New clues from old volcanic debris could help scientists better predict eruptions. According to a new report, ash that Mount St. Helens spewed during the months before its huge eruption on 18 May 1980 contains tiny crystals that show an explosive eruption was likely.
Many volcanoes go through a period of low-level activity that typically lasts months before a major eruption. But those same volcanoes also have periods of restlessness that never culminate in a big explosion. Telling the difference is one of the biggest challenges facing volcanologists. Previously, geologists trying to determine if magma was nearing the surface in preparation for a major eruption studied ash from minor eruptions. Researchers assumed that any evidence of rising magma would show up as glass because the magma would cool too quickly to allow crystals to form. (Contrary to appearances, glass is crystalline.) The lack of glass in the ash from Mount St. Helens in March and April 1980 misled volcanologists into thinking a major eruption was less likely.
But in the years since Helens blew her top, scientists have discovered that crystals can form if the quickly cooling magma contains water. Decompressing wet magma by bringing it up toward the surface allows the water to bubble out of the mixture like the carbonation in a popped bottle of champagne, says volcanologist Kathy Cashman of the University of Oregon in Eugene. This process also stabilizes some minerals, allowing them to form tiny crystals, sometimes in a matter of hours.
Armed with this new knowledge, Cashman and Richard Hoblitt of the U.S. Geological Survey's Hawaiian Volcano Observatory took another look at old samples of ash collected from Mount St. Helens in the months preceding the catastrophic 1980 explosion. They found traces of telltale crystals that indicated magma was welling up beneath the mountain. The crystals also revealed that the magma was the type that can cause explosive eruptions rather than lava flows. And it appears that the amount of crystals was increasing as the eruption neared, the team reports in the February issue of Geology.
"If scientists had been able to go out there and find this in the field before the eruption, then they would have predicted an eruption was more imminent," says volcanologist Malcolm Rutherford of Brown University in Providence, Rhode Island. Cashman says that looking for magma crystals may help with future predictions of any coastal volcano with wet magma, such as Mount Pinatubo in the Philippines and Mount Rainier near Seattle, Washington.