Gassy precursors for geyser eruptions?

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from John Ayers, Professor of Earth and Environmental Sciences at Vanderbilt University.

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Is there a way to predict when a geyser is about to erupt? Most geysers are not as predictable as Old Faithful. Take Steamboat Geyser, the highest erupting geyser in the world, as an example. During the last ten years Steamboat has gone from as few as three days to as much as three years between eruptions. Steamboat Geyser's erratic behavior can be frustrating for park visitors, who can spend days waiting for an eruption that does not happen. So, in the summer of 2019 we set out to look for eruption precursors, which are changes that occur consistently before an eruption (Permit # YELL-2019-SCI-8060). Precursors could be used to alert park rangers and visitors of an impending eruption. Our search for precursors focused on gas emissions at Steamboat Geyser. We made repeated measurements over time to look for sudden changes in gas emissions preceding a geyser eruption.

The gases we chose to study are carbon dioxide (CO₂) and methane (_CH4). In Yellowstone, carbon dioxide is a very common gas because it is released from magma deep beneath the caldera. These gases are also important because they are both greenhouse gases. To measure gas emissions, we sealed a chamber to the ground surface and then continuously measured the concentrations of these two gases within the chamber. The faster the concentrations in the chamber increased, the higher the level of gas emissions. We also continuously measured the carbon isotope compositions of these gases, expressed as the average ^13>C/¹²C isotope ratio of the carbon dioxide and methane molecules. This ratio should change if the gas source changes. For example, the proportion of gases derived from the magma may change just before an eruption of the geyser. We made our measurements at a safe distance from the geyser, about 150 feet away and next to the boardwalk.

We were extremely fortunate because Steamboat Geyser erupted just 90 minutes after we began making measurements on June 12, 2019. It impressive! Steam shot up over 300 feet into the air, and rock fragments went flying. The sound was as loud as standing next to a jet engine, testifying to the awesome power of the eruption. The eruption continued for at least 45 minutes, and we continued to make measurements until 210 minutes after the eruption started. Later, when we analyzed our data, we found that all four of our measured variables (the emission rates and carbon isotope compositions of CO₂ and CH₄) changed roughly 20 minutes before the eruption.

Our measurements do not explain why carbon dioxide and methane emissions change just before an eruption. Eruptions usually happen when water in the subsurface boils, and pressure builds up until the steam is ejected through a narrow conduit. Gases in the hydrothermal plumbing system beneath the geyser may be drawn toward the geyser conduit, decreasing their pressures in the subsurface surrounding the geyser and therefore changing emissions at the surface.

Because we made measurements over only one eruption cycle, it would be premature to say that gas emissions are a useful tool for geyser eruption prediction. But our initial results are promising. We hope to collect future measurements to see if our conclusions hold up. Stay tuned!

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