Geoscientists have for the first time revealed the magma plumbing beneath Mount St. Helens, the most active volcano in the Pacific Northwest. The emerging picture includes a giant magma chamber, between 5 and 12 kilometers below the surface, and a second, even larger one, between 12 and 40 kilometers below the surface. The two chambers appear to be connected in a way that could help explain the sequence of events in the 1980 eruption that blew the lid off Mount St. Helens.
So far the researchers only have a two-dimensional picture of the deep chamber. But if they find it extends to the north or south, that would imply that the regional volcanic hazard is more distributed rather than discrete, says Alan Levander, a geophysicist at Rice University in Houston, Texas, and a leader of the experiment that is doing the subterranean imaging. “It isn’t a stretch to say that there’s something down there feeding everything,” he adds.
Levander unveiled the results on 3 November at a meeting of the Geological Society of America in Baltimore, Maryland—the first detailed images from the largest-ever campaign to understand the guts of a volcano with geophysical methods. The campaign, “imaging magma under St. Helens” (iMUSH), started in 2014 when researchers stuck 2500 seismometers in the ground on trails and logging roads around the volcano. They then detonated 23 explosive shots, each with the force of a small earthquake. “You’d feel this enormous roll in the ground, and everyone would go, ‘Oh wow’,” Levander says.
IMUSH: MAGMA RESERVOIRS FROM THE UPPER CRUST TO THE MOHO INFERRED FROM HIGH-RESOLUTION VP AND VS TOMOGRAPHY BENEATH MOUNT ST. HELENS
The multi-disciplinary iMUSH (imaging Magma Under St. Helens) project is designed to investigate these and other fundamental questions associated with Mount St. Helens. Here we present the first high-resolution 2D Vp and Vs models derived from travel-time data from the iMUSH 3D active-source seismic experiment. Significant lateral heterogeneity exists in both the Vp and Vs models. Directly beneath Mount St. Helens we observe a high Vp/Vs body, inferred to be the upper/middle crustal magma reservoir, between 4 and 13 km depth. Southeast of this body is a low Vp column extending from the Moho to approximately 15 km depth. A cluster of low frequency events, typically associated with injection of magma, occurs at the northwestern boundary of this low Vp column. Much of the recorded seismicity between the shallow high Vp/Vs body and deep low Vp column took place in the months preceding and hours following the May 18, 1980 eruption. This may indicate a transient migration of magma between these two reservoirs associated with this eruption.
Outside of the inferred magma bodies that feed Mount St. Helens, we observe several other interesting velocity anomalies. In the lower crust, high Vp features bound the low Vp column. One explanation for these features is the presence of lower crustal cumulates associated with Tertiary ancestral Cascade volcanism. West of Mount St. Helens, high Vp/Vs regions in the upper and middle crust have eastern boundaries that are close to the eastern boundaries of the accreted Siletzia terrain inferred from magnetic data. Finally, a low Vp channel northeast of Mount St. Helens between 14 and 18 km depth correlates well with the location of the SWCC.
Its scarred and jagged crater is a reminder of the terrible devastation that Mount St Helens wrought over the Washington countryside 35 years ago.
Now a new study of the volcanic plumbing lurking beneath the 8,363ft (2,459 metre) summit suggests the volcano could yet again blow its top in an explosive eruption.
Geologists studying the volcano, which is responsible for the most deadly eruption in US history, have discovered a second enormous magma chamber buried far beneath the surface.
The IMUSH project has detected signs that a second larger magma chamber may lie beneath Mount St Helens, filling the chamber directly under the volcano from below (illustrated) through a series of earthquakes. The chamber may also connect Mount St Helens to other nearby volcanoes