The current perception of geysers—that their explosions are driven by hot water and steam—may not be exactly right. In a new paper published online on 7 March in Geology, researchers found that higher carbon dioxide (CO2) concentrations in geyser water preceded eruptions—leading them to suggest that CO2 is an important ingredient in triggering eruptions.
“The common perception about geysers is that it’s just water and steam—and that it’s just a process of boiling,” said Bethany Ladd, a research assistant at the University of Calgary in Alberta, Canada, and lead author on the paper. But “there are other things in the water, and they may have huge implications for the geyser.”
Blowing off More Than Steam
It’s rare to find a geyser on Earth—about 1000 of them exist, and most of those are in Yellowstone National Park in the western United States. Geysers erupt when the hot water deep below rises toward the Earth’s surface and suddenly chaotically boils.Deep underground, the high pressures keep the water locked into a calmer state, but as the water rises, pressure is relieved, and water vapor forms in a burst of activity called ebullition, sending jets of water shooting into the air for the enjoyment of spectators. Many geysers, like Old Faithful, are so predictable that they can be scheduled into a tourist’s day.
Past researchers have suggested that CO2 might be involved in these eruptions, but until now, no one has proven this empirically. To investigate this, Ladd and her team studied Spouter Geyser, located in the southwestern portion of the park. The video below shows the geyser during an active period.
Over 3 days, the researchers took 120 samples of discharge water from the geyser at different times of the day, then analyzed the samples in the lab. Of course, the discharge water can’t tell the researchers everything about CO2 concentrations in the geyser’s subsurface, which is difficult to observe directly. They had to extrapolate these data by figuring out its temperature when it was deep below.
That extrapolation involved looking at other clues in the water, such as its ion concentrations. Dissolved minerals raise the water’s boiling point, so knowing what’s in the water can help researchers estimate the temperature of the source water feeding the geyser. From there, the researchers use a complicated set of calculations to figure out how much CO2 was in the water prior to the eruption.
The researchers found that “CO2 concentrations increased and are highest immediately prior to eruption, and then they decrease quite drastically during the eruption,” Ladd said. This means that the CO2 degassed from the water as the geyser erupted.
The researchers suggest that dissolved CO2 gives the water a pressure boost that triggers the eruption. Below the geyser lies extremely hot water, but pressure from above—from any overlying water trapped in the enclosed space and from the atmosphere itself—prevents bubbles of water vapor from forming in the water and escaping. When CO2 from the massive magma pool below Yellowstone dissolves into this water, the combined pressure from the added CO2 gas and water molecules reaches a “bubbling pressure,” as Ladd calls it.
“Water vapor pressure does not reach bubbling pressure alone,” Ladd continued. “You need the contribution of CO2 dissolved gas pressure” to reach or exceed the bubbling pressure.
When bubbles form in the water, they “induce more bubbles, and then those bubbles induce more bubbles, and then you get this big event”—an eruption—Ladd said.
Hot Water Explosions
In a complementary paper published in Geology in February, Shaul Hurwitz, a research hydrologist at the U.S. Geological Survey in Menlo Park, Calif., looked at several Yellowstone geysers and found similar results.
These findings might help scientists better understand hydrothermal explosions, Ladd said. Such eruptions occur when a shallow reservoir of hot water suddenly flashes to steam and explodes, sending rocks and debris flying.
In September 2014, Japan’s Mount Ontake volcano erupted in a hydrothermal explosion, expelling a massive flow of ash and heat and killing 57 people. Earlier this year, a different team of researchers found that volcanic gases, including CO2, had been flowing into the mountain’s underground system for the last 10 years.
In Hurwitz’s study, the researchers suggest that this gas buildup could have triggered the eruption by ramping up pressure in subterranean water. They also suggest that seismic activity could help bring this gas into the subsurface plumbing—in Japan, the Ontake eruption occurred 2.5 weeks after seismicity in the area increased, Hurwitz and his colleagues reported.
In Yellowstone, the researchers also note, swarms of small tremors are common and can bring fluids—including gases like CO2—into the subsurface cracks. By monitoring changes in thermal waters, especially after earthquake swarms, scientists could possibly anticipate hydrothermal hazards.
—JoAnna Wendel, Staff Writer
Citation: Wendel, J. (2016), Can carbon dioxide trigger geyser eruptions?, Eos, 97, doi:10.1029/2016EO048289. Published on 16 March 2016.