Glacial erosion changes internal mountain structure, responses to plate tectonics
Intense glacial erosion of the spectacular St. Elias range in Alaska has elicited a structural response from deep within the mountain, according to new real-world data that support decades of model simulations of mountain evolution and the impact of climate on the distribution of deformation associated with plate tectonics.
The St. Elias range is a result of the North American plate pushing material up as it overrides the Pacific plate, then the material being worn down by glaciers. A million years ago, glacial conditions became more intense, creating larger and more erosive ice streams that changed the shape and evolution of the mountains. The process continues today, resulting in the particularly active and dramatic St. Elias “orogen” — geologists’ word for mountains that grow from the collision of tectonic plates.
James Spotila, associate professor of geosciences at Virginia Tech who has braved the mountains for many years, explains, “If you push snow with a plow, it will always pile up in front of the plow with the same shape,” called the Coulomb wedge. The increased glacial erosion across the top of the wedge of the past million years pushed the orogen to a tipping point as it struggled to maintain its wedge shape. “Rock faulting and folding has become more intense beneath the large glaciers as the orogen internally deforms to adjust to the focused erosion. Some faults, which previously responded to the push of the plow or tectonic plate, relocated to respond to the accelerated erosion.”
He concludes, “It is remarkable that climate can have such a profound impact on tectonics and the behavior of the solid earth.”
Aaron L. Berger, whose Ph.D. research contributed to the discovery, and Spotila reported their findings in Geology in July 2008 and, with co-authors from the National Science Foundation-funded St. Elias Erosion-Tectonics Project (STEEP), in Nature Geosciences in October.
For more information, see a Virginia Tech News story.
For the past few million years, the spectacular St. Elias mountain range has been covered by vast ice fields and glaciers like these on the leeward flank, leading to some of the fastest modern erosion rates on earth. Over the past few decades, the impact of climatically driven erosion on mountain formation and evolution has been a topic of considerable scientific debate. “Based on our research, the onset of intense glacial erosion is believed to have forced a structural reorganization of the orogen, supporting model predictions of orogenesis with real-world data,” says Aaron Berger, Virginia Tech Ph.D. graduate in geosciences. Photo by Aaron L. Berger.
This seismic station, shown being installed, is part of a network deployed to record, characterize, and locate the numerous earthquakes related to the St. Elias orogen each year. “Without the amazing capabilities of the helicopter pilots, the entire project would not have been possible,” says Aaron Berger. “While in the field, you quickly learned that you had to entrust your life to these competent people.” Photo by Aaron L. Berger.
The Bagley ice field is believed to cover a large fault (Bagley fault). Discovered by STEEP, the fault is thought to have become highly active in response to accelerated glacial erosion in the last million years. Resulting from differential rock motion across this structure, the large mountain in the background is moving up relative to the hill beneath the helicopter at about 10,000 feet per million years (plus or minus 3,000 feet). Photo by Aaron L. Berger.
Aaron Berger created this digital elevation model of the central St. Elias orogen in 2008 using ArcGIS. Brown colors represent high elevations (up to 18,008 feet), green colors represent low elevations, and the blue region depicts the Gulf of Alaska. This rugged landscape is the product of rock uplift due to the collision of the Pacific and North American plates and extreme glacial erosion. In the model, modern glaciers can be identified as regions of smoothed topography. Photo by Aaron L. Berger.