14 DAY TRIAL // How can you measure the speed of the mountain rises? A new and more precise technique for measuring this speed has surprised scientists. Apparently, mountains grow much more quickly than previously thought which makes geologists question their theories about the processes involved in the mountain rises.
Until recently determining the speed of mountain rises was based primarily on examining biological sediments. Plants that grow at different mountain heights belong to different species and thus, by studying and dating the leaf fossils one finds at a certain height one can guess the elevation history of that area. However, plant characteristics can change radically over millions of years, and changes in climate can also cause erosion, throwing a significant question mark into the equation.
Two new studies by Carmala Garzione, a University of Rochester researcher, used a different technique involving the products of erosion. As mountains are eroded, their sediment is carried down the slope in streams and collected at the base of the forming mountain range. As a mountain range rises, it experiences different atmospheric conditions simply due to its change in height. In her doctoral thesis Garzione showed how one could obtain the information about the atmosphere from the ancient sediments.
Thus, the sediment has recorded the history of the mountain uplift. We only need to know how to read this record.
The key concept has to do with the fact that there are two types of oxygen in the atmosphere, oxygen-16 (the most abundant) and oxygen-18. They have the same chemical properties, but their nuclei have a different number of neutrons. The point is that the ratio of oxygen-16 to oxygen-18 depends on the height in atmosphere. The composition of rainfalls at different heights reflects the variable ratio. As rain erodes the mountains various chemical reactions happen between water the minerals - thus, in the end, the sediments themselves reflect the oxygen-16 to oxygen-18 ratio.
By dating the sediments Garzione succeeded determining how fast the mountain has elevated. She concentrated on the Bolivian Altiplano, which is a large, high elevation basin in the Andes Mountains in South America. There she took samples of sedimentary rock that had accumulated between 12 million and 5 million years ago from erosion of the surrounding ranges. One type of mineral, carbonate, precipitates from surface water, so the composition of the carbonate is a good indicator of the composition of rainfall. A second method, used on the same Bolivian sediment, focused on the temperature at which the surface-forming carbonates were created. Atmosphere once again played a key role since air temperature decreases with altitude, meaning a temperature-based recording of the rocks' original altitude should be preserved.
The result of the studies was surprising: mountain ranges rise a kilometer per million years, several times faster than geologists have always thought.
"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 processes that caused the mountains to rise."
"These results really change the paradigm of understanding of how mountain belts grow," says Garzione. "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."