A hydrothermal explosion tore apart part of a boardwalk at Yellowstone National Park’s Biscuit Basin on July 23, 2024, throwing steam and debris hundreds of feet into the air near Old Faithful. No one was hurt, but the blast demonstrated that Yellowstone’s most frequent ground-rupturing hazard has nothing to do with the supervolcano most visitors associate with the park. The U.S. Geological Survey says small versions of that same hazard, hydrothermal explosions driven by boiling water rather than rising magma, happen somewhere in Yellowstone almost every year.
Steam, Not Magma, Drives the Blast
Hydrothermal explosions start when liquid water trapped underground suddenly flashes into steam. Michael Poland, scientist-in-charge of the Yellowstone Volcano Observatory, and research hydrologist Shaul Hurwitz have explained that mineral deposits can clog the narrow conduits that normally let geysers like Old Faithful vent steam in a controlled way. Pressure builds behind that blockage until expanding steam overwhelms the surrounding rock, and the ground gives way all at once rather than venting gradually.
That mechanism is entirely separate from the magmatic system that produces Yellowstone’s supervolcano eruptions and periodic ground uplift. A hydrothermal explosion draws its energy from shallow groundwater and heat already stored near the surface, not from fresh magma rising toward it, and the Yellowstone Volcano Observatory has found no link between the two kinds of events. The July 2024 blast at Biscuit Basin’s Black Diamond Pool illustrated the distinction directly: instrument networks that track Yellowstone’s volcanic system recorded nothing unusual beforehand. “No precursors to the event were detected by monitoring instruments,” Poland said of the explosion. A separate analysis of the blast, published by the science outlet Eos, noted that the absence of a nearby earthquake before or during the explosion confirmed it was pressure-driven rather than seismically triggered.
Small Blasts Happen Almost Every Year
USGS frequency estimates make clear that explosions of some size are a near-constant feature of the park rather than a rare event. Meter-scale craters, the smallest category, probably form somewhere in Yellowstone annually to a few times a year. Explosions on the scale of the 1989 Porkchop Geyser blast recur roughly every several years, while events the size of the Biscuit Basin explosion, sizable enough to destroy infrastructure, are estimated to happen once every decade or two.
The largest explosions in the geologic record dwarf anything documented in recent history. Craters roughly an acre in size form on average once every several hundred years, and the very largest, which can exceed 10 acres, appear to occur only once every several thousand years. Since the last glaciation roughly 14,000 years ago, at least 18 massive hydrothermal explosions have left craters between 300 and 2,500 meters across scattered through the park, according to the Eos analysis. Even at that scale, scientists attribute the mechanism to the same boiling-groundwater process, just acting on a much larger and longer-pressurized reservoir than the one beneath Biscuit Basin.
Most explosions on the smaller end of that scale go entirely unnoticed. A similar, more modest blast at Norris Geyser Basin’s Porcelain Terrace in April 2024 became the first hydrothermal explosion at Yellowstone ever captured by the park’s own seismic and acoustic monitoring network, according to the same analysis, underscoring how much of this hazard’s history predates any instrumental record at all.
Why Biscuit Basin’s Explosion Barely Hurt Anyone
The Biscuit Basin explosion happened at 10:19 a.m. local time, when visitors were nearby, according to an account from Yellowstone National Park. Rocks the size of grapefruit, and rock slabs roughly three feet across weighing hundreds of pounds, landed tens to hundreds of feet from Black Diamond Pool, and the blast destroyed the boardwalk crossing the area. Despite that, no injuries were reported, and park officials closed Biscuit Basin, including its parking area and boardwalks, before reopening any of it that season.
USGS researchers attribute that pattern, frequent explosions but rare injuries, to simple timing. Many hydrothermal explosions occur at night, during winter, or in remote backcountry sections of the park that draw few if any visitors. Yellowstone’s most heavily trafficked boardwalks sit above ground that is capable of this kind of rupture, but the odds of a person standing on the exact spot at the exact moment remain low even though the underlying hazard is common.
Monitoring at Biscuit Basin has continued well past the initial blast. Seismometers and acoustic sensors installed around Black Diamond Pool detected several dozen smaller eruptions in the months that followed, including two events, on November 5, 2024, and January 3, 2025, that threw water, mud, and rock 20 to 30 feet into the air. The observatory installed a pan-tilt-zoom webcam overlooking the pool on May 14, 2025, intended to run at least through that summer while geologists track whether a new geyser is forming.
Biscuit Basin itself remains closed to visitors while the park assesses the damage, one small, unresolved piece of a hazard the USGS expects to reassert itself again somewhere in Yellowstone within a few years, long before the park’s magma system does anything comparable.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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