Digging into the history of hydrothermal explosions at Yellowstone National Park

This is an aerial view of Black Opal Pool, Black Diamond Pool, and part of Wall Pool, all in Biscuit Basin, Yellowstone National Park, taken in 1959 sometime after the Hebgen Lake earthquake (which occurred on Aug. 17 of that year). The division between Black Diamond Pool and Wall Pool was less prominent at this time, which might be why park geologist George Marler used a single name to encompass both features. Note the many boulders strewn about in the bottom half of the image. (Courtesy of the Yellowstone National Park archives)

Scientists are hard at work investigating the hydrothermal explosion on July 23, 2024, at Black Diamond Pool in Biscuit Basin . An important part of understanding the recent activity involves investigating past similar events. And there have been several at Biscuit Basin.

At the turn of the 20th century, Biscuit Basin looked a little different than it does today. Sapphire Pool still had its biscuit-like geyserite formations made famous by the photography of F. Jay Haynes , but Black Diamond Pool apparently didn’t exist. No early maps of the area, including those made by the U.S. Geological Survey, show any indication of thermal features between Sapphire Pool and the Firehole River.

Although few details are known about the events that created Black Diamond Pool and its neighbors Wall Pool and Black Opal Pool, there is evidence to indicate that they were explosive.

The earliest documentation is a photograph taken by Haynes’ son Jack around 1912 showing what we now call Black Diamond Pool after an apparent hydrothermal explosion blew open its vent and threw large rocks out of its crater. The exact date of this first explosive episode is unknown, but the photograph and map dates suggest it happened between 1902 and 1912.

Documents preserved in the Yellowstone Heritage and Research Center indicate that more explosions occurred in 1918 and 1925, expanding the crater of Black Diamond Pool and forming Wall Pool. Additional explosions in 1934 and 1953 created and then enlarged the crater of Black Opal Pool.

Unfortunately, none of these explosions were witnessed, and records only describe the aftermath: new pool shapes, heavy wash, and freshly ejected boulders. No eruptive or explosive activity was reported at any of these pools for many decades after 1953.

Piecing together a history is often complicated by the many names applied to new features before one finally becomes the standard. Several early observers used the name “Black Pearl” for both Wall Pool and Black Opal Pool. This name actually belonged to a distinctive feature north of Jewel Geyser, also in Biscuit Basin, confusing matters even more.

By 1959, park geologist George Marler had applied the name “Black Opal Pool” to the easternmost pool due to its “dark opalescent coloration,” and he used the name “Wall Pool” to collectively refer to the two other pools due to the prominent escarpment along their edges. The name “Black Diamond” was used by park ranger Frank Childs in the 1930s but didn’t gain common usage as the name of the larger of the two Wall Pool features until the mid-2000s.

In early 2006, there were signs of increased heat flow at Biscuit Basin, with hot ground and new vents appearing between the three pools and the river. This turned out to be a prelude to the emergence of Black Diamond as an explosive feature in July 2006. The first eruption was witnessed by geyser enthusiast Kendall Madsen, who snapped a photo showing dark-colored water thrown to a height of 40–50 feet. Witnesses close to the pool heard loud thumps along with the explosive bursts.

For the next two months, Black Diamond erupted at irregular intervals of 2–5 days and occasionally as short as 5 hours. After mid-September 2006, events became sporadic. Except for a cluster of eruptions in late 2011, intervals of weeks or months between eruptions were typical. This phase seemed to end entirely by late 2012, and just three eruptions — one each in 2013, 2015, and 2016 — were reported before 2024.

Over the years, a number of these eruptions were seen by visitors, geyser enthusiasts, tour guides and park rangers. Most reached heights of 6–20 feet, lasted 10–20 seconds, and were accompanied by thumps and pops. Eruptions frequently threw small rock fragments and globs of mud.

In 2009, a group of geologists on a field trip witnessed an eruption matching the intensity of the first 2006 event, reaching 50 feet high and tossing rocks in all directions. Geyser enthusiast Bill Warnock saw and photographed an eruption he estimated to be 50 feet high in 2011 that expelled a huge amount of water toward the river. A few unverified reports from 2006 and 2011 suggest even larger eruptions of 60–100 feet. Finally, an eruption in April 2015 launched mud onto the boardwalk and killed a group of cormorants that had ventured too close at the wrong time.

With the possible exception of the unwitnessed hydrothermal explosions that created it, Black Diamond Pool’s most recent activity on July 23, 2024, was by far its largest. Its history points to scientific questions that will help guide the investigation into the 2024 explosion. For example, why was Black Diamond dormant for decades before reactivating as a violent geyser, and why do hydrothermal explosions reoccur in some places but not others?

Studying such events, past and present, will help to provide new insights into Yellowstone’s most common and underappreciated hazard .

This article was originally published by the Yellowstone Caldera Chronicles, a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory.