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 t news notes t news notes t news notes t news notes t news notes t news notes t news notes t news notes - 17 in

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C Mary ock EARTH R ed R Previous efforts to explain the East Next, Behn would like to see the new oodoos such as this one in H fornia’s help researchers better under stand ground past earthquakes. shaking during phenomena in other settings. “There are “There settings. other in phenomena lots of other places where there doesn’t this and plume mantle deep a be to seem might be a viable mechanism.” Ballmer also thinks the new mechanism hot where locations to applications mayhave such ridges, mid-ocean to close sit spots Louisville the near Ridge Hollister the as hot spot in the South Pacific. Massachusetts who was not explanation team’s involved The study. new the in “is plausible,” very he says. Pacific Rise volcanoes have included ideas such as a lithospheric crackingmodel, in which elongate cracks in the crust account for why the cones, discrete not and ridges as appear volcanoes and a density-dependent small-scale convection model. No past hypothesis has stood up to geochemical or geo physical testing, Ballmer says. to world, the in elsewhere applied model volcanic explain to used be could it if see Hole Oceanographic Institution ------leson O Woods Timothy Timothy In this instance, the ground motion “Many models have been proposed Shear-driven Shear-driven upwelling accounts for hot is which of one fluids, two When ing U.S. Geological Survey (USGS) seis (USGS) Survey Geological U.S. ing mic hazard assessment for the which area, is based on the the But behavior. past best-availableof understanding be always not mightassessments USGS case this and enough, precise or accurate study demonstrates how hoodoos can be useful to check existing predictions, the of Bulletin the in suggested team the Seismological America. Society of rock from which the pillars were made. were pillars the which from rock during motion ground that found They tothought earthquake — majorlast the have been a magnitude-7.4 event that occurred within the last 550 one-third about years exceed not did — probably the acceleration due to Earth’s grav ity (the conventional unit of measure ment to describe ground motion). Had it done so, the hoodoos likely would broken. have they determined agreed with the exist to explain these ridges and none been has convincing, untilMark Behn, a geodynamicist at now,” says says. So when the hotter materialthe Superswell of encounters the cooler mantle near the East Pacific Ridge, the elongating fingers create a channelized flow through the asthenosphere, mov ing from west to east. That then creates heterogeneityasthenospherethe in and pushes the source of the volcanism — the shear-driven upwelling — rapidly “This Plate. Nazca the toward eastward, would explain the fast age progression ridge,” mid-ocean the of direction the in Ballmer says. the the source of the magma, Ballmer to says. had team the motion, the explain to But of west looked They source. another find as known area an to Rise Pacific East the to thought Superswell, Pacific South the mantle. hot anomalously by underlain be ter and weaker than the other, in come contact they form fingers, Ballmer - - -

in earthquake-prone areas, so hort-term instrumental records records instrumental hort-term don’t necessarily accurate hazard provide assessments

OODOOS PROVIDE EARTHQUAKE CLUES CLUES EARTHQUAKE PROVIDE OODOOS Abdolrasool Anooshehpoor of the scientists are always on the lookout projections projections of the hoodoos and per mechan the of tests laboratory formed ical strength of pieces of the same hoodoos in particular, both located neartheGarlock FaultCalifornia’sin Red Rock Canyon. The researchers analyzed high-resolution 3-D digital U.S. Nuclear Regulatory Commission Regulatory Nuclear U.S. and colleagues from the University intacttwo studied Reno at Nevada of as hoodoos might serve as a helpful source of new data. past seismic activity. Now, a group of geophysicists has suggested that fragile, spindly rock pillars known for new physical evidence — rock falls falls rock — evidence physical new for — example for deposits, tsunami and understandingof our extend can that H Ballmer and his colleagues developed developed colleagues his and Ballmer

The other part of the enigma is that S

www.earthmagazine.org the seamounts of the East Pacific Rise decompression melting in the mantle, which creates the seamounts. mantle, lateral tension develops andcreates shearing motions in the plate. This shearing leads to upwelling and upwelling occurs as the Pacific Platemoves over heterogeneous underlying over moves plate the As asthenosphere. adjacent zones of weaker and stronger a new model to explain the source of the of source the explain to model new a unusual seamount ridges: shear-driven upwelling. In this case, shear-driven dentlyof the plate, being pushed very vigorously eastward.” But that’s not what the “According to team our model, found.it looks like indepen moving is asthenosphere the drags the asthenosphere, case the so asthenosphere in would have to this slowlybemoving thetowest,” says. he began forming away from the mid- ocean spreading ridge and the source migrated east towards the ridge.“The textbook thinking is that the plate

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