Mount St. Helens Gets a Huge Ultrasound

Researchers are using remote sensing to better understand the hidden passageways beneath one of the United States’ most dangerous active volcanoes.

Their work may begin to explain the enigmatic location of Mount St. Helens, which lies farther west than other peaks in the Cascades volcanic arc.

The paper, published in Nature Communications, analyzes compressional waves traveling through the crust and reflecting off the mantle below the volcano. Results show that on one side the mantle is largely serpentinite, a rare, moisture-absorbing, dark-green mineral that can look like a snake’s skin.

But the mantle below the eastern half of the mountain is mostly olivine, a common mineral that allows water—thought to play a key role in volcanic eruptions—to percolate up and into the overlying crust.

The paper presents the latest results from a July 2014 experiment that conducted a giant ultrasound of Mount St. Helens. The project, nicknamed iMUSH for imaging Magma Under St Helens, set off seismic waves to see how they travel under the mountain and generate a map of the volcano’s plumbing.

During that experiment, researchers from the University of New Mexico placed 900 autonomous seismographs within 15 kilometers (9.3 miles) of the crater, increasing the density of instruments right around the volcano. All sensors were deployed along the road and trail system at Mount St. Helens with an average spacing of 250 meters.

The University of New Mexico team placed instruments used for the new paper at the yellow dots, near the crater. The black dots are instruments placed by Rice University, while red stars are locations of the experimental explosions
The University of New Mexico team placed instruments used for the new paper at the yellow dots, near the crater. The black dots are instruments placed by Rice University, while red stars are locations of the experimental explosions

The iMUSH experiment set off 23 active-source explosions, with energy similar to small 2.0 magnitude earthquakes, over a two-week period. The resulting dataset provides high-resolution seismic imaging of deep crustal structure beneath this active arc volcano.

“We show that Mount St. Helens sits atop a sharp lateral boundary in Moho reflectivity,” says corresponding author Steven Hansen, a postdoctoral researcher at the University of New Mexico. The lack of reflections to the west can be explained if there is a relatively cold wedge of serpentinite to the west of the mountain, because the compressional wave speed of serpentine is not very different than that of the overriding crust.

In this context, cold is less than about 700 degrees C, or 1,300 degrees F. Below that temperature the serpentinite binds the water into a crystal structure. Above that temperature, however, serpentinite is not stable, and water can percolate up through the hot mantle unimpeded and into the overlying crust.

“The melt that supplies Mount St. Helens is probably formed to the east, in the mantle wedge below Mount Adams, and then moves west through the magmatic system somehow,” Hansen concludes.

“This is a nice result because it shows a very sharp boundary between where you have reflectivity and where you don’t, and that boundary between strong and weak reflectivity is pretty much directly beneath Mount St. Helens,” says coauthor Ken Creager, professor in the department of earth and space sciences at the University of Washington.

“The density of data lets us see that this boundary between where there is reflectivity and where there isn’t is very sharp. Presumably what it’s telling us is the temperature of the mantle.”

Water is locked in various minerals inside the subducting oceanic plate. As the slab goes down, the temperature and pressure increase and the water is squeezed out of the crystals. The water then migrates up into the mantle of the overriding continental plate, where it reacts with olivine to become serpentinite to the west of Mount St. Helens, or olivine to the east. The temperature is key, Creager says, because that indicates where water in the descending ocean plate could be mixing with the rock to lower the melting temperature and form volcano-creating magma.

“An important question is: where is the water, and where isn’t it?” Creager says.

“This adds to a variety of other experiments that suggest that where it’s cold, this water is basically getting soaked into the mantle and turning olivine into serpentine and not going anywhere, so it can’t get up into the crust to form volcanoes,” he says. “When you get up into where there is olivine, the temperature is hotter, serpentine isn’t stable, so the water can play its role in the volcanic process.”

An iMUSH study published in the spring that was led by Rice University suggested that most of the eruptive products came from one or more chambers at depths of 3 to 12 kilometers (about 2 to 7 miles), and that those chambers may be connected.

The full iMUSH experiment includes four different lines of analysis. The team led by the University of Washington deployed 70 3-component seismometers that detected the tiny earthquakes that happen about twice a day, either due to movement of tectonic plates or motion of magma within the volcano, over a two-year period. Researchers retrieved the instruments in August and are now analyzing their data. They hope to get a higher-resolution image of the deeper sections.

Overall, the goal is to create a more complete picture of the magma system beneath Mount St. Helens both to better understand and predict volcanic activity, and also to gauge the severity of the event when an eruption is imminent.

iMUSH is a National Science Foundation project.

Source: University of Washington

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