Many volcanologists point to the main chain of Cascades as being the true hazard in the region. After all, Mount St. Helens will take some time to properly recharge for another enormous eruption, whereas these have yet to blow their top after a considerably long period of inactivity.
Cascade Arc volcanoes that are deemed dormant or active are fueled by a range of magma sources almost directly beneath them. This is thanks to the wholesale destruction of the region’s oceanic crust.
Specifically, three small tectonic plates are subducting beneath the North American Plate. As they lose water and gradually disintegrate beneath the western seaboard, the chemistry of the mantle wedge above them changes. The liquidus – the temperature at which everything present is molten – of the superheated material there is subsequently lowered, which ultimately generates quite profuse and complex melting, and, as a result, often quite explosive volcanism at the surface.
Mount St. Helens is fueled by the same mechanism, but it’s incredibly strange that its plumbing system is stretched out towards the west.
Curiously, another recent seismic study seemed to reveal that there are two gigantic magma chambers directly beneath it, not off to the side. At present, it’s not clear how this new study can reconcile with the older one.
As unusual as this displaced magma source is, it highlights that the hell beneath our feet may be more wonky than we’ve previously thought. A recent study revealed that the hotspot beneath Hawaii – an upwelling plume of superheated material – has a history of contorting itself, weaving back and forth rather than just moving up vertically.
Mount St. Helens in 2015. NASA Earth Observatory