Editor's note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

Hydrothermal explosions, like that which occurred on July 23, 2024, in Biscuit Basin, are not just a hazard that occurs in Yellowstone National Park.

The July 23, 2024, hydrothermal explosion at Biscuit Basin was a spectacular and hazardous event, throwing water, rock, and mud for more than 330 feet (100 meters) around its source: Black Diamond Pool. The pool is known to be the location of numerous previous hydrothermal explosions, and it probably formed as a result of an explosion between 1902 and 1912.

Although seemingly rare because such an event had never been so well recorded in Yellowstone, hydrothermal explosions are relatively common. A few small events, leaving craters at least one to a few yards (meters) across, probably happen multiple times each year in some part of the park but are usually unwitnessed because they occur in backcountry areas, at night, or during the winter. Events the size of the July 23, 2024, Biscuit Basin explosion probably occur every decade to a few decades in Yellowstone.

Hydrothermal explosions, which are caused by pressure changes that result from the transition of liquid water to steam, are not restricted to the Yellowstone region. They also occur in hot-water systems located elsewhere in the USA and around the world.

One of the most spectacular hydrothermal explosions in the past century took place from an inconspicuous group of hot springs in Surprise Valley, near Lake City in the northeast corner of California. There is no recent volcanism in the immediate vicinity; the hot springs and mud volcanoes are probably associated with a large fault that is part of the tectonic extension occurring in the western USA. Sometime during the night of March 1–2, 1951, an explosion sent debris several thousand feet (about a thousand meters) into the air and caused fine particulates to fall 4 miles (over 6 kilometers) away. The explosion disturbed about 20 acres (0.03 square miles or 0.08 square kilometers) of the hot spring area and involved 300,000 tons of mud. There were no injuries thanks to the fact that the event occurred at night and few people lived in the region.

When occurring in a populated area, even small hydrothermal explosions can be deadly. Such an explosion occurred in October 1990 at the Agua Shuca thermal area, within a small community in El Salvador. The explosion caused about 25 deaths and many injuries, and it formed a crater that was 40 meters (131 feet) across. The same feature had also exploded in 1868 and probably many times previously based on geological evidence. That region of El Salvador is known for strong geothermal activity and occasional explosions.

Steam rises above a hot pool at Agua Shuca, one of many thermal areas of the Ahuachapán geothermal field of El Salvador. A sudden hydrothermal explosion at Agua Shuca in October 1990 ejected steam and debris within a 200-m-radius, and about 25 people living adjacent to the thermal area were killed. The Agua Shuca thermal area east-southeast of Laguna Verde volcano had historically been one of the most frequently visited areas at Ahuachapán. A previous explosion apparently took place in 1868. Photo by Pat Dobson, 1999 (Lawrence Berkeley National Laboratory). This image is made available under the Creative Commons BY-NC-ND 4.0 license terms.

Perhaps no place on Earth experiences hydrothermal explosions as frequently as the North Island of New Zealand, where events of some size occur nearly every year. Some of these explosions are noteworthy. For example, an explosion in a remote part of the North Island on April 19, 2005, resulted in a crater that was 164 feet (50 meters) across. The column of mud and debris reached 650 feet (200 meters) into the air and was visible from 6 miles (10 kilometers) away. In another instance, a hydrothermal explosion on the night of February 22, 1973 at the margin of Frying Pan Lake—one of the largest hot springs in the world—scalded or killed nearby vegetation and deposited mud up to a thickness of 1.5 feet (0.45 meters). Despite its size, this explosion was not heard by people sleeping only 850 meters (0.5 miles) away.

Frying Pan Lake (also called Waimangu Cauldron), in New Zealand, sits within Echo Crater, which formed during the 1886 eruption of Tarawera. A hydrothermal explosion from the crater in 1917 resulted in the formation of the hot spring "lake" by 1918. On the night of February 22, 1973, a hydrothermal explosion from the lake shore destroyed a set of silica terraces. USGS Photo by Mike Poland, February 7, 2023.

Hydrothermal explosions also occur in regions that are used as sources for geothermal power generation, including in New Zealand, the Philippines, and Central America. In such areas, changes in subsurface pressure can be caused by both extraction and injection of water, as well as the abandonment of wells.

Precursors to hydrothermal explosions are not well documented. Occasionally there are reports of changes in geyser and hot spring activity months to years before an explosion. For example, new springs formed on the margin of Frying Pan Lake in New Zealand months before the 1973 explosion there. Years of changes in water chemistry at Porkchop Geyser in Norris Geyser Basin preceded an explosion there in 1989. The April 15, 2024 explosion on Porcelain Terrace, also in Norris Geyser Basin, was preceded by about two years of more intense hot spring and geyser activity in that area. And at a few geothermal sites in Japan, localized ground deformation has occurred in areas of subsequent small explosions. Unfortunately, changes are common in hydrothermal regions and are usually recognized as precursors to explosions only in hindsight.

Frying Pan Lake (also called Waimangu Cauldron), in New Zealand, sits within Echo Crater, which formed during the 1886 eruption of Tarawera. A hydrothermal explosion from the crater in 1917 resulted in the formation of the hot spring "lake" by 1918. On the night of February 22, 1973, a hydrothermal explosion from the lake shore destroyed a set of silica terraces. USGS Photo by Mike Poland, February 7, 2023.

A common trait of these explosions is that they are not triggered directly by magma or magmatic gases interacting with shallow groundwater. Rather, when water boils, and the liquid-steam mixture is within a confined space that becomes sealed, pressure due to the expansion of steam bubbles eventually overcomes the strength of the rock, and an explosion occurs.

In natural thermal areas worldwide, the hazards due to hydrothermal explosions tend to be underappreciated, despite the loss of life that has occurred in some events. This hazard is an important focus for volcanologists, as indicated by the emphasis on hydrothermal explosions in monitoring and research by GNS Science in New Zealand, as well as in the 2022–2032 monitoring plan of the Yellowstone Volcano Observatory. Additional monitoring instruments, like those recently installed in Norris Geyser Basin in Yellowstone National Park, may help to better detect and potentially warn of such activity.