The discovery and imaging of a mammoth magma reservoir beneath the Yellowstone supervolcano is a capstone to decades of research, said geophysicist Robert B. Smith.
Smith should know - he’s been studying the Yellowstone system for 55 years.
Scientists have long known of a smaller magma chamber, but the new chamber - 4.4 times larger - helps answer the questions about the volcanic system.
“It’s the first time we’ve completed a whole 3-D image completely through the crust,” Smith said.
Smith, researcher and professor emeritus at the University of Utah, is a co-author of a paper discussing the study, published earlier this year in the journal, Science.
The study utilized seismic tomography from years of earthquake data to create images similar to a CT scan of the earth, he said.
The Largest Risk
While the discovery garnered headlines because of public fascination with the supervolcano, Smith said the study doesn’t change the risk assessment of a catastrophic eruption: 1 in 700,000 in any given year.
“That’s pretty small,” he said.
But it also shed light on a more immediate hazard, he said.
“People don’t appreciate the largest risk: earthquakes,” he said. “Earthquakes occur in and around these big faults with a frequency that is much higher. They are a much higher risk and higher hazard than volcanoes.
“A 7.3 earthquake in Yellowstone in 1959 killed 28 people,” he said. “There’s been no volcanic eruption in 70,000 years.”
Thomas A. Drean, AAPG member and director of the Wyoming State Geological Survey, agreed.
“The reason we monitor in Wyoming and monitor Yellowstone is not only curiosity. The biggest issue for us is public health and safety and associated geological hazards,” Drean said.
“The biggest takeaway (in the study) for people, and yes, a nice addition for us, is that it has not increased the risk or hazard probability,” Drean said.
A New Working Model
The newly found reservoir lies 12 to 28 miles below the supervolcano. In a University of Utah press release, co-author Jamie Farrell said the hot rock in it could fill the Grand Canyon 11.2 times over.
“For the first time, we have imaged the continuous volcanic plumbing system under Yellowstone,” said first author Hsin-Hua Huang, also a postdoctoral researcher in geology and geophysics. “That includes the upper crustal magma chamber we have seen previously plus a lower crustal magma reservoir that has never been imaged before and that connects the upper chamber to the Yellowstone hotspot plume below.”
“We now have a new working model” for Yellowstone and other volcanic systems, Smith said.
The magma chamber and reservoirs are filled with hot, mostly solid rock like a sponge, Smith said, with molten rock filling the pockets.
The upper chamber is about 2 percent melt while the upper chamber is about 9 percent melt. That matches earlier predictions, according to the authors.
Smith said the study is the result of advances in methodology and new data gathered over years by several cooperating entities.
A seismic array 200 and 300 kilometers wide uses data from about 60 stations, with installation begun in 1983, he said. In that time, seismic data has been gathered from some 45,000 Yellowstone quakes.
“It’s high-quality data collected in real time at the University of Utah,” he said. “We provide the data and the public has access to it immediately.”
Before the new discovery, researchers had envisioned partly molten rock moving upward from the Yellowstone hotspot plume via a series of vertical and horizontal cracks, known as dikes and sills, or as blobs, the authors said. They still believe such cracks move hot rock from the plume head to the magma reservoir and from there to the shallow magma chamber.
The new study employed a technique developed by Huang to combine seismic information from local quakes detected in Utah, Idaho, the Teton Range and Yellowstone by the University of Utah Seismograph Stations with data from more distant quakes detected by the National Science Foundation-funded EarthScope array, which was used to map the underground structure of the lower 48 states.
The Utah seismic network has closely spaced seismometers that are better at imaging the shallower crust beneath Yellowstone, while EarthScope’s seismometers are better at making images of deeper structures.
The Yellowstone System
As the authors explained, this is how the new study depicts the Yellowstone system, from bottom to top:
♦ Previous research has shown the Yellowstone hotspot plume rises from a depth of at least 440 miles within the Earth’s mantle. Some researchers suspect it originates 1,800 miles deep at Earth’s core.
The plume rises from the depths northwest of Yellowstone. The plume conduit is roughly 50 miles wide as it rises through the Earth’s mantle and then spreads out “like a pancake” as it hits the uppermost mantle about 40 miles deep.
Earlier Utah studies indicated the plume head was 300 miles wide. The new study suggests it may be smaller, but the data aren’t good enough to know for sure.
♦ Hot and partly molten rock rises in dikes from the top of the plume at 40 miles’ depth up to the bottom of the 11,200-cubic-mile magma reservoir, about 28 miles deep. The top of the newly discovered blob-shaped magma reservoir is about 12 miles deep, Huang said.
The reservoir measures 30 miles northwest to southeast and 44 miles southwest to northeast.
“Having this lower magma body resolved the missing link of how the plume connects to the magma chamber in the upper crust,” co-author Fan-Chi Lin said.
♦ The 2,500-cubic-mile upper magma chamber sits beneath Yellowstone’s 40-by-25-mile caldera, or giant crater. Farrell said it is shaped “like a gigantic frying pan” about three to nine miles beneath the surface, with a “handle” rising to the northeast.
The chamber is about 19 miles from northwest to southeast and 55 miles southwest to northeast. The handle is the shallowest, longest part of the chamber that extends 10 miles northeast of the caldera.
Scientists once thought the shallow magma chamber was 1,000 cubic miles. But at science meetings and in a published paper this past year, Farrell and Smith showed the chamber was 2.5 times bigger than once thought. That has not changed in the new study.
Discovery of the magma reservoir below the magma chamber solves a longstanding mystery: Why Yellowstone’s soil and geothermal features emit more carbon dioxide than can be explained by gases from the magma chamber.
Farrell said a deeper magma reservoir had been hypothesized because of the excess carbon dioxide, which comes from molten and partly molten rock.
Smith said the next step is integrating global positioning system data with GPS array paralleling the seismic.
He added that the reservoir discovery is gratifying, combining years of data collection and putting together computer programs and instrumentation from many people and agencies.
He said at least 30 graduate students have done theses based on the Yellowstone research.
“It’s a long record and it really paid off in this paper,” he said.
Now 76, Smith said he began studying Yellowstone “just out of high school.”
He maintains a full schedule of research, lecturing and field trips and still finds Yellowstone fascinating.
“There’s a new discovery every year,” he said. “That’s what keeps me going.”