yellowstone volcano research paper

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• Published: 19 March 2018

Lower-mantle plume beneath the Yellowstone hotspot revealed by core waves

• Peter L. Nelson 1 &
• Stephen P. Grand 1  

Nature Geoscience volume  11 ,  pages 280–284 ( 2018 ) Cite this article

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• Geodynamics
• Volcanology

The Yellowstone hotspot, located in North America, is an intraplate source of magmatism the cause of which is hotly debated. Some argue that a deep mantle plume sourced at the base of the mantle supplies the heat beneath Yellowstone, whereas others claim shallower subduction or lithospheric-related processes can explain the anomalous magmatism. Here we present a shear wave tomography model for the deep mantle beneath the western United States that was made using the travel times of core waves recorded by the dense USArray seismic network. The model reveals a single narrow, cylindrically shaped slow anomaly, approximately 350 km in diameter that we interpret as a whole-mantle plume. The anomaly is tilted to the northeast and extends from the core–mantle boundary to the surficial position of the Yellowstone hotspot. The structure gradually decreases in strength from the deepest mantle towards the surface and if it is purely a thermal anomaly this implies an initial excess temperature of 650 to 850 °C. Our results strongly support a deep origin for the Yellowstone hotspot, and also provide evidence for the existence of thin thermal mantle plumes that are currently beyond the resolution of global tomography models.

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Acknowledgements

We like to thank K. Tao and F. Zhang for helpful discussions about finite frequency tomography and S.-H. Hung for providing the tomography code. We also thank S. Yu and E. Garnero for providing the adaptive stacking travel time measurement code and B. Steinberger for useful discussion. Lastly, we thank P. Crotwell for help with S.O.D (Standing Order for Data) and the IRIS (Incorporated Research Institution for Seismology) Data Center and the Canadian National Data Center for providing the waveforms used in this experiment. This work was supported by the National Science Foundation grant EAR 1648770.

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Peter L. Nelson & Stephen P. Grand

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S.P.G. designed the project. P.L.N. undertook the data measurements and tomography. P.L.N. and S.P.G cowrote the manuscript.

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Supplementary figures showing the results of inversions using different starting models, the upper mantle results for the preferred model and additional resolution tests

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Nelson, P.L., Grand, S.P. Lower-mantle plume beneath the Yellowstone hotspot revealed by core waves. Nature Geosci 11 , 280–284 (2018). https://doi.org/10.1038/s41561-018-0075-y

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Yellowstone’s Supervolcano Is a Hot Spot, but It May Be Calming Down

Some researchers interpret a new timeline of some of the formation’s biggest eruptions as evidence that its activity is waning.

By Matt Kaplan

The volcano below Yellowstone National Park is among the largest on the planet and has a history of generating huge eruptions. There have long been fears — many exaggerated — that it is only a matter of time before it blows, taking much of North America with it.

But new research, published this month in the journal Geology , suggests that this volcanic menace may be losing its strength, and that you can be a bit less alarmed the next time you read a viral headline about that supervolcano out west.

Yellowstone’s volcano is the result of a hot spot, or a superheated area just below Earth’s crust. These regions burn through tectonic plates that glide over them.

This geological phenomenon is part of what gives Yellowstone its character, providing the steady flow of heat that warms groundwater and generates simmering, prismatic pools, caldrons of boiling mud and geysers found throughout the national park.

Unfortunately, ancient hot-spot eruptions on continents are much more difficult to study than similar eruptions that took place out at sea, because they are more explosive. While researchers can see the path that Yellowstone burned as the hot spot migrated from Oregon across Idaho and into Wyoming, discerning one eruption from another has been a chore as most volcanic deposits are scattered across vast landscapes in a chaotic jumble.

“My predecessors thought these messy deposits might be related to one another, but nobody was sure,” said Thomas Knott, the study’s lead author and a geochemist at the University of Leicester in England. He set about the grueling task of fingerprinting volcanic samples from 50 sites in Idaho.

Because each Yellowstone eruption would have involved different portions of the continent being melted, Dr. Knott reasoned that each eruption ought to be subtly different in its chemical profile.

To gain further resolution beyond chemical analysis, his team looked at paleomagnetics. Because the iron from the hot spot was liquid when ejected, it oriented itself toward magnetic north upon eruption, and then got locked into place when it cooled. Because magnetic north has moved throughout Earth’s history , Dr. Knott’s team was able to determine when this iron was erupted.

What they found transformed the timeline of some of the supervolcano’s eruptions. Instead of a series of small eruptions that geologists have long thought took place as the hot spot migrated across Idaho, there had actually been two very big outbursts.

One took place 8.72 million years ago. Based upon the magma that it erupted, it scored 8.8 on the volcanic explosivity index created by the U.S. Geological Survey . The other, which took place 8.99 million years ago, scored 8.6.

“Given that the index does not go higher than 9.0 and anything above 8.0 ranks as ‘mega-colossal,’ it is safe to say that these qualify as super-eruptions,” Dr. Knott said.

What his study means for Yellowstone’s future has set off considerable debate.

Dr. Knott suggests that these newly identified super-eruptions paint a picture of the hot spot's activity waning over time. Between 6 and 11 million years ago, giant eruptions once took place rather frequently, roughly every half million years. But his findings show that, since that time, such eruptions have become less frequent, occurring about every 1.5 million years.

Kari Cooper, a geochemist at University of California, Davis, is skeptical.

“We don’t have a lot of data about what makes a magmatic flare-up happen, especially in a hot spot. Whatever caused the flare-up 9 million years ago could happen again” she said.

Yet, for others, Dr. Knott’s proposal seems logical.

“It makes sense that Yellowstone would weaken as it leaves the relatively thin western crust and travels toward the thicker center of the continent,” said Michael Poland at the Yellowstone Volcano Observatory.

But other researchers say this interpretation only works if you look at the biggest of the big eruptions, or “mega-colossal ones,” as Kenneth Verosub, also at the University of California, Davis, puts it.

“If you also include supercolossal, which, let’s face, it would still bring devastation to a number of states, you suddenly get three big eruptions in the past two million years, and can then argue that the caldera was quiescent between 6 and 2 million years ago and is now just waking up again,” he said.

As for who is right, a few million more years of monitoring should prove most insightful.

Yellowstone Earthquake Information

Contemporary ground motions of yellowstone and the wasatch front, public web access tools for seismic & gps data visualization (a complete list of yellowstone links can be found in this pdf )., research on the yellowstone-teton-snake river plain region, research on the teton fault and teton range, biographical list of yellowstone-teton publications and presentations of the seismology and active tetonics research group., research on the wasatch fault, yellowstone and wasatch fault gps white paper, yellowstone geology book, yellowstone-teton gis site, robert b. smith's publication record, more information, research highlights, journal covers from papers on our yellowstone research, influence of the yellowstone plume on the western u.s. and ysrp.

Four papers in the Journal of Volcanology and Geothermal Research in November 2009 summarize the effects of the Yellowstone plume on the western U.S. geodynamics, the structure of the lithosphere, earthquakes, and earthquake harzards. These papers are included in the list below by date.

The complete yellowstone hotspot plumbing system from the mantle to the surface, the yellowstone magmatic system from the mantle plume to the upper crust, the yellowstone magma reservoir, tomography from 26 years of seismicity revealing that the spatial extent of the yellowstone crustal magma reservoir extends well beyond the yellowstone caldera, the 2010 madison plateau swarm, a fluid-driven earthquake swarm on the margin of the yellowstone caldera, repeating earthquakes in yellowstone, repeating earthquakes in the yellowstone volcanic field: implications for rupture dynamics, ground deformation, and migration in earthquake swarms, transient postseismic deformation and lithospheric strength in yellowstone region, effects of lithospheric viscoelastic relaxation on the contemporary deformation following the 1959 mw 7.3 hebgen lake, montana, earthquake and other areas of the intermountain seismic belt, geoelectrical imaging of the yellowstone mantle plume, three-dimensional inversion of large-scale earthscope magnetotelluric data based on the integral equation method: geoelectrical imaging of the yellowstone conductive mantle plume, extraordinary yellowstone caldera uplift from 2004 to 2010, an extraordinary episode of yellowstone caldera uplift, 2004-2010, from gps and insar observations, the 2008-2009 yellowstone lake swarm, dynamics and rapid migration of the energetic 2008-2009 yellowstone lake earthquake swarm, a white paper on university of utah gps monitoring, gps research and monitoring of the yellowstone volcanic system, wyoming, and the wasatch fault, utah: a white paper, more information:, discovery of first yellowstone explosive source earthquakes, seismic evidence for dilatational sources deformations accompanying the 2004-2008 yellowstone accelerated uplift episode, interaction of the yellowstone plume, north america lithosphere, and mantle flow, geodynamics of the yellowstone hotspot and mantle plume: seismic and gps imaging, kinematics, and mantle flow, yellowstone-snake river plain lithospheric structure, density and lithospheric strength models of the yellowstone-snake river plain volcanic system from gravity and heat flow data, earthquake swarms of yellowstone, earthquake swarm and b -value characterization of the yellowstone volcano-tectonic system, earthquake hazards of the teton-yellowstone area, seismicity and earthquake hazard analysis of the teton-yellowstone region, wyoming, national geographic article on yellowstone hotspot, when yellowstone explodes, microplate tectonics, western u.s. deformation, and the yellowstone hotspot, intraplate deformation and microplate tectonics of the yellowstone hot spot and surrounding western u.s. interior, unprecedented uplift of yellowstone, accelerated uplift and magmatic intrusion of the yellowstone caldera, 2004 to 2006, source modeling of yellowstone deformation, crustal deformation and source models of the yellowstone volcanic field from geodetic data, history of yellowstone crustal motions, crustal deformation of the yellowstone-snake river plain volcanic system: campaign and continuous gps observations, 1987-2004, the yellowstone plume, vp and vs structure of the yellowstone hot spot from teleseismic tomography: evidence for an upper mantle plume, hazards of the yellowstone volcanic system, steam explosions, earthquakes, and volcanic eruptions- what's in yellowstone's future, upper mantle anisotropy around the yellowstone hotspot, models of lithosphere and asthenosphere anisotropic structure of the yellowstone hot spot from shear wave splitting, analysis of yellowstone earthquakes triggered by 2002 denali fault earthquake, remotely triggered seismicity in the yellowstone national park region by the 2002 mw 7.9 denali fault earthquake, alaska, altered geyser activity and yellowstone earthquakes from denali fault earthquake in 2002, changes in geyser eruption behavior and remotely triggered seismicity in yellowstone national park produced by the 2002 m 7.9 denali fault earthquake, alaska, an improved yellowstone earthquake catalog, probabilistic earthquake relocation in three-dimensional velocity models for the yellowstone national park region, wyoming, the yellowstone magma chamber, evidence for gas and magmatic sources beneath the yellowstone volcanic field from seismic tomographic imaging, the yellowstone stress field, seismotectonics and stress field of the yellowstone volcanic plateau from earthquake first-motions and other indicators, the 1985 yellowstone swarm, seismic evidence for fluid migration accompanying subsidence of the yellowstone caldera, seismicity and tectonics of the hebgen lake-yellowstone region, seismicity and contemporary tectonics of the hebgen lake-yellowstone park region, seismicity and tectonics of the intermountain seismic belt, tectonics of the intermountain seismic belt, western united states: microearthquake seismicity and composite fault plane solutions, other links.

May 19, 2023

Yellowstone Supervolcano Eruptions Were Even More Explosive Than We Knew

The last caldera-forming eruption at Yellowstone “was much more complex than previously thought,” according to the annual report about activity at the supervolcano

By Hannah Osborne & LiveScience

The Grand Prismatic Spring at Yellowstone National Park sits in the Yellowstone Caldera, which formed 631,000 years ago.

Peter Adams/Getty Images

The last super-eruption at Yellowstone volcano, which occurred 631,000 years ago, was not one huge explosion. Instead, new research suggests it was a series of eruptions or multiple vents spewing volcanic material in rapid succession. 

According to the U.S. Geological Survey's (USGS) Yellowstone Volcano Observatory 2022 Annual Report , published May 4, fieldwork over the past year has provided new geological evidence that "the formation of Yellowstone Caldera was much more complex than previously thought." A caldera is a large crater that forms after the collapse of a volcano following an eruption. 

Yellowstone is one of the world's biggest volcanic systems. It sits above one of Earth's "hotspots" — areas in the mantle where hot plumes rise and form volcanoes on the crust above. It has produced three caldera-forming eruptions  in the past 3 million years: the Huckleberry Ridge Tuff eruption, 2.1 million years ago; the Mesa Falls eruption, 1.3 million years ago; and the Lava Creek eruption, 631,000 years ago. 

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What are super-eruptions?

The Huckleberry Ridge Tuff and Lava Creek events are considered super-eruptions because they expelled over 240 cubic miles (1,000 cubic kilometers) of material. The latter was responsible for the formation of the Yellowstone caldera. Mesa Falls erupted 67 cubic m (280 cubic km) of material, so — while still about 10 times bigger than the 1980 eruption of Mount St. Helens — is not considered a super-eruption.

Previous research  has shown that the Lava Creek super-eruption was not out of the blue; deposits at the Sour Creek Dome region east of the national park suggest that the giant blast was preceded by at least one eruption. Ignimbrite (volcanic rock formed via the deposits of the hot mix of material ejected during an eruption) found at the site had completely cooled before the main, mapped Lava Creek eruption took place.

To better understand the timeline of the eruption, scientists spent 2022 remapping and collecting samples at Sour Creek Dome.

"It had always been known that there were at least two geological units [a volume of rock distinct from those surrounding it] from the eruption, and it was thought that there was little to no time gap between them," Michael Poland , scientist-in-charge at the Yellowstone Volcano Observatory, told Live Science in an email. "Now, we think there are more units. And we're just not sure what the time gap might have been, if any."

So far, the team has found four previously unrecognized ignimbrite units at Sour Creek, suggesting at least four eruptive pulses. They also found two structures that appear to be eruptive vents, which may have been the sources of these rocks. 

"That could mean either several vents were active and/or there was time separation between the eruptions," Poland said. "But we don't yet have the data we need to answer those questions yet."

In 2020, scientists found the Huckleberry Ridge Tuff eruption  — which ejected more than twice the amount of volcanic material as Lava Creek did — was also a phased event. Analysis of rocks at the site suggests there were three separate eruptions, with weeks to months between the first two, and years to decades between the second and third. 

Yellowstone volcano is not expected to erupt anytime soon . However, the finding that the Lava Creek eruption may have followed a similar pattern to that of the Huckleberry Ridge Tuff eruption could give an idea of what to expect if and when Yellowstone does blow. "These major caldera-forming eruptions might not be single events at Yellowstone, but instead have multiple phases," Poland said. 

Researchers at the volcano now plan to carry out detailed examinations of the newly discovered units and the boundaries between them. This will allow them to paint a more detailed picture of what the Lava Creek eruption looked like — and maybe even what triggered it. 

Copyright 2023  LiveScience , a Future company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Steam rises off the Grand Prismatic Spring, one of the most stunning hydrothermal features in Yellowstone National Park.

Yellowstone Supervolcano May Rumble to Life Faster Than Thought

A new study of ancient ash suggests that the dormant giant could develop the conditions needed to blow in a span of mere decades.

If the supervolcano underneath Yellowstone erupts again, we may have far less advance warning time than we thought.

After analyzing minerals in fossilized ash from the most recent mega-eruption, researchers at Arizona State University think the supervolcano last woke up after two influxes of fresh magma flowed into the reservoir below the caldera.

And in an unsettling twist, the minerals revealed that the critical changes in temperature and composition built up in a matter of decades. Until now, geologists had thought it would take centuries for the supervolcano to make that transition.

A 2013 study, for instance, showed that the magma reservoir that feeds the supervolcano is about two and a half times larger than previous estimates. Scientists also think the reservoir is drained after every monster blast, so they thought it should take a long time to refill. Based on the new study, it seems the magma can rapidly refresh—making the volcano potentially explosive in the geologic blink of an eye.

“It’s shocking how little time is required to take a volcanic system from being quiet and sitting there to the edge of an eruption,” study co-author Hannah Shamloo told the New York Times .

Still, Yellowstone is one of the best monitored volcanoes in the world, notes Michael Poland , the current Scientist-in-Charge of the Yellowstone Volcano Observatory for the U.S. Geological Survey. A variety of sensors and satellites are always looking for changes, and right now, the supervolcano does not seem to pose a threat.

"We see interesting things all the time ... but we haven't seen anything that would lead us to believe that the sort of magmatic event described by the researchers is happening," says Poland via email, adding that the research overall is "somewhat preliminary, but quite tantalizing."

The new paper adds to a suite of surprises scientists have uncovered over the last few years as they have studied the supervolcano. (Also find out about a supervolcano under Italy that has recently been rumbling .)

Today, Yellowstone National Park owes much of its rich geologic beauty to its violent past. Wonders like the Old Faithful geyser and the Grand Prismatic Spring are products of the geothermal activity still seething below the park, which is driven in turn by the vast magma plume that feeds the supervolcano.

About 630,000 years ago, a powerful eruption shook the region, spewing forth 240 cubic miles’ worth of rock and ash and creating the Yellowstone caldera, a volcanic depression 40 miles wide that now cradles most of the national park.

That eruption left behind the Lava Creek Tuff, the ash deposit that Shamloo and her ASU colleague Christy Till used for their work, which they presented in August at a volcanology meeting in Oregon. The pair also presented an earlier version of their study at a 2016 meeting of the American Geophysical Union.

Based on fossil deposits like this one, scientists think the supervolcano has seen at least two other eruptions on this scale in the past two million years or so. Lucky for us, the supervolcano has been largely dormant since before the first people arrived in the Americas . While a handful of smaller belches and quakes have periodically filled the caldera with lava and ash, the last one happened about 70,000 years ago.

In 2011, scientists revealed that the ground above the magma chamber bulged by up to 10 inches in a span of about seven years.

"It's an extraordinary uplift, because it covers such a large area and the rates are so high," the University of Utah's Bob Smith , an expert in Yellowstone volcanism, told National Geographic at the time.

The swelling magma reservoir responsible for the uplift was too deep to create fears of imminent doom, Smith said, and instead the caldera’s gentle “breathing” offered valuable insights into the supervolcano’s behavior.

In 2012, another team reported that at least one of the past super-eruptions may have really been two events, hinting that such large-scale events may be more common than thought .

But almost everyone who studies Yellowstone’s slumbering supervolcano says that right now, we have no way of knowing when the next big blast will happen. For its part, the U.S. Geological Survey puts the rough yearly odds of another massive Yellowstone blast at 1 in 730,000—about the same chance as a catastrophic asteroid collision.

Volcanoes Around the World

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Science News

A hidden danger lurks beneath yellowstone.

Catastrophic hydrothermal explosions rocked the park in the past and could again in the future

The area around West Thumb Geyser Basin and Yellowstone Lake (in the background) has seen many hydrothermal explosions, which occur when hot water beneath the surface suddenly flashes into steam.

Martin Ruegner/Getty Images

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By Douglas Fox

May 8, 2024 at 9:30 am

Mount Ontake in Japan rises 3,067 meters above sea level — a windswept giant standing head and shoulders above densely forested hills. This ancient volcano is a popular trekking site. A trail traverses its ash- and boulder-strewn ridges. There are several huts and a shrine. On September 27, 2014, hikers took advantage of a blue sky and gentle wind. At 11:52 a.m., over a hundred of them stood on the summit, eating snacks and taking photos. Disaster struck with little warning.

The windows and doors of a nearby hut rattled, vibrated by a low-frequency shock wave inaudible to humans.

People glanced around curiously and quickly saw it — half a kilometer down the southwest slope, a gray cloud billowed from the mountain.

The ash cloud swept over the summit with a blast of hot air, leaving people shaken and blinded, but otherwise unhurt. Disoriented in that gray fog, they couldn’t see what arrived soon after.

Thud-thud. Thud. Rocks blasted out of the mountain rained down from the sky. The barren mountaintop offered no shelter to those who desperately sought it in the swirling, gagging dust.

The tempo of hail quickened, as millions of rocks came down — most smaller than baseballs but some as large as beach balls. More and more people fell.

Roughly a million tons of ash and rock spewed from the mountain that day, ejected through several craters that hadn’t existed a moment before. Fifty-eight people died, most killed by falling rocks. Five others were never found.

When scientists investigated the aftermath , they found no new lava flows and no freshly formed ash. What exploded from the mountain wasn’t lava or fire; it was water.

The explosion was powered by a seemingly innocuous pool of water, derived from rain and snowmelt, hidden beneath the surface. The water was suddenly heated from below, perhaps by a burp of hot gas from a deep magma chamber. The water flashed into steam.

Subterranean cracks were pried apart as this vaporized water expanded to hundreds of times its original volume. This high-pressure wedge drove the cracks to the surface — blowing out holes that widened into craters as the escaping vapor flung rocks and old ash into the air.

The tragedy at Ontake is not unique. A similar explosion killed 22 people and injured two dozen others on White Island off the coast of New Zealand in 2019 ( SN: 6/18/21 ). Steam explosions can happen in many other places around the globe, including Greece, Iceland and Northern California.

The ones that happen at active volcanoes are called phreatic explosions. They occur when underground water is suddenly heated by magma or gases. But similar steam explosions, called hydrothermal explosions, can happen in areas without active volcanoes. Like Ontake and White Island, destructive force comes from water expanding into steam.

Yellowstone National Park, where no magma eruption has happened in 70,000 years, has seen hundreds of hydrothermal explosions of various sizes. “In recorded history, it’s been only small ones,” says Paul Bedrosian, a geophysicist at the U.S. Geological Survey in Lakewood, Colo. “But we know [Yellowstone] is capable of creating whoppers.”

News stories often speculate on whether Yellowstone’s massive magma system will awaken and erupt , but these hydrothermal explosions represent a far greater risk today ( SN: 12/15/22 ).

Massive craters show that Yellowstone has seen explosions many times larger than the one at Mount Ontake. For a long time, scientists thought that Yellowstone’s huge explosions might have only happened under specific conditions that existed thousands of years ago at the close of the last ice age. But research in Yellowstone and other places where large hydrothermal explosions happen suggests that belief is misplaced.

“These [big] hydrothermal explosions are very, very dangerous,” says Lisa Morgan, a USGS scientist emerita and volcanologist in Denver who has spent 25 years studying the biggest explosions in Yellowstone’s history. “It could very well happen today.”

Hydrothermal explosions often occur with far less warning than regular magma eruptions. And reconstructing what triggers them, especially the largest ones, has proved challenging, says Shane Cronin, a volcanologist at the University of Auckland in New Zealand. “Globally, no one has really seen many of these happen,” he says. “They’re quite mysterious.”

But Morgan is getting a clearer picture of the triggers, and whether predicting the timing of these explosions might be possible. Exploring the bottom of Yellowstone’s largest lake, she and her colleagues have discovered a restless landscape dotted with hundreds of previously unknown hot vents, some of the world’s largest hydrothermal explosion craters and the brittle geologic pressure cookers that could one day unleash new explosions. While Yellowstone Lake has the most violent history, it’s becoming clear that other parts of the park could also produce large blasts.

Discovering Yellowstone’s explosive history

Yellowstone sits at the northeast end of the Snake River Plain — a conspicuous, flat corridor that plows through an otherwise mountainous region. This scar was created by a hot spot in Earth’s mantle — the geologic equivalent of a gas burner on a stove — which the North American tectonic plate is slowly sliding over, fueling a northeast-trending chain of massive volcanic eruptions over the last 17 million years ( SN: 1/6/22 ).

The most recent super-eruption occurred 640,000 years ago , vomiting forth enough lava to build several Mount Rainiers ( SN: 9/22/14 ). This blast emptied a huge underground chamber, which then collapsed — causing the landscape to slump into an oval-shaped caldera, roughly as big as Rhode Island and ringed with faults.

A magma chamber still sits beneath Yellowstone, left over from that huge eruption. It holds an estimated 10,000 cubic kilometers of magma. But the chamber is only about 15 to 20 percent liquid, making it far too viscous to erupt anytime soon.

Although magma underlies much of the park, it comes closest to the surface, within five kilometers, beneath the north edge of Yellowstone Lake. With magma temperatures above 800° Celsius, the heat flowing up through the ground is “just screaming high,” Bedrosian says. In some places, it’s 100 times the average on Earth’s surface.

In the park, rainwater and snowmelt percolating down toward the chamber are heated to over 250° Celsius but remain liquid because the immense pressure underground prevents the water from expanding into steam. That hot fluid, mixed with carbon dioxide and stinky hydrogen sulfide gas, spurts back up through cracks in the surrounding rocks — dissolving sodium, silica, chloride, arsenic and other minerals — and eventually reaches the surface where it feeds thousands of hot springs, geysers and bubbling mud pots that make Yellowstone a geologic wonder.

When a dome seals, “you’re going to have a pressure cooker.” Paul Bedrosian

Although scientists have studied Yellowstone’s hydrothermal system since the 1870s, not until 1966 did people start to realize that it could produce horrific explosions.

That summer, Patrick Muffler, then a young scientist with the USGS, stepped for the first time into Pocket Basin, near Yellowstone’s western edge. He was there to map the hydrothermal system for NASA, which wanted to understand the volcanic landscapes that future missions to Mars might find.

This broad, sagging meadow is pocked with bubbling hot springs that lace the air with the faint sour smell of hydrochloric acid. The basin is surrounded on three sides by a low ridge sprinkled with a few scraggly trees. As Muffler and his supervisor, Donald White, explored the landscape, White quickly recognized something familiar.

White was one of a handful of people around the world at the time who studied hydrothermal systems. In 1951, he had visited the small town of Lake City, Calif., five nights after a strange cataclysm had happened there. An inconspicuous cluster of hot springs feeding a lush, marshy meadow of bulrushes and grass had exploded, flinging 300,000 tons of mud and rock onto the surrounding fields.

Most of those rocks were jumbles of gravel and sand, cemented together with white zeolite and opal minerals. White knew that these materials form when mineral-­saturated hydrothermal waters reach the cooler surface and their dissolved substances crystallize. He concluded that the blast had been a hydrothermal explosion that was somehow triggered as underground water flashed into steam.

As White and Muffler walked up the ridge surrounding Pocket Basin, their boots crunched over similar rocks. White theorized that this basin was a hydrothermal explosion crater much larger than the one at Lake City — roughly the size of Yankee Stadium. The ridge was a heap of debris flung out of the hole.

But this explosion had not been triggered by a sudden injection of volcanic heat from below, White and Muffler believed. Instead, they surmised, it was caused by an environmental change on the surface.

The explosion debris sat directly atop rocks and gravel left behind when a glacier — the Pinedale ice cap — retreated at the close of the last ice age, around 13,500 years ago. While the glacier was present, the hot springs would have melted the ice overhead, creating an ice-dammed lake, says Muffler, who retired in 2001 but still works with USGS. The weight of that lake would have pressurized the hot springs beneath, preventing the water from boiling even if it was well over 100° C. Muffler and White speculated that as the glacier retreated, the ice dam burst and the lake’s water level plummeted.

“If you can get rid of that water instantly, that depressurizes the system — and bang, it goes off,” Muffler says. No longer constrained by pressure, the water expanded instantly into steam and blew apart the rocks enclosing the hot springs.

In 1971, Muffler and White proposed that at least 10 other large hydrothermal explosion craters might be scattered across Yellowstone . A few years later, geologists added one more crater to the list: Mary Bay, a lobe extending off the north edge of Yellowstone Lake. At 2.6 kilometers across, it remains the largest hydrothermal explosion crater ever found on Earth , forming around the same time as Pocket Basin.

These findings initiated a long-standing debate about whether these monster explosions in Yellowstone could only be caused by retreating ice, or whether other types of triggers could set off these blasts today.

Morgan, who started studying these explosions in the late 1990s, has slowly homed in on an answer .

What lies beneath Yellowstone Lake

In September 1999, an 8-meter-long aluminum boat traced slow, straight lines back and forth across the northern part of Yellowstone Lake. Two instruments were mounted on the stern of the boat. One scanned the lake bottom with narrow sonar beams, recording the echoes to capture the ups and downs of the lake bottom. The other fired periodic seismic shock waves into the lake. Those waves penetrated the lake floor before reflecting back, revealing a picture of the sediment and stone layers beneath the lake bottom.

Morgan organized this project with Pat Shanks, a USGS geochemist who had started studying hydrothermal vents in the lake. He was in bad need of a map of the lake floor to replace his time-consuming method of finding vents: venturing out onto the flat water in a boat early in the morning to see where gas bubbles rose from vents below.

Morgan, Shanks and several other scientists gathered each evening in a nearby building to review the new lake floor maps that the technicians were printing out. “It was like having cataracts taken off of your eyes,” Morgan says, “like night and day.”

Before long, these maps revealed an unknown structure southwest of Mary Bay. Now called Elliott’s Crater, this 830-meter-wide depression is the third-largest hydrothermal crater in world.

Yellowstone’s bottom

On the floor of Yellowstone Lake (colors indicate water depth) are hydrothermal explosion craters, like Elliott’s Crater and Mary Bay, plus domes like the North Basin Hydrothermal Dome, which mark where explosions could occur.

Later that month, people crowded inside the boat’s cabin to watch live video as a remote-controlled submersible descended some 50 meters underwater for a closer look. The inner walls of the crater loomed nearly vertical in the murky water. Foot-long suckerfish “lined up like airplanes” on the edge of the crater, Morgan recalls. “They love the hot water.”

The submersible explored several smaller craters , some twice as wide as a football field, nested within Elliott’s Crater. Inside them were hydrothermal vents. These vents were often flanked by microbial mats; small crustaceans cavorted about just outside the plumes of searing water, grazing on drifting microbes, while trout darted in and out, hunting the crustaceans.

The ROV’s mechanical arm grabbed rocks from the bottom. Examining them later, Shanks found the rocks mottled in greens and blues — signs of iron- and magnesium-rich chlorite minerals, which formed as hydrothermal waters altered rocks lying beneath the lake or welded together sediments on the lake bottom. These samples, presumably, were shards of rock blasted into the air by the explosion, some of which fell back into the crater.

The team spent the next three Septembers mapping the rest of the lake floor. “We found it to be a far more hydrothermally and tectonically active lake than anyone had ever expected,” Morgan says.

Several active faults run through the lake . Over 250 hydrothermal vents nestle within V-shaped depressions that hot water had either dissolved or blasted out of the lake floor. In addition to Elliott’s Crater, the team discovered two other craters at least half a kilometer across plus numerous ones smaller than 200 meters.

Here and there, rounded domes protruded from the lake floor. Seismic profiles revealed them to be soft sediments draped on top of a hard crust. Each dome probably marks where hydrothermal waters had emerged from one or more vents and fused sediments together with silicate and chlorite minerals. Over time, an impermeable barrier formed, allowing less and less water to exit the vents. As pressure built up beneath, the cap gradually arched up, Bedrosian says.

When such a dome seals, “you’re going to have a pressure cooker as opposed to a pot boiling on the surface,” Bedrosian says. It may set the stage for catastrophe.

In fact, during ROV dives, Morgan and Shanks saw what appear to be the blasted edges of a dome on the fringes of Elliott’s Crater. They also found hundreds of intact domes. Most were less than 2 meters across — but some much bigger.

The North Basin Hydrothermal Dome, for instance, spans 750 meters and rises seven stories above the lake floor. Hot water exits the dome through dozens of small vents, at least for now. “But over time, that’s going to change, and those open spaces will seal with silica,” Morgan says. Once that happens, “it’s a perfect candidate for a potential hydrothermal explosion.”

What triggers Yellowstone’s hydrothermal explosions?

As the mapping of Yellowstone Lake was still under way in 2000, Morgan sought approval to pluck cores from the lake bottom to pinpoint when the largest explosions had occurred and what triggered them. Getting that permit from the National Park Service took 16 years. “One of their biggest concerns was that you put a corer [into the lake floor] and we have an explosion,” she says.

In 2016, she and collaborators finally retrieved eight sediment cores, without incident. These cores plus some others from additional field campaigns revealed debris deposits from at least 16 different hydrothermal explosions stacked atop one another, with intervening layers of mud representing peaceful times in between. These include the Elliott’s Crater and Mary Bay explosions and previously unknown smaller ones. Based on estimates of how quickly mud accumulates on the lake floor, three of the smaller explosions happened in the last 350 years or so — the most recent, around 1860.

Analyses of the larger explosions, which Morgan, Shanks and their colleagues published in GSA Bulletin in 2022, suggest that they were not set off by the retreat of the Pinedale ice cap , as previously suspected.

The debris layer from Elliott’s Crater sits just below a well-known volcanic ash layer derived from the eruption of Mount Mazama, which formed Crater Lake in Oregon 7,600 years ago. Morgan’s team estimates that Elliott’s Crater exploded 8,000 years ago, triggered by a major earthquake that happened around the same time. The quake caused a fault that runs through the lake to slip 2.8 meters and could have easily cracked the hydrothermal dome, bursting it like a party balloon.

This dovetails with other research suggesting that two major explosion craters near the lake also formed well after the Pinedale ice cap retreated, one about 9,400 years ago and the other 2,900 years ago. “We don’t think the recession of glacial ice is a big factor,” Morgan says.

Even the Mary Bay explosion, which lake cores confirm occurred around when the ice cap retreated, was probably triggered by something else. Geologic evidence points to a roughly magnitude 6.5 quake that unleashed a tsunami.

Morgan and colleagues think the wave swept into the north end of the lake, past its present-day shoreline, and washed out a pile of rocks and earth that had dammed the north end. The hills surrounding the lake preserve evidence of what happened next.

Eroded into these slopes are two stranded shorelines, one above the other, formed by the lake when its water level was higher in the past. The lower shoreline is younger, with an estimated age of roughly 13,000 years, suggesting that the lake level suddenly dropped from the higher shore to the lower shore, right around the time of the earthquake.

“The lake dropped suddenly 14 meters,” Morgan says. “That’s huge.”

It would have lowered the water pressure over Mary Bay by around 20 or 30 percent. If the lake floor overlying that hot water was already strained to its limit, then that sudden drop in pressure could have caused a catastrophic rupture.

Danger zone

Measuring the reflections of seismic shock waves sent into the bottom of Yellowstone Lake has allowed scientists to get a detailed view of this hotbed of hydrothermal activity. A cross section of Elliott’s Crater reveals vents where hydrothermal fluids and gases rise up. Domes show where sediments are fusing over vents and blocking the release of fluid. If a dome ever breaks, it could set off an explosion.

Seismic profile through Elliott’s Crater

Lauren Harrison, a geologist at Colorado State University in Fort Collins, recently discovered another kind of event that can instigate these explosions. She has carefully studied the Twin Buttes explosion crater, a broad divot the size of an 18-hole golf course, located roughly 40 kilometers west of Yellowstone Lake. Its debris field spills a kilometer down a mountainside, with washing machine–sized boulders jumbled at the bottom. When Harrison used airborne lidar to create a 3-D map of the debris, she realized that it came from two separate events. First, a landslide swept down the mountain, carrying the boulders. Then explosion debris rained down on top of the landslide.

The landslide, she argues, marks the collapse of a massive, rickety pile of rocks that formed over a cluster of thermal vents while the Pinedale ice cap still existed. Rocks being carried by that glacier were gradually cemented together by silicate minerals burbling out of the vents. After the ice cap retreated, the pile could no longer support its own weight and collapsed.

“That [landslide] is a perfect, immediate depressurization event,” Harrison says. The superheated water, no longer buried under rocks, flashed explosively into steam. So this explosion may have been caused indirectly by ice retreat, but the precipitating event was a landslide .

What unifies all of these events — earthquakes, tsunamis and landslides — is that they can happen today, without warning, Morgan says ( SN: 10/25/22 ). But there’s more to learn. Cronin wonders, for example, whether one hydrothermal explosion can trigger another.

He is studying an ominous example in New Zealand, where a cluster of at least 25 explosion craters runs along a 10-kilometer section of the Ngapouri-Rotomahana Fault, through a quilted landscape of farms and forest. “You’re looking at craters up to 300 to 500 meters wide, and [fallen debris] extending out at least a kilometer in many cases,” Cronin says.

The blasts all happened about 700 years ago. His team is trying to pin down the exact timing. He believes they may have unfolded over a period of months or years, with each explosion triggering the next one, possibly by creating new cracks in the bedrock that destabilized other hydrothermal areas. The notion of such a domino effect is alarming. But the idea that a single earthquake might have triggered them simultaneously is even more so. “It’s important for us to figure out if they are all happening at the same time,” Cronin says. “This kind of scenario is far more hazardous” than a single explosion.

Sizing up the danger at Yellowstone

The 2014 Ontake disaster might seem like the worst-case outcome of either a phreatic or hydrothermal explosion. But far worse things can happen.

Morgan estimates that the Mary Bay explosion ejected roughly a quarter of a cubic kilometer of mud, sand and water-saturated rock from its crater. That is 100 to 400 times the volume ejected by the Ontake explosion. It is also roughly 50 times the volume of sand and rock ejected in the Storax Sedan nuclear test, when the U.S. military detonated a 104-kiloton bomb underground in the Nevada desert in 1962.

The Mary Bay explosion also tossed refrigerator-sized boulders out of the water and sent smaller debris up to two kilometers into the air — landing as far as 20 kilometers away. The blast sent a wave of boiling mud surging onto the lake shore, forming a pile up to eight stories tall.

The explosion unfolded as a chain reaction, Morgan says. As the top layer of rock exploded off the lake floor, the removal of its weight depressurized the water-saturated rock below, allowing it to explode, which in turn depressurized yet another layer of rock and fluids farther down — and so on. Layers in the lake cores suggest that three main explosions occurred, probably within minutes, Morgan says, with smaller explosions perhaps continuing “on and off for hours or days.”

She and others are now studying hydrothermal domes in and around Yellowstone Lake that could explode. In 2016, Bedrosian and Carol Finn, a USGS geophysicist, peered inside the North Basin Hydrothermal Dome and other structures in Yellowstone using a remote sensing technique called electrical resistivity , which hints at the chemical composition of minerals and the presence of water in the subsurface.

This effort revealed some sort of hot material with high resistivity hidden beneath the dome’s hard cap. Bedrosian, who is still analyzing the data, thinks it’s primarily steam, since salty water would have lower resistance.

That’s good news. It suggests that the hydrothermal fluid rising beneath this dome is already boiling much farther down — and what reaches the dome is mostly vapor, rather than superheated liquid. If the dome were to become destabilized, there’s not enough liquid water present to expand into vapor and power a major explosion, though a small blast would be possible. But if fluid circulation changes, it could fill the dome with superheated liquid water, creating a more dangerous situation.

Some of the ingredients for a big explosion may already exist in other parts of the park. In the Lower Geyser Basin, where the massive Pocket Basin and Twin Buttes craters reside, water burbling from the ground is high in sodium chloride. This chemical profile indicates that the fluids have not boiled before reaching the surface, and therefore they retain their full explosive potential. The same is true of Norris Geyser Basin, which hosts three other big explosion craters, and Upper Geyser Basin, where Old Faithful sits.

Even if monitoring for signs of impending hydrothermal explosions is not yet possible, scientists aren’t arguing that people should avoid visiting Yellowstone. In the same way, most people don’t avoid visiting Los Angeles just because they are worried about earthquakes. The chances that a massive quake or hydrothermal explosion will happen on any given day are quite low.

But if a rare, huge explosion did occur, it would cause extreme damage.

So even as Morgan studies other explosion craters, she keeps an eye on places that might someday explode, including Storm Point, on the north shore of Yellowstone Lake, near Mary Bay. This dome, 800 meters across, often has snow-free areas during winter due to the heat seeping from it. The ground can reach 50° C in some low, sandy spots, similar to a hot summer sidewalk. Plants are scarce and the gravelly ground is hard and unforgiving, cemented with hydrothermal minerals. Hot water still bubbles out of vents along the edges of the dome, so for now it still has a safety valve that can vent pressure.

But if it seals off, “it’s like a ticking time bomb,” Morgan says. Then, it will only need a sudden trigger, like an earthquake — “and everything’s going to explode.”

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Integrated geoscience studies in the Greater Yellowstone Area - Volcanic, tectonic, and hydrothermal processes in the Yellowstone geoecosystem

• Document: Index Page (html)
• Chapter A : The Yellowstone hotspot, Greater Yellowstone ecosystem, and human geography
• Chapter H : The question of recharge to the deep thermal reservoir underlying the geysers and hot springs of Yellowstone National Park: Chapter H in Integrated geoscience studies in Integrated geoscience studies in the Greater Yellowstone Area—Volcanic, tectonic, and hydrothermal processes in the Yellowstone geoecosystem
• Download citation as: RIS | Dublin Core

Yellowstone National Park, rimmed by a crescent of older mountainous terrain, has at its core the Quaternary Yellowstone Plateau, an undulating landscape shaped by forces of volcanism, tectonism, and later glaciation. Its spectacular hydrothermal systems cap this landscape. From 1997 through 2003, the United States Geological Survey Mineral Resources Program conducted a multidisciplinary project of Yellowstone National Park entitled Integrated Geoscience Studies of the Greater Yellowstone Area, building on a 130-year foundation of extensive field studies (including the Hayden survey of 1871, the Hague surveys of the 1880s through 1896, the studies of Iddings, Allen, and Day during the 1920s, and NASA-supported studies starting in the 1970s—now summarized in USGS Professional Paper 729 A through G) in this geologically dynamic terrain. The project applied a broad range of scientific disciplines and state-of-the-art technologies targeted to improve stewardship of the unique natural resources of Yellowstone and enable the National Park Service to effectively manage resources, protect park visitors from geologic hazards, and better educate the public on geologic processes and resources. This project combined a variety of data sets in characterizing the surficial and subsurface chemistry, mineralogy, geology, geophysics, and hydrothermal systems in various parts of the park.

The sixteen chapters presented herein in USGS Professional Paper 1717,   Integrated Geoscience Studies in the Greater Yellowstone Area—Volcanic, Tectonic, and Hydrothermal Processes in the Yellowstone Geoecosystem , can be divided into four major topical areas: (1) geologic studies, (2) Yellowstone Lake studies, (3) geochemical studies, and (4) geophysical studies. The geologic studies include a paper by Ken Pierce and others on the influence of the Yellowstone hotspot on landscape formation, the ecological effects of the hotspot, and the human experience and human geography of the greater Yellowstone ecosystem as influenced by the Yellowstone hotspot. Another paper by Paul Carrara describes the recent movement of a large landslide block dated by tree-ring analyses in the Tower Falls area. The section under Yellowstone Lake studies begins with a classic paper by J. David Love and others on ancestral Lake Yellowstone. Other papers in this section include results and interpretation of the high-resolution bathymetric, seismic reflection, and submersible studies by Lisa Morgan and others. Ken Pierce and others describe results from their studies of shorelines along Yellowstone Lake and their interpretation of inflation-deflation cycles, tilting, and faulting in the Yellowstone caldera. The influence of sublacustrine hydrothermal vent fluids on the geochemistry of Yellowstone Lake is described by Laurie Balistrieri and others. In Pat Shanks and others’ chapter, hydrothermal reactions, stable-isotope systematics, sinter deposition, and spire formation are related to the geochemistry of sublacustrine hydrothermal deposits in Yellowstone Lake. The geochemical studies section considers park-wide geochemical systems in Yellowstone National Park. In Bob Rye and Alfred Truesdell’s paper, the question of recharge to the deep thermal reservoir underlying the geysers and hot springs of Yellowstone National Park is discussed. Irving Friedman and Dan Norton report on the chloride flux emissions from Yellowstone in their paper questioning whether Yellowstone is losing its steam. Wildlife issues as addressed by examining trace-element and stable-isotope geochemistry are discussed in a chapter by Maurice Chaffee and others. In another chapter by Chaffee and others, natural and anthropogenic anomalies and their potential impact on the environment using geochemistry is reported. Pam Gemery-Hill and others present geochemical data for selected rivers, lake waters, hydrothermal vents, and subaerial geysers for the time interval of 1996–2004. The life cycle of gold deposits near the northeast corner of the park is discussed by Brad Van Gosen. Under the geophysical studies segment, Ray Kokaly and others use AVIRIS (Airborne Visible and Infrared Spectroscopy) data to map vegetation cover and microbial communities in Yellowstone National Park. Eric Livo and others report their results using AVIRIS data on hydrothermally altered rock and hot-spring deposits. In his final paper following a half century of scientific research, Irving Friedman presents data on monitoring changes in geothermal activity at Norris Geyser Basin using satellite telemetry. These papers summarize a near-decade-long effort by the USGS from the late 1990s to mid-2000s.

In 2001, the USGS in cooperation with the National Park Service (Yellowstone National Park) and the University of Utah established the Yellowstone Volcano Observatory, the 5th volcano observatory in the United States.

Is the Yellowstone supervolcano really 'due' for an eruption?

Yellowstone's supervolcano last erupted 70,000 years ago. Will it erupt again anytime soon?

Beneath Yellowstone National Park, a vast region of spectacular wilderness visited by around 3 million people annually , lurks one of the largest volcanoes in the world. 

The Yellowstone Caldera — the cauldron-like basin at the summit of the volcano — is so colossal that it is often called a "supervolcano," which, according to the Natural History Museum in London, means it has the capacity to "produce a magnitude-eight eruption on the Volcanic Explosivity Index, discharging more than 1,000 cubic kilometers [240 cubic miles] of material." 

To put that into perspective, the 1991 eruption of Pinatubo in the Philippines, arguably the most powerful volcanic eruption in living memory, was rated a 6 on the Volcanic Explosivity Index, making it, according to the Natural History Museum, "around 100 times smaller than the benchmark for a supervolcano."

So should we be worried? Will Yellowstone erupt anytime soon?

Is Yellowstone "due" for an eruption?

Media reports have often claimed that Yellowstone is due to erupt. They claim that because the last eruption of the supervolcano was 70,000 years ago , it's bound to blow soon. But that's not how volcanoes work.

"This is perhaps the most common misconception about Yellowstone, and about volcanoes in general. Volcanoes don't work on timelines," Michael Poland , a geophysicist and the scientist-in-charge at Yellowstone Volcano Observatory, told Live Science in an email. "They erupt when there is enough eruptible magma beneath the surface, and pressure to cause that magma to ascend. 

"Neither condition is in place at Yellowstone right now," he added. "It's all about that magma supply. Cut that off, and the volcano won't erupt."

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Many volcanoes go through cycles of activity and inactivity, Poland said. More often than not, a volcano's activity is a direct consequence of the magma supply. "Some volcanoes do seem to have regular eruptions, but this is because the magma supply is relatively constant — think Kilauea in Hawaii or Stromboli in Italy," Poland said.

Related: The 11 biggest volcanic eruptions in history So where does the idea of Yellowstone being "overdue" for an eruption come from, then?

"I suspect the 'overdue' idea comes from the concept of earthquakes," Poland said. "Earthquakes happen as stress accumulates on faults, and in many places this stress accumulates at relatively constant rates due to, for example, plate motion. That being the case, you might expect earthquakes to occur at somewhat regular intervals. It is, of course, more complicated than that — there are many variables at play — but for that reason, it makes more sense to say that a fault is 'overdue' for an earthquake."

Poland also noted that "supervolcanoes" — a term he considers somewhat crude and sensationalist — are "no more or less temperamental" than other volcanoes. So, how do experts keep an eye on Yellowstone's subterranean activity so that, in the case of a major volcanic eruption, warnings can be given?

"Yellowstone is very well monitored by a variety of techniques," Poland said. "It is covered in terms of seismicity and ground deformation. We track the temperatures of some thermal features, although this is not an indicator of volcanic activity, but rather of the behavior of specific hydrothermal areas. We look at overall thermal emissions from space, collect gas and water to assess chemistry over time, and track stream/river flow and chemistry."

So what might indicate that a massive eruption is imminent?

"Having thousands of earthquakes in a short period of time (a few weeks), with many felt events, would be noteworthy, as long as it was not an aftershock sequence from a tectonic event," Poland said. "That seismicity would need to be coupled with really extreme ground deformation (tens of centimeters over the same short period), park-wide changes in geyser activity, and thermal/gas emissions. The ground rises and falls normally by 2-3 cm [0.8 to 1.2 inches] every year, and there are typically ~2000 quakes annually in the area, so it would have to rise far beyond these normal background levels."

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While Yellowstone is relatively stable right now and has not displayed any unusual seismic activity lately, if it were to erupt, the consequences could be extreme. Volcanologists have suggested that the ramification they are most concerned about is wind-flung ash, which could end up coating a surrounding region 500 miles (800 kilometers) across in more than 4 inches (10 centimeters) of ash. This could, experts predict, result in the short-term destruction of Midwest agriculture, and would leave scores of watercourses clogged. According to the U.S. Department of the Interior , "the surrounding states of Montana, Idaho, and Wyoming that are closest to Yellowstone would be affected by pyroclastic flows, while other places in the United States would be impacted by falling ash." Poland added that the effects would also be felt beyond the United States' borders. "If there were a very large explosive eruption, it could impact the global climate by emitting ash and gas into the stratosphere, which could block sunlight and lower global temperatures by a few degrees for a few years," Poland explained. 

Research published in the journal Science in December 2022 found that the Yellowstone caldera holds more liquid molten rock than previously estimated. Given that volcanoes tend to erupt only when a vast amount of magma is readily available, should this finding be a cause for concern?

"This [research] really just confirms what we already know about Yellowstone," Poland said. "Initial findings were that the magma chamber beneath Yellowstone was only 5-15% molten. The new research, which uses more advanced techniques but the same data, suggests it is closer to 16-20% molten. The take-home message is that the magma chamber is mostly solid. And that means there is far less likelihood of a consequential eruption. I find this result reassuring."

Joe Phelan is a journalist based in London. His work has appeared in VICE, National Geographic, World Soccer and The Blizzard, and has been a guest on Times Radio. He is drawn to the weird, wonderful and under examined, as well as anything related to life in the Arctic Circle. He holds a bachelor's degree in journalism from the University of Chester. 

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Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing (2017)

Chapter: 1 introduction, 1 introduction.

Volcanoes are a key part of the Earth system. Most of Earth’s atmosphere, water, and crust were delivered by volcanoes, and volcanoes continue to recycle earth materials. Volcanic eruptions are common. More than a dozen are usually erupting at any time somewhere on Earth, and close to 100 erupt in any year ( Loughlin et al., 2015 ).

Volcano landforms and eruptive behavior are diverse, reflecting the large number and complexity of interacting processes that govern the generation, storage, ascent, and eruption of magmas. Eruptions are influenced by the tectonic setting, the properties of Earth’s crust, and the history of the volcano. Yet, despite the great variability in the ways volcanoes erupt, eruptions are all governed by a common set of physical and chemical processes. Understanding how volcanoes form, how they erupt, and their consequences requires an understanding of the processes that cause rocks to melt and change composition, how magma is stored in the crust and then rises to the surface, and the interaction of magma with its surroundings. Our understanding of how volcanoes work and their consequences is also shared with the millions of people who visit U.S. volcano national parks each year.

Volcanoes have enormous destructive power. Eruptions can change weather patterns, disrupt climate, and cause widespread human suffering and, in the past, mass extinctions. Globally, volcanic eruptions caused about 80,000 deaths during the 20th century ( Sigurdsson et al., 2015 ). Even modest eruptions, such as the 2010 Eyjafjallajökull eruption in Iceland, have multibillion-dollar global impacts through disruption of air traffic. The 2014 steam explosion at Mount Ontake, Japan, killed 57 people without any magma reaching the surface. Many volcanoes in the United States have the potential for much larger eruptions, such as the 1912 eruption of Katmai, Alaska, the largest volcanic eruption of the 20th century ( Hildreth and Fierstein, 2012 ). The 2008 eruption of the unmonitored Kasatochi volcano, Alaska, distributed volcanic gases over most of the continental United States within a week ( Figure 1.1 ).

Finally, volcanoes are important economically. Volcanic heat provides low-carbon geothermal energy. U.S. generation of geothermal energy accounts for nearly one-quarter of the global capacity ( Bertani, 2015 ). In addition, volcanoes act as magmatic and hydrothermal distilleries that create ore deposits, including gold and copper ores.

Moderate to large volcanic eruptions are infrequent yet high-consequence events. The impact of the largest possible eruption, similar to the super-eruptions at Yellowstone, Wyoming; Long Valley, California; or Valles Caldera, New Mexico, would exceed that of any other terrestrial natural event. Volcanoes pose the greatest natural hazard over time scales of several decades and longer, and at longer time scales they have the potential for global catastrophe ( Figure 1.2 ). While

the continental United States has not suffered a fatal eruption since 1980 at Mount St. Helens, the threat has only increased as more people move into volcanic areas.

Volcanic eruptions evolve over very different temporal and spatial scales than most other natural hazards ( Figure 1.3 ). In particular, many eruptions are preceded by signs of unrest that can serve as warnings, and an eruption itself often persists for an extended period of time. For example, the eruption of Kilauea Volcano in Hawaii has continued since 1983. We also know the locations of many volcanoes and, hence, where most eruptions will occur. For these reasons, the impacts of at least some types of volcanic eruptions should be easier to mitigate than other natural hazards.

Anticipating the largest volcanic eruptions is possible. Magma must rise to Earth’s surface and this movement is usually accompanied by precursors—changes in seismic, deformation, and geochemical signals that can be recorded by ground-based and space-borne instruments. However, depending on the monitoring infrastructure, precursors may present themselves over time scales that range from a few hours (e.g., 2002 Reventador, Ecuador, and 2015 Calbuco, Chile) to decades before eruption (e.g., 1994 Rabaul, Papua New Guinea). Moreover, not all signals of volcanic unrest are immediate precursors to surface eruptions (e.g., currently Long Valley, California, and Campi Flegrei, Italy).

Probabilistic forecasts account for this uncertainty using all potential eruption scenarios and all relevant data. An important consideration is that the historical record is short and biased. The instrumented record is even shorter and, for most volcanoes, spans only the last few decades—a miniscule fraction of their lifetime. Knowledge can be extended qualitatively using field studies of volcanic deposits, historical accounts, and proxy data, such as ice and marine sediment cores and speleothem (cave) records. Yet, these too are biased because they commonly do not record small to moderate eruptions.

Understanding volcanic eruptions requires contributions from a wide range of disciplines and approaches. Geologic studies play a critical role in reconstructing the past eruption history of volcanoes,

especially of the largest events, and in regions with no historical or directly observed eruptions. Geochemical and geophysical techniques are used to study volcano processes at scales ranging from crystals to plumes of volcanic ash. Models reveal essential processes that control volcanic eruptions, and guide data collection. Monitoring provides a wealth of information about the life cycle of volcanoes and vital clues about what kind of eruption is likely and when it may occur.

1.1 OVERVIEW OF THIS REPORT

At the request of managers at the National Aeronautics and Space Administration (NASA), the National Science Foundation, and the U.S. Geological Survey (USGS), the National Academies of Sciences, Engineering, and Medicine established a committee to undertake the following tasks:

• Summarize current understanding of how magma is stored, ascends, and erupts.
• Discuss new disciplinary and interdisciplinary research on volcanic processes and precursors that could lead to forecasts of the type, size, and timing of volcanic eruptions.
• Describe new observations or instrument deployment strategies that could improve quantification of volcanic eruption processes and precursors.
• Identify priority research and observations needed to improve understanding of volcanic eruptions and to inform monitoring and early warning efforts.

The roles of the three agencies in advancing volcano science are summarized in Box 1.1 .

The committee held four meetings, including an international workshop, to gather information, deliberate, and prepare its report. The report is not intended to be a comprehensive review, but rather to provide a broad overview of the topics listed above. Chapter 2 addresses the opportunities for better understanding the storage, ascent, and eruption of magmas. Chapter 3 summarizes the challenges and prospects for forecasting eruptions and their consequences. Chapter 4 highlights repercussions of volcanic eruptions on a host of other Earth systems. Although not explicitly called out in the four tasks, the interactions between volcanoes and other Earth systems affect the consequences of eruptions, and offer opportunities to improve forecasting and obtain new insights into volcanic processes. Chapter 5 summarizes opportunities to strengthen

research in volcano science. Chapter 6 provides overarching conclusions. Supporting material appears in appendixes, including a list of volcano databases (see Appendix A ), a list of workshop participants (see Appendix B ), biographical sketches of the committee members (see Appendix C ), and a list of acronyms and abbreviations (see Appendix D ).

Background information on these topics is summarized in the rest of this chapter.

1.2 VOLCANOES IN THE UNITED STATES

The USGS has identified 169 potentially active volcanoes in the United States and its territories (e.g., Marianas), 55 of which pose a high threat or very high threat ( Ewert et al., 2005 ). Of the total, 84 are monitored by at least one seismometer, and only 3 have gas sensors (as of November 2016). 1 Volcanoes are found in the Cascade mountains, Aleutian arc, Hawaii, and the western interior of the continental United States ( Figure 1.4 ). The geographical extent and eruption hazards of these volcanoes are summarized below.

The Cascade volcanoes extend from Lassen Peak in northern California to Mount Meager in British Columbia. The historical record contains only small- to moderate-sized eruptions, but the geologic record reveals much larger eruptions ( Carey et al., 1995 ; Hildreth, 2007 ). Activity tends to be sporadic ( Figure 1.5 ). For example, nine Cascade eruptions occurred in the 1850s, but none occurred between 1915 and 1980, when Mount St. Helens erupted. Consequently, forecasting eruptions in the Cascades is subject to considerable uncertainty. Over the coming decades, there may be multiple eruptions from several volcanoes or no eruptions at all.

The Aleutian arc extends 2,500 km across the North Pacific and comprises more than 130 active and potentially active volcanoes. Although remote, these volcanoes pose a high risk to overflying aircraft that carry more than 30,000 passengers a day, and are monitored by a combination of ground- and space-based sensors. One or two small to moderate explosive eruptions occur in the Aleutians every year, and very large eruptions occur less frequently. For example, the world’s largest eruption of the 20th century occurred approximately 300 miles from Anchorage, in 1912.

In Hawaii, Kilauea has been erupting largely effusively since 1983, but the location and nature of eruptions can vary dramatically, presenting challenges for disaster preparation. The population at risk from large-volume, rapidly moving lava flows on the flanks of the Mauna Loa volcano has grown tremendously in the past few decades ( Dietterich and Cashman, 2014 ), and few island residents are prepared for the even larger magnitude explosive eruptions that are documented in the last 500 years ( Swanson et al., 2014 ).

All western states have potentially active volcanoes, from New Mexico, where lava flows have reached within a few kilometers of the Texas and Oklahoma borders ( Fitton et al., 1991 ), to Montana, which borders the Yellowstone caldera ( Christiansen, 1984 ). These volcanoes range from immense calderas that formed from super-eruptions ( Mastin et al., 2014 ) to small-volume basaltic volcanic fields that erupt lava flows and tephra for a few months to a few decades. Some of these eruptions are monogenic (erupt just once) and pose a special challenge for forecasting. Rates of activity in these distributed volcanic fields are low, with many eruptions during the past few thousand years (e.g., Dunbar, 1999 ; Fenton, 2012 ; Laughlin et al., 1994 ), but none during the past hundred years.

1.3 THE STRUCTURE OF A VOLCANO

Volcanoes often form prominent landforms, with imposing peaks that tower above the surrounding landscape, large depressions (calderas), or volcanic fields with numerous dispersed cinder cones, shield volcanoes, domes, and lava flows. These various landforms reflect the plate tectonic setting, the ways in which those volcanoes erupt, and the number of eruptions. Volcanic landforms change continuously through the interplay between constructive processes such as eruption and intrusion, and modification by tectonics, climate, and erosion. The stratigraphic and structural architecture of volcanoes yields critical information on eruption history and processes that operate within the volcano.

Beneath the volcano lies a magmatic system that in most cases extends through the crust, except during eruption. Depending on the setting, magmas may rise

___________________

1 Personal communication from Charles Mandeville, Program Coordinator, Volcano Hazards Program, U.S. Geological Survey, on November 26, 2016.

directly from the mantle or be staged in one or more storage regions within the crust before erupting. The uppermost part (within 2–3 km of Earth’s surface) often hosts an active hydrothermal system where meteoric groundwater mingles with magmatic volatiles and is heated by deeper magma. Identifying the extent and vigor of hydrothermal activity is important for three reasons: (1) much of the unrest at volcanoes occurs in hydrothermal systems, and understanding the interaction of hydrothermal and magmatic systems is important for forecasting; (2) pressure buildup can cause sudden and potentially deadly phreatic explosions from the hydrothermal system itself (such as on Ontake, Japan, in 2014), which, in turn, can influence the deeper magmatic system; and (3) hydrothermal systems are energy resources and create ore deposits.

Below the hydrothermal system lies a magma reservoir where magma accumulates and evolves prior to eruption. Although traditionally modeled as a fluid-filled cavity, there is growing evidence that magma reservoirs may comprise an interconnected complex of vertical and/or horizontal magma-filled cracks, or a partially molten mush zone, or interleaved lenses of magma and solid material ( Cashman and Giordano, 2014 ). In arc volcanoes, magma chambers are typically located 3–6 km below the surface. The magma chamber is usually connected to the surface via a fluid-filled conduit only during eruptions. In some settings, magma may ascend directly from the mantle without being stored in the crust.

In the broadest sense, long-lived magma reservoirs comprise both eruptible magma (often assumed to contain less than about 50 percent crystals) and an accumulation of crystals that grow along the margins or settle to the bottom of the magma chamber. Physical segregation of dense crystals and metals can cause the floor of the magma chamber to sag, a process balanced by upward migration of more buoyant melt. A long-lived magma chamber can thus become increasingly stratified in composition and density.

The deepest structure beneath volcanoes is less well constrained. Swarms of low-frequency earthquakes at mid- to lower-crustal depths (10–40 km) beneath volcanoes suggest that fluid is periodically transferred into the base of the crust ( Power et al., 2004 ). Tomographic studies reveal that active volcanic systems have deep crustal roots that contain, on average, a small fraction of melt, typically less than 10 percent. The spatial distribution of that melt fraction, particularly how much is concentrated in lenses or in larger magma bodies, is unknown. Erupted samples preserve petrologic and geochemical evidence of deep crystallization, which requires some degree of melt accumulation. Seismic imaging and sparse outcrops suggest that the proportion of unerupted solidified magma relative to the surrounding country rock increases with depth and that the deep roots of volcanoes are much more extensive than their surface expression.

1.4 MONITORING VOLCANOES

Volcano monitoring is critical for hazard forecasts, eruption forecasts, and risk mitigation. However, many volcanoes are not monitored at all, and others are monitored using only a few types of instruments. Some parameters, such as the mass, extent, and trajectory of a volcanic ash cloud, are more effectively measured by satellites. Other parameters, notably low-magnitude earthquakes and volcanic gas emissions that may signal an impending eruption, require ground-based monitoring on or close to the volcanic edifice. This section summarizes existing and emerging technologies for monitoring volcanoes from the ground and from space.

Monitoring Volcanoes on or Near the Ground

Ground-based monitoring provides data on the location and movement of magma. To adequately capture what is happening inside a volcano, it is necessary to obtain a long-term and continuous record, with periods spanning both volcanic quiescence and periods of unrest. High-frequency data sampling and efficient near-real-time relay of information are important, especially when processes within the volcano–magmatic–hydrothermal system are changing rapidly. Many ground-based field campaigns are time intensive and can be hazardous when volcanoes are active. In these situations, telemetry systems permit the safe and continuous collection of data, although the conditions can be harsh and the lifetime of instruments can be limited in these conditions.

Ground-based volcano monitoring falls into four broad categories: seismic, deformation, gas, and thermal monitoring ( Table 1.1 ). Seismic monitoring tools,

TABLE 1.1 Ground-Based Instrumentation for Monitoring Volcanoes

including seismometers and infrasound sensors, are used to detect vibrations caused by breakage of rock and movement of fluids and to assess the evolution of eruptive activity. Ambient seismic noise monitoring can image subsurface reservoirs and document changes in wave speed that may reflect stress. changes. Deformation monitoring tools, including tiltmeters, borehole strainmeters, the Global Navigation Satellite System (GNSS, which includes the Global Positioning System [GPS]), lidar, radar, and gravimeters, are used to detect the motion of magma and other fluids in the subsurface. Some of these tools, such as GNSS and lidar, are also used to detect erupted products, including ash clouds, pyroclastic density currents, and volcanic bombs. Gas monitoring tools, including a range of sensors ( Table 1.1 ), and direct sampling of gases and fluids are used to detect magma intrusions and changes in magma–hydrothermal interactions. Thermal monitoring tools, such as infrared cameras, are used to detect dome growth and lava breakouts. Continuous video or photographic observations are also commonly used and, despite their simplicity, most directly document volcanic activity. Less commonly used monitoring technologies, such as self-potential, electromagnetic techniques, and lightning detection are used to constrain fluid movement and to detect

ash clouds. In addition, unmanned aerial vehicles (e.g., aircraft and drones) are increasingly being used to collect data. Rapid sample collection and analysis is also becoming more common as a monitoring tool at volcano observatories. A schematic of ground-based monitoring techniques is shown in Figure 1.6 .

Monitoring Volcanoes from Space

Satellite-borne sensors and instruments provide synoptic observations during volcanic eruptions when collecting data from the ground is too hazardous or where volcanoes are too remote for regular observation. Repeat-pass data collected over years or decades provide a powerful means for detecting surface changes on active volcanoes. Improvements in instrument sensitivity, data availability, and the computational capacity required to process large volumes of data have led to a dramatic increase in “satellite volcano science.”

Although no satellite-borne sensor currently in orbit has been specifically designed for volcano monitoring, a number of sensors measure volcano-relevant

TABLE 1.2 Satellite-Borne Sensor Suite for Volcano Monitoring

NOTE: AIRS, Atmospheric Infrared Sounder; ALOS, Advanced Land Observing Satellite; ASTER, Advanced Spaceborne Thermal Emission and Reflection Radiometer; AVHRR, Advanced Very High Resolution Radiometer; CALIPSO, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation; COSMO-SkyMed, Constellation of Small Satellites for Mediterranean Basin Observation; GOES, Geostationary Operational Environmental Satellite; IASI, Infrared Atmospheric Sounding Interferometer; MISR, Multi-angle Imaging SpectroRadiometer; MLS, Microwave Limb Sounder; MODIS, Moderate Resolution Imaging Spectroradiometer; OMI, Ozone Monitoring Instrument; OMPS, Ozone Mapping and Profiler Suite; SAGE, Stratospheric Aerosol and Gas Experiment.

parameters, including heat flux, gas and ash emissions, and deformation ( Table 1.2 ). Thermal infrared data are used to detect eruption onset and cessation, calculate lava effusion rates, map lava flows, and estimate ash column heights during explosive eruptions. In some cases, satellites may capture thermal precursors to eruptions, although low-temperature phenomena are challenging to detect. Both high-temporal/low-spatial-resolution (geostationary orbit) and high-spatial/low-temporal-resolution (polar orbit) thermal infrared observations are needed for global volcano monitoring.

Satellite-borne sensors are particularly effective for observing the emission and dispersion of volcanic gas and ash plumes in the atmosphere. Although several volcanic gas species can be detected from space (including SO 2 , BrO, OClO, H 2 S, HCl, and CO; Carn et al., 2016 ), SO 2 is the most readily measured, and it is also responsible for much of the impact of eruptions on climate. Satellite measurements of SO 2 are valuable for detecting eruptions, estimating global volcanic fluxes and recycling of other volatile species, and tracking volcanic clouds that may be hazardous to aviation in near real time. Volcanic ash cloud altitude is most accurately determined by spaceborne lidar, although spatial coverage is limited. Techniques for measuring volcanic CO 2 from space are under development and could lead to earlier detection of preeruptive volcanic degassing.

Interferometric synthetic aperture radar (InSAR) enables global-scale background monitoring of volcano deformation ( Figure 1.7 ). InSAR provides much higher spatial resolution than GPS, but lower accuracy and temporal resolution. However, orbit repeat times will diminish as more InSAR missions are launched, such as the European Space Agency’s recently deployed Sentinel-1 satellite and the NASA–Indian Space Research Organisation synthetic aperture radar mission planned for launch in 2020.

1.5 ERUPTION BEHAVIOR

Eruptions range from violently explosive to gently effusive, from short lived (hours to days) to persistent over decades or centuries, from sustained to intermittent, and from steady to unsteady ( Siebert et al., 2015 ). Eruptions may initiate from processes within the magmatic system ( Section 1.3 ) or be triggered by processes and properties external to the volcano, such as precipitation, landslides, and earthquakes. The eruption behavior of a volcano may change over time. No classification scheme captures this full diversity of behaviors (see Bonadonna et al., 2016 ), but some common schemes to describe the style, magnitude, and intensity of eruptions are summarized below.

Eruption Magnitude and Intensity

The size of eruptions is usually described in terms of total erupted mass (or volume), often referred to as magnitude, and mass eruption rate, often referred to as intensity. Pyle (2015) quantified magnitude and eruption intensity as follows:

magnitude = log 10 (mass, in kg) – 7, and

intensity = log 10 (mass eruption rate, in kg/s) + 3.

The Volcano Explosivity Index (VEI) introduced by Newhall and Self (1982) assigns eruptions to a VEI class based primarily on measures of either magnitude (erupted mass or volume) or intensity (mass eruption rate and/or eruption plume height), with more weight given to magnitude. The VEI classes are summarized in Figure 1.8 . The VEI classification is still in use, despite its many limitations, such as its reliance on only a few types of measurements and its poor fit for small to moderate eruptions (see Bonadonna et al., 2016 ).

Smaller VEI events are relatively common, whereas larger VEI events are exponentially less frequent ( Siebert et al., 2015 ). For example, on average about three VEI 3 eruptions occur each year, whereas there is a 5 percent chance of a VEI 5 eruption and a 0.2 percent chance of a VEI 7 (e.g., Crater Lake, Oregon) event in any year.

Eruption Style

The style of an eruption encompasses factors such as eruption duration and steadiness, magnitude, gas flux, fountain or column height, and involvement of magma and/or external source of water (phreatic and phreatomagmatic eruptions). Eruptions are first divided into effusive (lava producing) and explosive (pyroclast producing) styles, although individual eruptions can be simultaneously effusive and weakly explosive, and can pass rapidly and repeatedly between eruption styles. Explosive eruptions are further subdivided into styles that are sustained on time scales of hours to days and styles that are short lived ( Table 1.3 ).

Classification of eruption style is often qualitative and based on historical accounts of characteristic eruptions from type-volcanoes. However, many type-volcanoes exhibit a range of eruption styles over time (e.g., progressing between Strombolian, Vulcanian, and Plinian behavior; see Fee et al., 2010 ), which has given rise to terms such as subplinian or violent Strombolian.

1.6 ERUPTION HAZARDS

Eruption hazards are diverse ( Figure 1.9 ) and may extend more than thousands of kilometers from an active volcano. From the perspective of risk and impact, it is useful to distinguish between near-source and distal hazards. Near-source hazards are far more unpredictable than distal hazards.

Near-source hazards include those that are airborne, such as tephra fallout, volcanic gases, and volcanic projectiles, and those that are transported laterally on or near the ground surface, such as pyroclastic density currents, lava flows, and lahars. Pyroclastic density currents are hot volcanic flows containing mixtures of gas and micron- to meter-sized volcanic particles. They can travel at velocities exceeding 100 km per hour. The heat combined with the high density of material within these flows obliterates objects in their path, making them the most destructive of volcanic hazards. Lava flows also destroy everything in their path, but usually move slowly enough to allow people to get out of the way. Lahars are mixtures of volcanic debris, sediment, and water that can travel many tens of kilometers along valleys and river channels. They may be triggered during an eruption by interaction between volcanic prod-

TABLE 1.3 Characteristics of Different Eruption Styles

ucts and snow, ice, rain, or groundwater. Lahars can be more devastating than the eruption itself. Ballistic blocks are large projectiles that typically fall within 1–5 km from vents.

The largest eruptions create distal hazards. Explosive eruptions produce plumes that are capable of dispersing ash hundreds to thousands of kilometers from the volcano. The thickness of ash deposited depends on the intensity and duration of the eruption and the wind direction. Airborne ash and ash fall are the most severe distal hazards and are likely to affect many more people than near-source hazards. They cause respiratory problems and roof collapse, and also affect transport networks and infrastructure needed to support emergency response. Volcanic ash is a serious risk to air traffic. Several jets fully loaded with passengers have temporarily lost power on all engines after encountering dilute ash clouds (e.g., Guffanti et al., 2010 ). Large lava flows, such as the 1783 Laki eruption in Iceland, emit volcanic gases that create respiratory problems and acidic rain more than 1,000 km from the eruption. Observed impacts of basaltic eruptions in Hawaii and Iceland include regional volcanic haze (“vog”) and acid rain that affect both agriculture and human health (e.g., Thordarson and Self, 2003 ) and fluorine can contaminate grazing land and water supplies (e.g., Cronin et al., 2003 ). Diffuse degassing of CO 2 can lead to deadly concentrations with fatal consequences such as occurred at Mammoth Lakes, California, or cause lakes to erupt, leading to massive CO 2 releases that suffocate people (e.g., Lake Nyos, Cameroon).

Secondary hazards can be more devastating than the initial eruption. Examples include lahars initiated by storms, earthquakes, landslides, and tsunamis from eruptions or flank collapse; volcanic ash remobilized by wind to affect human health and aviation for extended periods of time; and flooding because rain can no longer infiltrate the ground.

1.7 MODELING VOLCANIC ERUPTIONS

Volcanic processes are governed by the laws of mass, momentum, and energy conservation. It is possible to develop models for magmatic and volcanic phenomena based on these laws, given sufficient information on mechanical and thermodynamic properties of the different components and how they interact with each other. Models are being developed for all processes in volcanic systems, including melt transport in the mantle, the evolution of magma bodies within the crust, the ascent of magmas to the surface, and the fate of magma that erupts effusively or explosively.

A central challenge for developing models is that volcanic eruptions are complex multiphase and multicomponent systems that involve interacting processes over a wide range of length and time scales. For example, during storage and ascent, the composition, temperature, and physical properties of magma and host rocks evolve. Bubbles and crystals nucleate and grow in this magma and, in turn, greatly influence the properties of the magmas and lavas. In explosive eruptions, magma fragmentation creates a hot mixture of gas and particles with a wide range of sizes and densities. Magma also interacts with its surroundings: the deformable rocks that surround the magma chamber and conduit, the potentially volatile groundwater and surface water, a changing landscape over which pyroclastic density currents and lava flows travel, and the atmosphere through which eruption columns rise.

Models for volcanic phenomena that involve a small number of processes and that are relatively amenable to direct observation, such as volcanic plumes, are relatively straightforward to develop and test. In contrast, phenomena that occur underground are more difficult to model because there are more interacting processes. In those cases, direct validation is much more challenging and in many cases impossible. Forecasting ash dispersal using plume models is more straightforward and testable than forecasting the onset, duration, and style of eruption using models that seek to explain geophysical and geochemical precursors. In all cases, however, the use of even imperfect models helps improve the understanding of volcanic systems.

Modeling approaches can be divided into three categories:

• Reduced models make simplifying assumptions about dynamics, heat transfer, and geometry to develop first-order explanations for key properties and processes, such as the velocity of lava flows and pyroclastic density currents, the height of eruption columns, the magma chamber size and depth, the dispersal of tephra, and the ascent of magma in conduits. Well-calibrated or tested reduced models offer a straightforward ap-

proach for combining observations and models in real time in an operational setting (e.g., ash dispersal forecasting for aviation safety). Models may not need to be complex if they capture the most important processes, although simplifications require testing against more comprehensive models and observations.

• Multiphase and multiphysics models improve scientific understanding of complex processes by invoking fewer assumptions and idealizations than reduced models ( Figure 1.10 ), but at the expense of increased complexity and computational demands. They also require additional components, such as a model for how magma in magma chambers and conduits deforms when stressed; a model for turbulence in pyroclastic density currents and plumes; terms that describe the thermal and mechanical exchange among gases, crystals, and particles; and a description of ash aggregation in eruption columns. A central challenge for multiphysics models is integrating small-scale processes with large-scale dynamics. Many of the models used in volcano science build on understanding developed in other science and engineering fields and for other ap-

plications. Multiphysics and multiscale models benefit from rapidly expanding computational capabilities.

• Laboratory experiments simulate processes for which the geometry and physical and thermal processes and properties can be scaled ( Mader et al., 2004 ). Such experiments provide insights on fundamental processes, such as crystal dynamics in flowing magmas, entrainment in eruption columns, propagation of dikes, and sedimentation from pyroclastic density currents ( Figure 1.11 ). Experiments have also been used successfully to develop the subsystem models used in numerical simulations, and to validate computer simulations for known inputs and properties.

The great diversity of existing models reflects to a large extent the many interacting processes that operate in volcanic eruptions and the corresponding simplifying assumptions currently required to construct such models. The challenge in developing models is often highlighted in discrepancies between models and observations of natural systems. Nevertheless, eruption models reveal essential processes governing volcanic eruptions, and they provide a basis for interpreting measurements from prehistoric and active eruptions and for closing observational gaps. Mathematical models offer a guide for what observations will be most useful. They may also be used to make quantitative and testable predictions, supporting forecasting and hazard assessment.

Volcanic eruptions are common, with more than 50 volcanic eruptions in the United States alone in the past 31 years. These eruptions can have devastating economic and social consequences, even at great distances from the volcano. Fortunately many eruptions are preceded by unrest that can be detected using ground, airborne, and spaceborne instruments. Data from these instruments, combined with basic understanding of how volcanoes work, form the basis for forecasting eruptions—where, when, how big, how long, and the consequences.

Accurate forecasts of the likelihood and magnitude of an eruption in a specified timeframe are rooted in a scientific understanding of the processes that govern the storage, ascent, and eruption of magma. Yet our understanding of volcanic systems is incomplete and biased by the limited number of volcanoes and eruption styles observed with advanced instrumentation. Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing identifies key science questions, research and observation priorities, and approaches for building a volcano science community capable of tackling them. This report presents goals for making major advances in volcano science.

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