New Imagery Reveals That Mount St. Helens Is Sitting Atop a Small Sea of Magma

My home is in the Cascade Mountains of Washington state, roughly equidistant from three of the Northwest's largest active and potentially active volcanoes: Mt. Hood, Mt. Adams, and the ever-rumbling Mt. St. Helens. Mt. Rainier isn't much further, and all are visible on a good day. Everything here is touched by volcanism, from landslide-sheared mountains to kilometers-long lava tube caves to the brittle wasteland of the Great Lava Bed. In geologic time, it's a landscape shaped just yesterday by a molten realm not far underground.

The catastrophic 1980 eruption of Mount St. Helens was really more like just "this morning" in geologic time, while its mid-00s reawakening might have occurred around the time you started reading this—again, in geologic time. What exactly is happening far underneath the mountain and its surroundings right now has remained somewhat an open question.

New research presented last week at the annual meeting of the Geological Society of America offers the first images of the largest ever campaign to understand the innerworkings of a volcano using geophysical methods, known as the "imaging magma under St. Helens" or iMUSH campaign. What they reveal is that there are two vast and almost certainly connected magma chambers located in the vicinity of the mountain between 5 and 12 kilometers underground.

The shallower chamber is located under the eastern flank of Mt. St. Helens itself, while the deeper one is a ways away, proximate to St. Helens but also Mt. Adams and a constellation of nine or so short sharp cinder cones and dozens more "eruptive centers" known together as the Indian Heaven Volcanic Field. The upshot: It would appear that the two chambers are capable of fueling more volcanic activity than just St. Helens.

Seismic research in the area has been ongoing since Mt. St. Helens blew in 1980, an event that directly killed 57 people, reduced the elevation of the mountain by 1,300 feet, and reduced hundreds of square miles to a gnarled wasteland. However, imaging at depths beyond 10 kilometers or so has offered only low resolutions. The iMUSH project is the first to probe deeper at sufficiently high resolutions, which it's accomplished thanks to thousands of seismic probes set to record both the mountain's normal small earthquake rumblings and also the acoustic reflections produced by dozens of controlled detonations.

Shallow observations can offer geologists only limited information about what's known as the Southern Washington Cascades Conductor (SWCC). This is a vast electromagnetic anomaly—a region of unusually high electrical conductivity—first discovered in the late 1980s. Its origins have so-far remained unclear, but unusual electromagnetic activity has been associated with volcanism via a number of processes, including but not limited to piezomagnetic effects (how applied stress affects magnetism), electrokinetic effects (electrical charges induced by molten magma flowing against solid rock), and the changes in magnetism that occur as molten material crosses a certain threshold known as the Curie Point.

A controversial study in 2009 took the leap and argued that the source of the anomaly is really a vast pool of magma, perhaps indicating the possibility of a latent super-volcano akin to that beneath Yellowstone National Park, which is also underpinned by a magma reservoir. At the time of the study's release there wasn't much other evidence, particularly on the surface, to support the conclusion that the SWCC traces to magma pockets.

As noted in a Science News piece, the new observations may help explain some features of St. Helens' 1980 eruption. In the months prior to the event, a series of small earthquakes were detected along a peculiar path near the mountain, but the relationship to the blast has until now remained mysterious. The discovery of the magma chambers suggests the possibility that the quakes originated in a sort of geologic pumping action that helped move magma from the lower chamber to the upper chamber (and then on to the volcano itself).

Similar quakes detected in the future could serve as an eruption early warning.

As for the SWCC, it's still not a slam dunk that it traces back to magma reservoirs. For one thing, it stretches for well over a hundred kilometers underneath Washington state. Another component of the iMUSH campaign is the placement of electrodes and magnetometers in more than 100 locations, hopefully offering geologists more detail on the anomaly.