Using vibrations to see into Yellowstone's magma reservoir

(Press-News.org) Beneath Yellowstone lies a magma reservoir, pulsing with molten and superheated rock and exsolved gases. Scientists have long known about the chamber’s existence, but have yet to precisely locate its uppermost boundary and characterize the contents of the chamber closest to the surface—information crucial for understanding the potential perils this volcanic feature poses.

That changed this week with new research by seismologists from the University of Utah and the University of New Mexico (UNM) who used hundreds of portable seismometers and a mechanical vibration source to render 2D seismic reflection images of the ground beneath Yellowstone’s caldera.

Using artificial seismic waves, the team determined that the top of the chamber is 3.8 kilometers, or about 12,500 feet, below Earth’s surface, and it is sharply delineated from the rock strata above, according to findings published in the journal Nature. The researchers also determined the portion of the uppermost magma chamber that is comprised of volatile gases and liquids.

“The depth of 3.8 kilometers is important,” said coauthor Jamie Farrell, a U research associate professor of geology and geophysics and chief seismologist for the Yellowstone Volcano Observatory, operated by the U.S. Geological Survey. “We know what pressures are going to be and how bubbles are going to come out of the magma. One thing that makes these eruptions so devastating is that if gases are trapped, they become very explosive as they decompress.”

The good news is that these findings indicate the long-dormant Yellowstone Volcano is in no immediate danger of eruption.

This is because much of the volatile gas released from the magma escapes through Yellowstone’s surface geothermal features, such as Mud Volcano, without accumulating to dangerous levels, according to coauthor Fan-Chi Lin, professor in Utah’s Department of Geology & Geophysics.

“When the magma rises from the deeper crust, volatile materials such as CO2 and H2O exsolve from the melt. Due to their buoyancy, they tend to accumulate at the top of the magma chamber,” he said. “But if there’s a channel, they can escape to the surface.”

A high-silica type of igneous rock called rhyolite makes up Yellowstone’s magma chamber, which spans an area 55 miles by 30 miles, dropping to a depth of 10 miles below the surface. Beneath it is an even larger reservoir made of low-silica basalt and containing far less molten rock, according to a University of Utah study published in 2015 in Science.

The volcano blew catastrophically 630,000 years ago and many wonder if it’s getting ready for another eruption. Such fears are unwarranted, and the new findings are further evidence of that, according to Farrell.

For decades, scientists have studied Yellowstone’s intriguing magmatic system that drives the geysers, mudpots and thousands of other hydrothermal features that draw millions of visitors to Yellowstone National Park each year.

The U’s Seismograph Stations oversee a network of fixed seismometers at Yellowstone to monitor its frequent earthquakes. Seismic waves from these natural events have long helped scientists characterize the magma chamber, similar to the way CT scans image tissue inside the human body, but the representations are blurry. To achieve greater resolution in the new study, Farrell and Lin’s team deployed an array of 650 portable geophones along Yellowstone National Park’s roads at 100- to 150-meter intervals. Instead of waiting for earthquakes to happen, they brought in a Vibroseis truck, typically used in oil and gas exploration to image subsurface formations and deposits.

“In a sense, we're causing our own earthquakes, and we record all that data on the seismometers,” Farrell said. “And since we put so many out, we can get a higher resolution image of the subsurface.”

The team vibrated the ground at 110 locations, delivering 20 treatments lasting 40 seconds each.

Seismic waves propagate in two forms, known as S-waves and P-waves, which travel at different speeds and behave differently when they strike molten rock. Leveraging the properties of these waves, the researchers were able to locate the top of the chamber and determined that 86% of the upper portion is solid rock, with pore spaces comprising the remaining 14%. These pore spaces are about half filled with molten material and half with volatile gases and liquid, the researchers discovered.

This research is providing crucial clues about the structure of the magma body, according to USGS’s Mike Poland, the scientist in charge at the Yellowstone Volcano Observatory.

“That helps us understand more about the heat engine that's powering Yellowstone and about how melt is distributed. That can have ramifications for how we perceive the volcanic hazard,” he said. “Yellowstone in many ways is a laboratory volcano, and what we learn at Yellowstone can be used to better understand volcanoes in other parts of the world that are a lot more active, but are harder to study. Examples might be Campi Flegrei in Italy or Santorini in Greece, which is mostly submarine.”

He likened recent breakthroughs in seismic imaging to advances in digital cameras that have enabled vast leaps in photographic resolution. Prior studies, which relied on natural seismic events, led by Farrell, Lin, and other seismologists, pictured the magma chamber as an “amorphous blob” beneath Yellowstone. Now it’s coming into sharper focus with the help of artificially generated seismic waves.

“Over the years, different techniques have used the older data, and then there have been new data collection efforts like the one that in geoscientists from Utah and New Mexico that have allowed for increased resolution,” Poland said. “Similar techniques are being used in other places where you put out huge numbers of seismometers and then you record both the background earthquakes and you make your own seismic energy, which allows you to target specific things. These developments are allowing us to see into volcanoes in just really unprecedented ways.”

The study, titled “A sharp volatile-rich cap to the Yellowstone magmatic system,”  was published April 16 In the journal Nature. The research was supported by grants from the National Science Foundation and the Brinson Foundation. Lead authors include Chenglong Dan and Wenkai Song of the University of New Mexico and Brandon Schmandt of Rice University.

about University of Utah’s geophysical research on the Yellowstone magma chamber and hydrothermal features.

U scientists plumb the depths of the world’s tallest geyser

A pool at Yellowstone is a thumping thermometer

Scientists see a deeper Yellowstone magma

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