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, and Shaul Hurwitz, research hydrologist with the U.S. Geological Survey.

How often do hydrothermal explosions occur in Yellowstone National Park? It all depends on the size of the event.

The July 23, 2024, hydrothermal explosion at Biscuit Basin in Yellowstone National Park was dramatic and extremely hazardous. It was the first such explosion in the park to be captured clearly on video, and it highlighted what has often been underappreciated as a source of hazard.

A hydrothermal explosion is triggered when liquid water flashes to steam in an underground cavity. When the cavity (or reservoir) is connected to the surface, a geyser can form to release that pressure. When the reservoir becomes sealed and confined, mainly due to deposition of silica, the liquid and steam mixture increases pressure in the confined space, similar to a pressure cooker. If the pressure in the reservoir increases to the point that it exceeds the strength of the surrounding rocks, the rocks break and an explosion occurs. The force of the explosion creates a crater (or enlarges an existing one) and hurls rocks that can cause damage and injury.

“Dirty” geyser eruptions that throw out rocks and mud, like the one that was photographed at Wall Pool in Biscuit Basin in 2009, are considered to be on the small end of the explosion spectrum. The 2018 eruption of Ear Spring in the Upper Geyser Basin, although small, also hurled rocks a short distance but did not enlarge the existing pool, so whether or not it should be classified as a hydrothermal explosion is debatable. Regardless, both of those events were caused by a pressure perturbation in the shallow hydrothermal system.

Small hydrothermal explosion, or a "dirty eruption," in 2009 from Wall Pool at Biscuit Basin in Yellowstone National Park. These types of events are the most common geologic hazard to occur in Yellowstone National Park.

At the largest end of the spectrum of hydrothermal explosions are craters that formed thousands of years ago, many of which can be found in and around Yellowstone Lake. These craters are hundreds of meters (thousands of feet) across and include Indian Pond, Turbid Lake, Duck Pond, Elliott’s Crater, and Pocket Basin, among others. Mary Bay is the largest of these craters, and at 1.5 miles (2.5 kilometers) across is the largest hydrothermal explosion crater in the world.

Hydrothermal explosions in Yellowstone are defined by their size, which can be approximated by the dimensions of their craters. Determining crater size, however, can be challenging because some are submerged, some have been eroded, and some developed sequentially over time. To know how often explosions of different sizes occur, crater ages need to be determined. Although some explosions, like that of July 23, 2024, were witnessed, most were not, and ages may not be well known.

Despite the uncertainties with dating many of the explosion craters and determining their sizes, some patterns can still be extracted from the data. Small hydrothermal explosions that create craters only a meter (a few feet) across or less probably happen annually to a few times per year. The occurrence of the largest craters is not random in time, as they may be triggered by external events, like major earthquakes or landslides that cause pressure changes in the shallow hydrothermal system. All of the largest craters in Yellowstone formed several thousand years ago, in many cases not long after the last glaciation, which ended about 14,000 years ago. Any explosions that might have occurred before that time were erased from the geologic record by all that ice.

Map showing different types of thermal areas in Yellowstone National Park. Also shown are the rim of Yellowstone Caldera, resurgent domes, and large hydrothermal explosion craters.

Information gathered on crater size and age allows geologists to develop a model for the frequency of small and large explosions and the energy they release. The model underestimates the frequency of the smallest events, for which data are lacking, and the uncertain ages of prehistoric events introduce further uncertainty. But for single significant events like that of July 23, 2024, in Biscuit Basin, the model provides a reasonable way to estimate how often such explosions might occur.

An important unknown regarding the July 23, 2024, explosion is the size of the crater, which is filled with water and rapidly enlarged after the explosion as the side walls slid inward. Considering a range of reasonable crater sizes suggests that explosions of that magnitude might occur once every decade to few decades on average.

Larger events, forming craters an acre in size, might occur once every several hundred years, on average, and the largest events, greater than 10 acres (more than 40,000 square meters, or 435,000 square feet!) in size, might occur once every several thousand years. Hydrothermal explosions with sizes like the one that occurred at Porkchop Geyser in 1989 probably occur every several years. Smaller events, like the April 15, 2024, explosion at Porcelain Terrace in Norris Geyser Basin, may happen every year in Yellowstone. The probabilities of someone being injured by a hydrothermal explosion are lower than the probabilities of their occurrence, given that explosions can occur during the night or in winter months, when few people are present, or in backcountry areas that are rarely visited.

Plot of size versus annual probability for hydrothermal explosion craters in Yellowstone National Park. The line is a model based on the energy required to form a crater of a specific size, and it is fit to known hydrothermal explosion craters in Yellowstone National Park. Red boxes are example hydrothermal explosion craters in Yellowstone that were observed or were identified from geological studies, with plausible uncertainty in crater sizes given by the error bars. The yellow oval marks the July 23, 2024, hydrothermal explosion at Black Diamond Pool in Biscuit Basin. Figure modified from Christiansen and others (2007).

Additional data on the timing and locations of small hydrothermal explosions could be aided by monitoring data, like that provided by the station installed in Norris Geyser Basin in September 2023. Using these data together with knowledge of past explosions and models that quantify the amount of energy they release can provide an important means of better understanding how often explosions occur, the danger they pose to visitors, and whether there may be precursory signals that can be used to provide warning of these hazardous events.