CO₂ shakes up theory of how geysers spout

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Monitoring the innards of Yellowstone’s gurgling geysers, scientists report in two new studies  that carbonation helps the geysers erupt like shaken cans of soda.

During the buildup to an eruption of Yellowstone’s Spouter Geyser, carbon dioxide accumulates in the geyser water, researchers report online March 7 in Geology. The dissolved gas lowers the water’s boiling point and triggers an eruption. This phenomenon may occur elsewhere in Yellowstone. Several of the park’s other geysers, including Old Faithful, also contain abundant CO₂ and other dissolved gases, a separate research team reports in the March Geology.

The findings overturn the 150-year-old explanation that hot water alone fuels geyser eruptions, says Jacob Lowenstern, a volcanologist at the U.S. Geological Survey in Menlo Park, Calif., who was not involved in either study. “People always assumed that water was the end of the story,” he says. “If CO₂ was completely absent, many of these geysers would still erupt. But they erupt more regularly and frequently because of the dissolved CO₂ gas.”

Geysers can spew thousands of liters of water tens of meters into the air. For geysers in Yellowstone National Park in the western United States, the heat from underground magma fuels these spritzers (SN: 5/16/15, p. 16). Measurements in the 1970s, however, revealed that many Yellowstone geysers, including Old Faithful, aren’t hot enough to boil pure water.

Uncovering what’s going on inside Yellowstone’s geysers is challenging. The geyser water is scorching hot and often acidic. Hydrogeologists Bethany Ladd and Cathryn Ryan of the University of Calgary in Canada, authors of the March 7 study, lost equipment to these harsh conditions while attempting to monitor several of Yellowstone’s geysers. At the nonacidic Spouter Geyser, the researchers had better luck. Using special glass jars, the researchers sampled the geyser’s water every 10 to 20 minutes from a side vent that branches off the geyser’s main channel. The measurements allowed the researchers to track the amount of dissolved CO₂ in the geyser over the course of several eruptions.

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The abundance of CO₂ in the geyser starts low, the researchers found. During the one- to two-hour interval between eruptions, however, CO₂ levels steadily increase as gases from Yellowstone’s magma enter the geyser water through permeable rocks. As CO₂ increases, the gas lowers the water’s boiling point. Eventually the boiling point drops below the water’s actual temperature and bubbles of steam and CO₂ form. As these bubbles climb the geyser column toward the surface, they expand, displacing water and lowering the pressure inside the geyser. That pressure drop lowers the boiling point even further, causing a runaway reaction that triggers a full-blown eruption similar to that of a shaken soda can. These eruptions can last for hours. When the eruption finally fizzles, CO₂ levels have dropped to about half their peak value just before eruption and the cycle begins anew.

“People typically think of geysers as hot-water features that only emit water and steam,” Ladd says. “But there are other things in the water such as CO₂ that have huge implications for geyser eruptions.”

In the other study, Shaul Hurwitz, a hydrogeologist at the U.S. Geological Survey in Menlo Park, and colleagues discovered CO₂ and other dissolved gases such as nitrogen in many of Yellowstone’s other geysers. Understanding the role gas plays in steam eruptions is also important, Hurwitz says, because geysers aren’t the only way water erupts. The rapid boiling of an enclosed underground water reservoir can generate an explosive blast. In September 2014, a steam eruption rocked Japan’s Mount Ontake volcano without warning and killed 57 people. If CO₂ helps fuel steam eruptions, then monitoring gas levels in groundwater could provide early warning, Hurwitz says.