Michael Poland’s photograph of Black Sand Pool shows blue water, rising steam, and dry grass near Old Faithful. What the camera leaves out is the thud.
Every few minutes, something inside the spring delivers a knock that travels through the water and surrounding ground. The sound comes from steam bubbles collapsing within the pool, sometimes with enough force to make the surface jump.
Black Sand Pool is one of several places where visitors can encounter this phenomenon in Yellowstone National Park, according to a new Yellowstone Caldera Chronicles column by Poland, a U.S. Geological Survey geophysicist and scientist-in-charge of the Yellowstone Volcano Observatory. Researchers studying Doublet Pool, another of Yellowstone’s thumping springs, found that changes in the timing can reflect how heat enters and leaves the pool.
Why Yellowstone’s Hot Springs Thump
Hot water and steam move upward through Yellowstone’s hydrothermal plumbing. As a steam bubble rises, it expands and the pressure inside it falls. Near the surface, cooler water causes some of the vapor to condense back into liquid. With less steam inside, the bubble abruptly collapses.
That collapse produces a pressure pulse that moves through the spring. If it is powerful enough, people nearby may hear the resulting knock or feel the ground move.
Explore All OptionsOnly a small number of Yellowstone’s hot springs produce these thumps. Specific temperature and pressure conditions must develop, and the bubbles must become large enough.
Among those springs are Black Sand Pool and Doublet Pool in the Upper Geyser Basin. Visitors can hear and feel the thumping at both, while Doublet Pool’s water may even appear to jump.
Firehole Spring in the Lower Geyser Basin offers a view of the cause. Through its clear water, visitors can watch bubbles rise from a vent and disappear before reaching the surface.
Read More: Melting Ice Didn't Trigger Yellowstone Eruptions — but It May Have Sparked Hydrothermal Explosions
Researchers Listen to Doublet Pool
To determine why Doublet Pool’s rhythm changes, a team placed seismic instruments around the spring during seven field deployments between 2015 and 2021. Researchers later lowered temperature and water-level sensors into the pool.
Their study, published in Geophysical Research Letters, showed that the pauses between thumping episodes changed over hours, days, and years.
The quiet period lasted about 30 minutes during measurements in November 2016. It dropped to approximately 13 minutes in September 2018, then returned to around 20 minutes by November 2021.
The timing also shifted with the wind, as gusts carried heat away from the surface, delaying the next round of thumping. When more heat remained in the system, the next episode arrived sooner.
A Pool That Keeps Time With Yellowstone’s Heat
When Ear Spring erupted for the first time since 1957 in September 2018, Doublet Pool, about 200 feet (60 meters) away, had its shortest measured pause and boiled afterward.
Researchers think increased heat beneath Geyser Hill may explain the quicker rhythm. Because the timing responds to heat, the pool could help scientists track wider changes in the area’s hydrothermal system. A shorter interval would not reveal the cause on its own, but it could prompt researchers to check nearby springs and geysers for related changes.
Each thump gives scientists a clue about changing heat below, while nearby visitors hear the knock and feel it travel through the ground.
Read More: Yellowstone Cleanup Crews Tackle Litter That Causes Hot Springs to Clog and Change Colors
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from the USGS: Yellowstone's Thumping Hot Springs
- This article references information from Geophysical Research Letters: Thumping Cycle Variations of Doublet Pool in Yellowstone National Park, USA
