The Mystery of Yellowstone’s Vanishing SO2
Yellowstone Caldera Chronicles brings together insights from scientists and collaborators at the Yellowstone Volcano Observatory. In this week’s piece, Jennifer Lewicki, a Research Geologist with the USGS California Volcano Observatory, explains a curious puzzle about gas emissions at Yellowstone.
When people imagine active volcanoes, they often picture huge plumes of gas rising into the sky. Those plumes are rich in sulfur dioxide (SO2), a gas that creates volcanic smog and can be spotted by satellites from space. Volcanoes such as Kīlauea in Hawai‘i routinely release hundreds to thousands of metric tons of SO2 every day, forming a visible haze that can affect air quality across the islands. Mount Etna in Italy, Ambrym in Vanuatu, and many other active systems also produce substantial SO2 emissions, and these gases are essential for volcanologists to monitor volcanic activity.
So why doesn’t Yellowstone show the same SO2 signals? Despite lying atop one of the planet’s largest magmatic systems, Yellowstone releases virtually no detectable SO2. This seems counterintuitive, especially since Yellowstone is one of the most prolific sources of carbon dioxide (CO2) on Earth. The park’s hydrothermal zones constantly degas, creating steam plumes and the familiar rotten-egg scent of hydrogen sulfide (H2S) at places like Norris Geyser Basin and the Mud Volcano area. With so much gas activity, where does the SO2 go?
A key clue lies in the depth of Yellowstone’s magma. Geophysical studies reveal two main magma bodies beneath the park: an upper rhyolitic chamber and a much larger, deeper basaltic reservoir. The rhyolitic chamber sits roughly 4–17 kilometers (about 2.5–10 miles) below the surface, while the deeper basaltic source extends from around 20–50 kilometers (12–30 miles) down.
This arrangement is deeper than in many erupting volcanoes. As magma ascends and pressure drops, dissolved gases begin to exsolve, or separate, from the melt. Different gases come out of solution at different depths. Carbon dioxide, which dissolves relatively poorly in magma, starts to bubble away at great depths—40 kilometers or more. Sulfur dioxide, on the other hand, typically begins to exsolve much closer to the surface, often within a few kilometers.
At Yellowstone, the shallowest magma pockets remain several kilometers below the surface. Any SO2 that does escape from the magma must travel upward through the crust before it can reach the atmosphere. Along the way, it encounters Yellowstone’s vast, water-rich hydrothermal system—the world’s most extensive network of hot springs, geysers, and steam features, spanning more than 100 thermal areas with over 10,000 individual features.
This hydrothermal system acts like a giant chemical scrubber. As magmatic gases rise through the water-saturated environment, they undergo reactions that remove certain gases before they can reach the surface. Specifically, SO2 reacts with liquid water in a process called scrubbing, ultimately converting SO2 into hydrogen sulfide (H2S), dissolved sulfate, and sometimes elemental sulfur. That’s why you can see yellow deposits and smell H2S at many Yellowstone features—the sulfur has been reworked along the way.
From a monitoring perspective, the lack of surface SO2 is actually reassuring. A sudden appearance of SO2 in the atmosphere would signal a notable change: magma would have begun rising to shallower depths and forming dry gas pathways through the hydrothermal system, indicating heightened volcanic unrest.
For now, Yellowstone Volcano Observatory tracks gases that do reach the surface, mainly CO2 and H2S. CO2 is released early and deeply and is less easily scrubbed by water, while H2S can arise from multiple processes, including the transformation of any preceding SO2. A multi-gas station at Mud Volcano continuously measures these gas concentrations, enabling scientists to detect shifts in the volcanic system’s behavior in real time.
So, the next time you visit Yellowstone and catch that distinctive sulfur odor wafting from a steaming fumarole, remember you’re witnessing a natural, highly efficient chemical factory at work. SO2 isn’t truly missing—it’s being transformed as it travels through Yellowstone’s extraordinary hydrothermal regime, turning into the H2S you detect at the surface.
