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.
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.
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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