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EAST PACIFIC RISE VOLCANOES FINALLY LINE up News Notes Notes News T EAST PACIFIC RISE VOLCANOES FINALLY LINE UP news notes notes news t awaii’s island chain makes study in Geology. “These ridges display seven centimeters a year, the seamounts scientific sense: As the Pacific an age progression that’s much faster of the East Pacific Rise display an age plate moves westward over than the rate of plate motion.” progression closer to 22 centimeters per a stationary deep mantle Unlike in Hawaii, where the age pro- year, a discrepancy “that has been a Hplume known as a hot spot, upwellings gression of the islands matches up with long-standing enigma in geophysics,” of magma have left a trail of islands a rate of Pacific Plate motion of about Ballmer says. whose relative ages match up with the news notes notes news t rate of plate motion. But elsewhere in the Pacific, the relationship between plate motion and volcanic chains is not as clear. Near the East Pacific Rise in the southeast Eurasian North American Pacific, for example, a trail of seamounts Plate Plate appears to have formed not only much Juan de Fuca faster than the Pacific Plate is moving, but Plate also in the opposite direction. Many theories have attempted to news notes notes news t explain how the Pukapuka, Hotu-Matua Caribbean Filipino Plate and Sojourn seamounts formed in par- Plate allel, west-east trending ridges perpen- Cocos Plate dicular to the boundary between the EQUATOR Pacific Plate and the Nazca Plate. But South none has been able to account for either American the rapid pace or the direction of ridge Plate propagation or the directionality of the news notes notes news chain, until now. Indo-Australian Paci!c Plate Nazca Plate t The westward-moving Pacific Plate Plate diverges from the eastward-moving Nazca Plate at a mid-ocean ridge spread- ing center. The seamounts of the East Pacific Rise sit mainly on the Pacific Plate with a few trailing onto the Nazca Plate. “The East Pacific Rise is distinct Antarctic Plate from many other volcano chains in the news notes notes news world,” says Maxim Ballmer, a geo- t physicist at the University of Hawaii at Manoa and lead author of the new The source of a series of par- 0 4˚S allel volcanic ridges running -2000 perpendicular to the mid- -4000 ocean ridge between the Depth (m) -6000 Pacific and Nazca plates may 8˚S news notes notes news now be explained. t East Paci!c Marquesas Rise Sojourn 12˚S Hotu-Matua I Nazca Plate 16˚S Seamounts AG Tuamotus Pukapuka antner, antner, C news notes notes news t Rano-Rahi 20˚S athleen athleen K Paci!c Plate Nazca Plate Both: 140˚W 130˚W 120˚W 110˚W 16 Q EARTH June 2013 www.earthmagazine.org news notes news notes notes news t news notes notes news HOODOOS PROVIDE EARTHQUAKE CLUES hort-term instrumental records rock from which the pillars were made. t don’t necessarily provide They found that ground motion during notes news accurate hazard assessments the last major earthquake — thought to in earthquake-prone areas, so have been a magnitude-7.4 event that Sscientists are always on the lookout occurred within the last 550 years — for new physical evidence — rock falls probably did not exceed about one-third and tsunami deposits, for example — the acceleration due to Earth’s grav- that can extend our understanding of ity (the conventional unit of measure- past seismic activity. Now, a group ment to describe ground motion). Had of geophysicists has suggested that it done so, the hoodoos likely would t fragile, spindly rock pillars known have broken. notes news as hoodoos might serve as a helpful In this instance, the ground motion source of new data. they determined agreed with the exist- Abdolrasool Anooshehpoor of the ing U.S. Geological Survey (USGS) seis- U.S. Nuclear Regulatory Commission mic hazard assessment for the area, and colleagues from the University which is based on the best-available of Nevada at Reno studied two intact understanding of past behavior. But the hoodoos in particular, both located USGS assessments might not always be near the Garlock Fault in California’s accurate or precise enough, and this case t Red Rock Canyon. The researchers study demonstrates how hoodoos can Hoodoos such as this one in Cali- notes news analyzed high-resolution 3-D digital be useful to check existing predictions, fornia’s Red Rock Canyon might projections of the hoodoos and per- the team suggested in the Bulletin of the help researchers better under- formed laboratory tests of the mechan- Seismological Society of America. stand ground shaking during ical strength of pieces of the same Timothy Oleson past earthquakes. t The other part of the enigma is that Shear-driven upwelling accounts for Hole Oceanographic Institution in notes news the seamounts of the East Pacific Rise the source of the magma, Ballmer says. Massachusetts who was not involved began forming away from the mid- But to explain the motion, the team had to in the new study. The team’s explanation ocean spreading ridge and the source find another source. They looked west of “is very plausible,” he says. migrated east towards the ridge.“The the East Pacific Rise to an area known as Previous efforts to explain the East textbook thinking is that the plate the South Pacific Superswell, thought to Pacific Rise volcanoes have included drags the asthenosphere, so in this be underlain by anomalously hot mantle. ideas such as a lithospheric cracking case the asthenosphere would have to When two fluids, one of which is hot- model, in which elongate cracks in the be slowly moving to the west,” he says. ter and weaker than the other, come crust account for why the volcanoes t But that’s not what the team found. in contact they form fingers, Ballmer appear as ridges and not discrete cones, notes news “According to our model, it looks like says. So when the hotter material of and a density-dependent small-scale the asthenosphere is moving indepen- the Superswell encounters the cooler convection model. No past hypothesis dently of the plate, being pushed very mantle near the East Pacific Ridge, the has stood up to geochemical or geo- vigorously eastward.” elongating fingers create a channelized physical testing, Ballmer says. Ballmer and his colleagues developed flow through the asthenosphere, mov- Next, Behn would like to see the new a new model to explain the source of the ing from west to east. That then creates model applied elsewhere in the world, to unusual seamount ridges: shear-driven heterogeneity in the asthenosphere and see if it could be used to explain volcanic upwelling. In this case, shear-driven pushes the source of the volcanism — phenomena in other settings. “There are t news notes notes news upwelling occurs as the Pacific Plate the shear-driven upwelling — rapidly lots of other places where there doesn’t moves over heterogeneous underlying eastward, toward the Nazca Plate. “This seem to be a deep mantle plume and this asthenosphere. As the plate moves over would explain the fast age progression might be a viable mechanism.” Ballmer adjacent zones of weaker and stronger in the direction of the mid-ocean ridge,” also thinks the new mechanism may mantle, lateral tension develops and Ballmer says. have applications to locations where hot nooshehpoor A creates shearing motions in the plate. “Many models have been proposed spots sit close to mid-ocean ridges, such This shearing leads to upwelling and to explain these ridges and none has as the Hollister Ridge near the Louisville decompression melting in the mantle, been convincing, until now,” says hot spot in the South Pacific. bdolrasool bdolrasool t A which creates the seamounts. Mark Behn, a geodynamicist at Woods Mary Caperton Morton notes news www.earthmagazine.org EARTH June 2013 Q 17.
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