between tectonic plates and climate
Rocks from the Chugach and St Elias mountain ranges
in southeast Alaska collected and dated...
May 27, 2004
"Like an ice palace cheese grater, glaciers sweep and grind rocks off of mountains. No matter how fast the plate pushes the rock up, the glacier will erode it just as fast," said James A. Spotila of Blacksburg, assistant professor of geosciences at Virginia Tech. The National Science Foundation-funded research is reported in the June issue of Geology, in the article, "Long-term glacial erosion of active mountain belts: Example of the Chugach-St. Elias Range, Alaska," by Spotila; Jamie Buscher of Los Angeles, a Ph.D. candidate in geosciences at Virginia Tech; Andrew Meigs of Oregon State University, and Peter Reiners of Yale.
The research team collected and dated rocks from the rugged, dangerous Chugach and St Elias mountain ranges in southeast Alaska, where a bend in the tectonic plate boundary results in collision of a plate fragment. This fragment, the Yakutat microplate, is sandwiched between the colliding Pacific and North American plates, which move closer together by about 5 centimeters per year, producing massive earthquakes and towering mountains. This collision of tectonic plates has pushed the Chugach-St. Elias Range up faster and higher than any other mountain belt this close to the sea. The range has the greatest coastal relief on earth. Mt. St. Elias rises to a height of 18,008 feet in a distance of less than 20 miles from the sea, despite eroding and discharging one of the highest rate of sediment in the world into the ocean.
It is not faults and plates that make Alaska's Chugach-St. Elias Range unique, however. "It is that the erosion is by glaciers," Spotila said. "The tidewater glaciers of southeast Alaska, so familiar to cruise liners, form because of the northern latitude, but are so prominent because of the several meters of annual precipitation along the coast."
The combination of tectonic and climatic conditions provides a huge laboratory for the study of the impact of the earth's surface processes on tectonics.
"In the last decade of research, there has been ground breaking recognition that processes at the surface have an effect on processes deep in the earth. The rapid removal of mass in one spot can affect where rock is going when there is mountain building and collision," Spotila said.
"We've seen hints of this with rivers," Spotila said, "but glaciers are more effective at erosion than rivers." If erosion by rivers can locally keep pace with tectonics under some conditions, erosion by glaciers may do so much more extensively. The impact of erosion on plate motions could thus be more widely felt during ice ages. Geoscientists describe this possibility as the "glacial buzz saw" hypothesis, a term coined in 1997 by University of Southern California researchers (Brozovic, N., Burbank, D.W., and Meigs, A.J., Climatic limits on landscape development in the northwestern Himalaya: Science, v.276.). Meigs continued his pursuit of this work in Alaska, eventually inviting Spotila to join his team.
Helicopters and floatplanes delivered the researchers throughout the range to collect rocks. The Chugach-St. Elias Range is not the most remote mountain range. It is limited in area and no spot is greater than 50-100 miles away from roads or small towns, "but storms and fog make the logistics very fluid," Spotila said. The helicopters would wait to take the scientists out at a moment's notice. "Sometimes the research required teams -- typically, graduate students -- to remain camped out for weeks at lakes or fjords in the range," he said.
From the rocks, the researchers could determine the rate of exhumation, that is, the rate at which underlying material is pushed to the surface, and how fast it is being stripped off and deposited in the ocean.
"We know that temperature increases with depth in the earth. Different chemical systems are sensitive to temperature. By measuring the different isotopes of different minerals, we can reconstruct their cooling history," Spotila said. "Thus, we figured out how fast rocks rose to the surface. The technique we use measures the helium .produced by radioactive decay."
The researchers report that over the entire mountain range, the rate of exhumation is one to two millimeters per year, or one to two kilometers of rock per million years is pushed to the surface. "This is consistent with findings of other mountain ranges formed by tectonic collision," Spotila said.
And the affect of climate and glaciers on tectonics? "When we look at the rate of exhumation and compare it to the rate at which rock is coming in tectonically, it matches. Glaciers are keeping pace. This supports the buzz-saw hypothesis. The rock mass built into mountains by tectonics is redistributed by the action of glacial erosion," Spotila said..
A remaining mystery is a difference in time scale when rates of erosion are observed. "We see rocks rising to the surface over millions of years, but researchers who have looked at sediment in fjords over 50 and 100 years, by comparing present glacial retreat to photographs from 50 and 100 years ago, report erosion rates 50 to 100 times faster than our measurements," Spotila said. "As climates fluctuate over time, the average rate of movement is going to be lower. But we need to do more research to understand better the system of mountain building, glacial erosion, and climate change. It is possible that the erosion rate has increased."
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