The early April earthquake of magnitude 8.6 that shook Sumatra wasa grim reminder of the devastating earthquakes and tsunami thatkilled tens of thousands of people in 2004 and 2005. Now a new study, funded by the National Science Foundation, showsthat the residents of that region are at risk from yet anotherpotentially deadly natural phenomenon - major volcanic eruptions. Researchers from Oregon State University working with colleagues inIndonesia have documented six major volcanic eruptions in Sumatraover the past 35,000 years - most equaling or surpassing inexplosive intensity the eruption of Washington's Mount St. Helensin 1980. |
Results of the research have just been published in the Journal ofVolcanology and Geothermal Research. "Sumatra has a number of active and potentially explosive volcanoesand many show evidence of recent activity," said Morgan Salisbury,lead author on the study, who recently completed his doctoralstudies in OSU's College of Earth, Ocean, and Atmospheric Sciences."Most of the eruptions are small, so little attention has been paidto the potential for a catastrophic eruption. "But our study found some of the first evidence that the region hasa much more explosive history than perhaps has been appreciated,"he added. Until this study, little was known about Sumatra's volcanic history- in part because few western scientists have been allowed accessto the region. The most visible evidence of recent volcanicactivity among the estimated 33-35 potentially active volcanoes aretheir steep-sided cones and lack of vegetation, indicating at leastsome minor eruptive processes.
But in 2007, an expedition led by OSU's Chris Goldfinger waspermitted into the region and the Oregon State researchers andtheir Indonesian colleagues set out to explore the earthquakehistory of the region by studying sediment cores from the IndianOcean. Funded by the National Science Foundation, it was the firstresearch ship from the United States allowed intoIndonesia/Sumatran waters in nearly 30 years. While searching the deep-sea sediment cores for "turbidites" -coarse gravel deposits that can act as a signature for earthquakes- they noticed unmistakable evidence of volcanic ash and beganconducting a parallel investigation into the region's volcanichistory. "The ash was located only in certain cores, so the activity waslocalized," said Adam Kent, a professor of geosciences at OSU andan author on the study. "Yet the eruptions still were capable ofspreading the ash for 300 kilometers or more, which gave us anindication of how powerful the explosive activity might have been." Salisbury and his colleagues found evidence of six major eruptionsand estimated them to be at least from 3.0 to 5.0 on the VolcanicExplosivity Index.
Mount St. Helens, by comparison, was 5.0. The Indian Ocean region is certainly known to have a violentvolcanic history. The 1883 eruption of Krakatoa between Sumatra andJava is perhaps the most violent volcanic explosion in recordedhistory, measuring 6.0 on the VEI and generating what manyscientists believe to have been one of the loudest noises everheard on Earth.
Sumatra's own Toba volcano exploded about 74,000 years ago,generating a major lake - not unlike Oregon's own Crater Lake, butmuch larger. "It looks like a giant doughnut in the middle ofSumatra," said Jason "Jay" Patton, another OSU doctoral student andauthor on the study. Sumatra's volcanoes occasionally belch some ash and smoke, andprovide comparatively minor eruptions, but residents there may notbe fully aware of the potential catastrophic nature of some of itsresident volcanoes, Goldfinger said. "Prior to 2004, the risk from a major earthquake were not widelyappreciated except, perhaps, in some of the more rural areas,"Goldfinger said.
"And earthquakes happen more frequently than majorvolcanic eruptions. If it hasn't happened in recent memory..." Kent said the next step in the research is to work with scientistsfrom the region to collect ash and volcanic rock from the island'svolcanoes, and then match their chemical signature to the ash theydiscovered in the sediment cores. "Each volcano has a subtly different fingerprint," Kent said, "soif we can get the terrestrial data, we should be able to link thesix major eruptions to individual volcanoes to determine the onesthat provide the greatest risk factors." In addition to the Oregon State University scientists, twoIndonesian researchers were authors on the journal article: YusufDjadjadihardja and Udrekh Hanif, of the Agency for the Assessmentand Application of Technology in Jakarta.
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