A hydrothermal blast rocked Yellowstone. It won’t be the last.
Hydrothermal explosions are a regular occurrence at Yellowstone and other volcanic areas. But scientists aren’t sure exactly what triggers them or how to predict the next big one.
The boardwalk trail of Yellowstone National Park’s Biscuit Basin has long been a peaceful place to stroll and wonder at the geysers and hydrothermal pools that draw crowds of tourists every year. Last Tuesday morning, however, an explosion reminded everyone that the park encloses a geologically active wonderland where water, heat, and pressure are not always visitor-friendly.
The blast wasn’t a regular geyser eruption or the rumblings of Yellowstone’s supervolcano but a hydrothermal explosion. Videos showed rock and debris tossed hundreds of feet into the air, including blocks of stone about three feet wide and weighing hundreds of pounds.
No one was hurt. Nevertheless, park officials have closed the Biscuit Basin trail for the rest of the summer. The blast damaged the boardwalk and altered the underground crevices where water and heat interact to create Yellowstone’s celebrated features, including the nearby Black Diamond Pool.
“Given the recent changes to the hydrothermal plumbing system,” park officials noted in a press release, “small explosions of boiling water from this area in Biscuit Basin continue to be possible over the coming days to months.”
What causes hydrothermal explosions?
The explosion is far from the first in the park, or in Biscuit Basin. Geologists recorded a small explosion in the vicinity of Black Diamond pool in 2009, spurting mud and debris in the immediate area. The latest explosion was much bigger.
“When I saw the first video online, I was just shocked by the height of the water jets and the number of large projectiles,” says University of California, Berkeley geologist Mara Reed. Visitors in the immediate area were lucky some features of explosions like these, such as jets of boiling hot water, did not shower the fleeing crowd. “This was likely the largest hydrothermal explosion to occur near a trail in the park’s modern history, and I think it’s lucky that there were no injuries,” Reed says.
Changes underground caused the sudden blast. The critical event that precipitates the hydrothermal explosions is when water beneath the ground suddenly “flashes” to steam due to a sudden drop in pressure. The quick change from water to steam, in turn, creates an incredible amount of pressure – enough to blow through the crust and send three-foot-wide rocks high into the air.
“The hot water in hydrothermal systems is under pressure and can be very near the boiling point,” Reed says. Something as simple as a new crack forming or debris a tourist tossed into a hydrothermal feature can alter the pressure inside, setting conditions up for a flash. That steam takes up more space in the underground chambers than water does, Reed notes, and with nowhere to go the trapped gas causes an explosion. The conditions that trigger the instantaneous change from water to steam vary depending on unknown details below the ground.
While the explosion has grabbed national attention, it’s far from the first of its kind. Hydrothermal explosions are a regular part of the interaction between water, heat, and rock that create and reshape the pools, geysers, mud pots, and other features that make Yellowstone unlike any place else in the United States.
Minor hydrothermal explosions happen just about every year. Just the day before the explosion of Biscuit Basin, in fact, geologists from the Yellowstone Volcano Observatory announced that they’d discovered evidence of a small hydrothermal explosion in the park’s Norris Geyser Basin that happened on April 15, 2024. The hydrothermal pop left a crater around three feet across and dried out some nearby hot springs.
Sometimes, however, geologists have recorded larger blasts like the one at Biscuit Basin or even bigger ones like the famous explosion of the park’s Porkchop Geyser in 1989. The bottom of Yellowstone Lake contains a crater record of even more ancient hydrothermal explosions. The largest of these craters formed 13,800 years ago and spans 1.5 miles wide.
How are these explosions different from geysers?
The bangs and showers of rock from hydrothermal explosions are not a warning sign of something deeper, but a consequence of the very conditions that create the park’s geothermal features to begin with. The same plumbing system that powers the blasts also powers the park’s iconic geysers that often shoot water and steam into the air. “There is an interval between each eruption of the geyser where the water has to refill and heat up to boiling,” says Denison University geologist Erik Klemetti Gonzalez. But subtle alterations to that process can cause explosions with no way to predict when or where they might occur.
Yellowstone’s hydrothermal features—from splashy geysers to drier, steamier fumaroles—each represents varying interactions between water and rock on the surface. What’s most important in the lead-up to an explosion, however, is what’s going on underground. “At some volcanoes there is little evidence beyond fumaroles of the hydrothermal system percolating underground,” Klemetti Gonzalez says, but “they can have steam blasts as easily as geysers.”
The underground systems sometimes go off predictably, as they do at geysers like Old Faithful. Then again, Klemetti Gonzalez notes, hydrothermal areas can also build to unexpected disasters, such as a hydrothermal explosion on Japan’s Mount Ontake in 2014 that killed 63 people or one at New Zealand’s White Island in 2019 that killed 22 people.
Geologists don’t know exactly what triggers hydrothermal explosions and often can’t investigate until after the fact. Past hydrothermal explosions have been linked to earthquakes and landslides, all part of the geology that continually reshapes the park. But the recent explosion at Biscuit Basin, and others like it, are not a sign of an eruption or other volcanic activity. “The volcanic system at Yellowstone is stable and doesn’t directly influence geyser activity,” Reed says
Geologists are still eager to learn more about the specific causes of hydrothermal explosions and other features of the geyser fields within the park. “We still don’t know a lot about what geyser pumping systems look like,” Reed says, as electronic equipment usually cannot withstand the combination of moisture and heat that creates hydrothermal features.
A hydrothermal explosion like that at Biscuit Basin effectively removes the cover off part of that system, like scraping off the surface of an ant nest to see the chambers below. “These events are rare opportunities to study what’s beneath the surface,” Reed says, a hidden system of pockets and channels that create beautiful, and sometimes frightening, geology.
