“Mount St. Helens is pretty unusual,” said Steve Hansen, about the volcano responsible for the deadliest eruption in US history, which appears to be stealing its fire from somewhere else. Hansen, a geoscientist at the University of New Mexico and a partner in the Imaging Magma Under St Helens (iMUSH) project adds: “It’s telling us something about how the arc system is behaving, and we don’t yet know what that something is.”
Mount St. Helens is one of the most active volcanoes of the Cascade Arc, a string of eruptive mountains that runs parallel to the Cascadia subduction zone from northern California to British Columbia.
Unlike most major volcanoes of the Cascade Arc that lie along a north-south line, where the wedging of the Juan de Fuca tectonic plate beneath the North American plate forces hot mantle material to rise, Mount St. Helens, lies to the west, in a geologically quiescent region called the forearc wedge.
“We don’t have a good explanation for why that’s the case,” said Hansen.
Hansen led a seismic mapping survey of Mount St. Helens in the summer of 2014, deploying thousands of sensors to measure motion in the ground around the volcano. They drilled nearly two dozen holes, packed the holes full of explosives, triggered a handful of minor quakes, and watched as seismic waves ricocheted beneath the mountain.
“We’re looking at what seismic energy propagates off in the subsurface,” Hansen explained. “It’s a bit like a CAT scan.”
In their analysis Hansen and his colleagues learned that the types of minerals present at the boundary between Earth’s crust and mantle are markedly different to the east and west of Mount St. Helens, confirming that this area is geologically special.