This boulder is the largest confirmed to be part of the July 23, 2024 hydrothermal explosion at Black Diamond Pool, Biscuit Basin, Yellowstone National Park. The measuring tape is 50 centimeters (20 inches) long. Black Diamond Pool and the boardwalk are visible in the background. Smaller rocks that were part of the explosion are in the foreground. (Photo by Lauren Harrison, Colorado State University)

Shortly before 10 a.m. July 23, an explosion of hot water, mud and rocks occurred at the Black Diamond Pool in Biscuit Basin, Yellowstone National Park, just about 2 miles northwest of Old Faithful. Monitoring equipment did not detect any precursors to the event. Dramatic videos posted on social media showed a plume of water and rock fragments rising about 600 feet (180 meters) into the air as people ran for safety. The explosion severely damaged a nearby boardwalk, and the pool remains closed while geologists assess the activity. Fortunately, no one was injured.

Geologists studying the deposit noted that the rocks thrown up by the explosion were composed of glacial materials, sandstones, siltstones, and gravels that directly underlie the silica sinter that forms the surface shell. None of the rhyolitic basement rocks, which lie about 175 feet (50 meters) below the surface (based on drilling from the 1960s), were found. This indicates that the explosion was generated at a much shallower depth so as not to disturb the bedrock. This is not surprising, since hydrothermal vents occur primarily at shallow levels below the surface in Yellowstone.

The explosion was largely directed northeast toward the Firehole River (away from the boardwalk), and the largest rocks—some several feet in diameter and weighing hundreds of pounds!—fell in that direction. This fortunate directionality was probably the reason no one standing on the boardwalk at the time of the incident was injured.

Hydrothermal explosions occur when liquid water boils into steam in the shallow subsurface. This type of transformation occurs all the time in established geyser systems, such as Old Faithful or Steamboat Geyser, where well-defined conduit systems allow steam and hot liquid water to move freely to the surface, resulting in a geyser explosion. However, when the liquid and steam mixture is in a confined space that has been sealed and without a well-defined conduit system, the pressure caused by the expansion of steam bubbles eventually overcomes the strength of the rock and an explosion occurs.

In the case of the Black Diamond Pool, the July 23 explosion was likely caused by a change in a hot water reservoir in the shallow subsurface. Silica deposits can plug lines or “pipes” in the reservoir, leading to a buildup of steam and pressure. Data that geologists collect from the debris from the explosion will provide even more detail about the exact conditions at the time of the event.

Hydrothermal explosions are more common than you might think in Yellowstone National Park. They are the most common but least destructive hazard, compared to strong earthquakes, lava flows and domes, and caldera-forming eruptions. On average, several hydrothermal explosions of varying sizes occur in Yellowstone National Park each year, often in remote areas where they can go unnoticed.

Some of the largest hydrothermal explosions in the last 150 years at Yellowstone National Park occurred in the 1880s at Excelsior Geyser, which is adjacent to the Grand Prismatic Spring in the Midway Geyser Basin. The September 1989 explosion of Porkchop Geyser at Norris Geyser Basin may be one of the most famous hydrothermal explosions in the known area. It was visible to several visitors but caused no injuries. Another small explosion occurred at Norris Geyser Basin recently, on April 15, 2024. This event was detected by nearby monitoring instruments that were specifically designed for this purpose. Without the data from these instruments, this unnoticed event may never have been recognized. In the Yellowstone Monitoring Plan, which was released in 2022, the Yellowstone Volcano Observatory specifically identified expanded hydrothermal monitoring as a goal to address the threat posed by these small but dangerous hydrothermal eruptions. The first step in achieving this goal is to install monitoring equipment in the Norris Geyser Basin in September 2023.

Much smaller events are even more common—for example, the rare 2018 eruption at Ear Spring near Old Faithful Geyser, which brought decades of human debris to the surface. But larger events can happen, too. Since the last ice age ended in the Yellowstone region about 14,000 years ago, more than a dozen hydrothermal explosions have left craters hundreds of feet across. The largest such crater—in fact, the largest hydrothermal explosion crater known on Earth!—is about 1.5 miles (2.5 kilometers) across, formed by an event about 13,000 years ago at Mary Bay, along the northern end of Yellowstone Lake.

It’s too early to tell what might happen next at Biscuit Basin. The explosion clearly altered the shallow hydrothermal flow paths in the area, and it’s unclear how the thermal features will respond. Did we witness the birth of a new geyser? Will activity return to its previous, calmer state, and will the quiet hot spring be reborn? Data collected by geologists will help us better understand the July 23 event and how the area might evolve in the future.

THANKS:Many YVO scientists contributed to the preparation of this article, especially Shaul Hurwitz and Mark Stelten (USGS), as well as Lauren Harrison (Colorado State University), Mary Reed (UC Berkeley), and Jeff Hungerford, Kiernan Folz-Donahue, and Elle Blom (Yellowstone National Park), who led the field work at the July 23 explosion site.

Yellowstone Caldera Chronicles is a weekly column written by scientists and contributors to the Yellowstone Volcano Observatory. This week’s article comes from Michael Poland, a geophysicist with the U.S. Geological Survey and executive scientist at the Yellowstone Volcano Observatory.