YELLOWSTONE NATIONAL PARK — The Yellowstone Volcano Observatory (YVO) has published its September video update, with Scientist-in-Charge Mike Poland describing monitoring data that occurred over the last month. Recorded observations included a large hydrothermal explosion in a newly formed pool in Biscuit Basin.
Poland starts out by describing the science behind the stinky smells that come from some iconic mudpots in Yellowstone National Park (YNP): “That rotten-egg smell is hydrogen sulfide … and it’s very common in geothermal systems like the ones we have here in Yellowstone.”
The smell originates in magma chambers deep beneath the surface, according to Poland. He compares magma to a soda, which has dissolved gases inside that are trying to get out. One of those gases is sulfur dioxide. As the compound rises away from the magma chamber, it will typically encounter water and a chemical reaction will occur, changing the sulfur dioxide to hydrogen sulfide.
“That’s one of the reasons we monitor the gas emissions at volcanoes in general,” Poland said in the video. “If we see sulfur dioxide, that might be an indication that magma is on the rise or that there’s a clear pathway between the magma and the surface with no water in the way.”
Monitoring equipment recorded 77 earthquakes in the region, with a 3.4-magnitude quake noted as the largest on Sept 7. Poland said most seismic activity took place in an east-west band from Norris Geyser Basin to Hebgen Lake. Over the course of the month, a couple dozen earthquakes occurred just north of West Yellowstone. When it came to ground deformation, Poland reported that there was not much change over the last several months. Major net changes have not been detected since February.
Hydrothermal behavior in the park over the last month included minor activity from Steamboat Geyser and one eruption from Echinus Geyser. Giant Geyser had two eruptions, on Sept. 10 and 26, which brings its total to five this year. Poland said Giant Geyser seems to be entering a phase of relatively frequent eruptions. A lot of activity in Biscuit Basin was recorded in September, including in Black Diamond Pool, which was the source of the major explosion in 2024. That pool had several eruptions, including one that sent water and mud into the air on Sept. 23.
“The small pool that formed via collapse after the June 2026 explosion also had several eruptions,” Poland said. “And then on the night of September 27, just after 8:12 p.m. Mountain Time, the pool was the source of a small hydrothermal explosion that threw boiling water, steam, mud, rocks at least 100 feet or so into the air. And that is the first such significant explosion from that feature. Fortunately, that event was very well recorded by seismic, acoustic and camera monitoring stations.”
