Oct. 22, 2001 MINNEAPOLIS / ST. PAUL -- It won't happen overnight, but eventually, the floor of the Atlantic Ocean will plunge beneath the North American continent, forming a deep trench about 2,000 miles long and possibly generating volcanoes, according to research at the University of Minnesota and the Swiss Federal Institute of Technology in Zurich.
Formation of the trench will begin in three million to 10 million years and is projected to take about three million years. It will be triggered by water seeping into offshore rock--built up by river sediments--and making the rock softer and more pliable. The study will be published in the Oct. 19 issue of Science.
Plates of the Earth's crust slide under other plates in a process known as subduction. Subduction has formed deep trenches in the Pacific Ocean floor and raised mountains around the world, including the string of volcanoes between British Columbia and Oregon. A similar fate awaits the East Coast, but it will happen on a much larger scale, said author David Yuen, professor of geology and geophysics at the University of Minnesota.
"The West Coast trenches are small because there are numerous small plates there," said Yuen. "That area is called the Cascadian Subduction Zone, and it extends only about 370 miles. But the East Coast zone is 2,000 miles."
Yuen and his colleagues simulated the fate of the offshore sediments along the East Coast, where rivers have been dumping sediment for 100 million years. They found that this has built up layers of sedimentary rock tens of miles thick that presses down on the ocean bottom. But the critical factor is likely to be seepage of water into the rock, which is quite porous. When water seeps in, it lubricates the rock and makes it softer and less resistant to being deformed or pushed.
The push comes from the middle of the Atlantic, where undersea mountains are rising and exerting pressure on the rock, driving it toward North America. At some point in the next several million years, the ocean bottom will succumb to the downward and westward pressure and start to slide under the continent. Instead of a gently sloping continental shelf, the coast will develop a deep oceanic trench. And inland volcanoes may rise roughly parallel to the trench.
To perform the simulation, the researchers used equations that describe how water in rock makes it more pliable. David Kohlstedt, also a professor of geology and geophysics at the university, headed the team that derived the equations.
Yuen's colleagues in the study were Klaus Regenauer-Lieb, a research assistant at the Swiss Federal Institute of Technology, and Joy Branlund, who at the time was a graduate student at the University of Minnesota.
"When the oceanic slab begins to slide under the continent, the process will spread up and down the coast like a crack propagating," said Yuen. "That will take about three million years. We used data from East Coast sediments because there are no other places in the world where sediments are loading like that. This model leads to the prediction that geologically, the East Coast will eventually resemble the Pacific Northwest coast."
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