The melt source region for Mount St Helens may lie to the east, near a neighbouring mountain, rather than directly below the volcano according to a new study published in Nature Communications this week. New seismic experiments find that the mantle below the volcano, where most would expect hot magma to be present, is stone cold. This research challenges the assumptions of where melt may be sourced from in active volcanoes.
The largest recent eruption of Mount St Helens, located in Southern Washington along the North American Cascade Arc, occurred in 1980. During the eruption the side of the volcano collapsed, which redefined our knowledge of how volcanoes behave during eruption. Despite extensive research, the melt source region has remained enigmatic, especially as Mount St Helens lies 50 km west of the main volcanic arc.
A large high-resolution seismic imaging experiment, where the seismic waves from 23 explosions around Mount St Helens were analysed to generate images beneath the volcano was undertaken by Steven Hansen and colleagues. They discovered that a cold wet wedge of the mantle lies beneath the volcano, which could not be the source of the magma. Instead the authors suggest that clusters of deep earthquakes in the crust (23-44 km depth) southeast of Mount St Helens (towards Mount Adams) reflecting magma movement are more likely to be the melt source region.