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AMERICAN GEOPHYSICAL UNION | 13–17 DECEMBER 2010 | SAN FRANCISCO, CALIFORNIA Tectonic Blow Ended Mountain Building, Fired Up Volcanoes Fifty-fi ve million years ago, Siletzia arrived 25 September 2009, p. 1620). “We now have on North American shores with nary a bump. a lot more data,” says Humphreys. “Now I But the 500-kilometer-wide chunk of drift- believe the structures we see.” ing oceanic plate had far-reaching geologic Most geologists think the thicker Silet- effects across the continent’s western third, zia section of the Farallon plate jammed at according to a presentation at the meeting. the edge of the continent and stopped sub- New seismic imaging shows Siletzia throw- duction across the Pacifi c Northwest. In the ing a monkey wrench into the gears of the OU scenario, however, the Farallon plate plate tectonics machine and thus reshaping didn’t just stop; it tore clean through beneath Still hangin’. Seismic imaging reveals a “curtain” western North America. the southern edge of Siletzia. That left the of old ocean plate (blue, right) left dangling since Before Siletzia arrived, things tectonic remaining shallowly subducting plate to the Siletzia arrived 40 million years ago. were humming along nicely. The Farallon south to peel off the base of North America oceanic plate was creeping northeastward and progressively fall away southward. and diving into the mantle beneath North As evidence for this scenario, the scien- America. But unlike most subducting plates, tists point to a vertical “curtain” of colder What Heated Up the Farallon plate wasn’t diving very deeply. rock they see stretching 600 kilometers Instead, it was leveling off and sliding hard down into the mantle beneath Idaho. The the Eocene? on June 24, 2013 against the underside of the North American curtain, they say, is the dangling remnant of plate. The drag of the Farallon plate tended the subducted plate that tore away from the Talk about your global warming. Fifty-fi ve to squeeze the overlying plate, crumpling rest of the Farallon plate. million years ago, carbon dioxide gushed its surface into mountains like a wrinkled All of this fits with what geologists into the atmosphere over as little as a millen- rug. The mountain building extended as have inferred happened at the surface, nium, acidifying the ocean and scorching the far as the Black Hills of South Dakota and Humphreys said. The ocean crust melting world of the Eocene epoch with a 5˚C green- included the Laramie Mountains of Wyo- on the hanging slab would have fueled the house warming. At least two more progres-

ming. But starting about 50 million years so-called Challis volcanism through Idaho sively weaker “hyperthermals” would strike www.sciencemag.org ago, the Farallon plate began diving more and into Canada. The broad loss of shallowly during the next few million years. So where steeply into the mantle, ending the era of subducting plate would have triggered the did all that carbon dioxide come from? so-called Laramide mountain building. switch from North American compression The possibilities are expanding, from the Geologists wondered why. and mountain building to the crustal stretch- sea fl oor to an ice-free Antarctica. In one The reason subduction steepened, seis- ing seen today. And the progressive falling presentation, earth systems modeler Andrew mologist Eugene Humphreys of the Uni- away of Farallon plate would have allowed Ridgwell of the University of Bristol in the versity of Oregon (UO), Eugene, said at the hot mantle rock to rise and drive the fl are-up United Kingdom and his colleagues sug- meeting, was the arrival of a thicker patch of volcanism seen to the south. gested an ocean source. Earth is always wob- Downloaded from of the plate called Siletzia. Humphreys and The sharper view of the mantle “ties the bling on its axis like a spinning top, and its UO colleague Brandon Schmandt reached tectonic history of the crust with what you orbit about the sun is rhythmically stretching that conclusion by using seismic waves can now see in the mantle,” says geologist into an ellipse and relaxing to more of a cir- from earthquakes to image what remains Ray E. Wells of the U.S. Geological Sur- cle. By changing how much sunlight falls at of the Farallon plate in the mantle beneath vey in Menlo Park, California. It also helps different latitudes in different seasons, these North America. Using seismic waves the tie the geology in the north with that in the orbital variations can make for exceptionally way radiologists use x-rays to form CT south, he says: “Now that we can see things warm summers with extra-cold winters, for

images, they drew on the newest records under there, they’re starting to integrate example. 2011) © GEOLOGICAL SOCIETY AMERICA OF 5 JANUARY DATE, (PRE-ISSUE PUBLICATION from a dense network of seismometers processes across wide areas.” Ridgwell and his colleagues included Science

known as the USArray project ( , –R.A.K. orbital variations in a computer model of GEOLOGY early Eocene climate, when the world was slowly warming under a strengthening green- house. When the orbital variations in the model combined to make an extreme contrast between summer and winter temperatures, the model’s ocean circulation would slow and stagnate. That allowed the deep ocean to warm, warming the sea fl oor. And that’s where natural methane hydrates reside—the

Shutdown. The Farallon plate delivered Siletzia, stopping the sinking of the older plate (darker green). “ice that burns.” Warm them and they decom- CREDITS: B. SCHMANDT AND E. HUMPHREYS,

142 14 JANUARY 2011 VOL 331 SCIENCE www.sciencemag.org Published by AAAS