Deep magma chambers seen beneath Mount St. Helens
Dean J. Koepfler/MCT/Newscom
Geophysicists have imaged the magma chambers that blew the lid off Mount St. Helens in its 1980 eruption.
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.
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.
![[Visit Client Website]](https://gsa.confex.com/img/gsa/banner.jpg)
2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)
Paper No. 181-7
Presentation Time: 9:35 AM
IMUSH: MAGMA RESERVOIRS FROM THE UPPER CRUST TO THE MOHO INFERRED FROM HIGH-RESOLUTION VP AND VS TOMOGRAPHY BENEATH MOUNT ST. HELENS
Seismic
investigations following the 1980 eruption of Mount St. Helens have led
to a detailed model of the magmatic and tectonic structure directly
beneath the volcano. These studies suffer from limited resolution below
~10 km, making it difficult to estimate the volume of the shallow magma
reservoir beneath the volcano, the regions of magma entry into the lower
crust, and the connectivity of this magma system throughout the crust.
The latter is particularly interesting as one interpretation of the
Southern Washington Cascades Conductor (SWCC) suggests that the Mount St
Helens and Mount Adams volcanic systems are connected in the crust
(Hill et al., 2009).
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.
...................................................................................................
Could Mount St Helens be about to erupt? Massive magma chamber found below the volcano may reveal clues about future explosions
- Geologists have discovered a second magma chamber beneath volcano
- They believe this feeds the smaller chamber directly below the mountain
- Earthquakes in the area may be a sign of magma pumping between them
- Geologists still consider Mount St Helens to be of high risk of erupting
Lava,
steam and ash began erupting from Mount St Helens in 2004 but it fell
silent again in 2008. Geologists have been closely monitoring the
volcano for signs that the unrest may begin again
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
Rescuer: Houses exploded from the mud
