kilometers into the Earth; whereas we can easily see the deformation on the surface,” says Garzione. “Some geologists have guessed that the dense lower crust and mantle are removed continuously and evenly during mountain building. Our data argue that this dense material just accumulates down there until some critical moment when it becomes unstable and drops off.”
Garzione is seeking even more accurate measurements of mountain growth speeds. She has begun new research in northern Tibet that brings together what she describes as one of the largest collaborative efforts between climatologists and geologists yet assembled.
“This study is a first of its kind,” says Garzione. “We’re studying the Tibetan Plateau to answer how mountain formation changed the Earth’s climate in the region, and how that climate change in turn affected the mountains as they formed. In terms of the breadth of research, this is the biggest proposal that the earth sciences and atmospheric sciences programs at the National Science Foundation have ever supported.
“It’s really exciting to see how our field is changing,” she says. “We’re able to ask bigger questions, and we need researchers from across disciplines to come together to answer them.”
Jonathan Sherwood ’04 (MA) ’09S (MBA) is a senior science writer for University Communications.
