CORVALLIS, Ore. - Researchers for the first time have attempted to measure all the material leaving and entering a mountain range over millions of years and discovered that glacial erosion can, under the right circumstances, wear down mountains faster than plate tectonics can build them.
A study of the St. Elias Mountains on the Alaskan coast by researchers from The University of Texas at Austin, University of Florida, Oregon State University and elsewhere found that erosion accelerated sharply about one million years ago.
The study adds insight into a longstanding debate over the balance of climate and tectonic forces that influence mountain building, which defines how landscapes are shaped by and in turn influence climate. The findings will be published this week in the Proceedings of the National Academy of Sciences.
The international research team, working under the Integrated Ocean Drilling program, included Oregon State University Professors Alan Mix and Joe Stoner and postdoctoral researcher Maureen Walczak as well as other scientists from the U.S., Germany, Brazil, Norway, India, China, Japan, Canada, Australia and the United Kingdom.
The seagoing expedition was the culmination of more than a decade of field work. On a previous expedition, the researchers first mapped a huge submarine sediment fan in the Gulf of Alaska built by sediment eroded from the nearby mountains. Next, they recovered sediment cores to understand the fan environments and recent history. The cores are now archived in the national repository at Oregon State.
Most recently, the researchers collected and dated almost four kilometers of drill cores from the floor of the gulf and the Alaskan continental shelf, revealing millions of years of geologic history.
"It turned out most sediments were younger than we anticipated, implying that erosion was higher than we expected," said lead author and co-chief scientist Sean Gulick of the University of Texas Institute for Geophysics.
Mountain ranges form when tectonic plates thrust into one another over millions of years and scrunch up the Earth's outer crust. But even as mountains are built by these titanic forces, other agents work to wear them down.
"About a million years ago, short, 40,000-year climate oscillations jumped into a new mode with stronger, 100,000-year long glacial cycles, and erosion of the mountains accelerated under attack from the ice," Gulick said. "In fact, more rock was eroded than tectonics has replaced."
Co-chief scientist John Jaeger of the University of Florida added: "People often see mountain ranges as permanent, but they aren't really. If more rock is pushed in, they grow, and if more rock is eroded away, they shrink."
Since the mid-Pleistocene, erosion rates have beaten tectonic inputs by 50 to 80 percent, demonstrating that climatic processes that ultimately drive the glaciers can outstrip mountain building over a span of a million years. The findings highlight the pivotal role climate fluctuations play in shaping Earth's landforms.
"We were pleasantly surprised by how well we could establish ages of the sediment sequences and the composition of the sediment gave clear evidence of when the glaciation started and then expanded, in sync with global climate trends," said co-author Mix of OSU's College of Earth, Ocean, and Atmospheric Sciences.
"Only by drilling the sea floor where the sediment accumulates could we see these details in focus."
The study was funded by the U.S. National Science Foundation and the Integrated Ocean Drilling Program.