Bubbles burst prevailing theory about volcano chemistry

As magma squeezes its way out of a volcano, bubbles of lava-coated hot gas form. Volcanologists now report these emissions tell a more complex story of the chemistry going on during volcanic eruptions than previously thought.

With a suite of experiments that fall into the “don’t try this at home” category, Clive Oppenheimer from the University of Cambridge and colleagues measured the infrared spectra of gases spewing out of Kīlauea volcano on Hawaii’s Big Island. The ongoing eruption at Kīlauea began in May 2018, but Oppenheimer’s fieldwork took place in 2013. His team precariously placed a Fourier transform infrared spectrometer on the crater of Kīlauea’s lava lake and took measurements when the lava lake was sedate, releasing occasional bubbles, as well as when the volcano was degassing more vigorously.

Magma contains dissolved gases, and bubbles rising from volcanoes carry with them the gases that escape. Scientists thought these gases were directly related to the magma’s composition before eruptions. But Oppenheimer’s measurements suggest the picture is more complicated (Nat. Geosci. 2018, DOI: 10.1038/s41561-018-0194-5).

From their measurements, the team could also calculate the temperature of the gas coming out of the bubbles as they burst at the top of the lava lake.

Despite all the bubbles coming from the same molten lake, they found a range of temperatures from 900 to 1150 °C. However, at 900 °C, lava is solid. “That posed the question, how can the gas be colder than the lava it is passing through?” Oppenheimer says.

Larger bubbles from the most vigorous activity were coldest, and these held a surprise: The gas composition wasn’t what was expected if the oxidation state were being controlled by the liquid magma, explains Oppenheimer. The gases in the larger bubbles were more oxidized than expected, seen by a higher ratio of CO₂ to CO, as well as different ratios of other redox-sensitive gases including SO₂/H₂S and H₂O/H₂.

The bubbles cool as they expand, Oppenheimer says, and as they shoot up to the lava lake’s surface, the bubble walls can’t respond quickly enough to maintain thermal equilibrium. “At the same time, for larger bubbles, the gas in the interior of the bubbles loses contact with the molten lava, and it follows its own redox path as a closed system as the gas expands and cools,” Oppenheimer says.

The complexity of this magmatic degassing could have implications for volcano monitoring, Oppenheimer says. Averaging data from these kinds of volcano measurements is typical, he explains, because researchers assume that the variation reflects errors in making the measurements. “If you average a set of data, you simply miss this incredible variability that speaks to the complexity of the thermodynamics of magma degassing,” he says. Kenneth Rubin, a volcanologist at University of Hawaii, says that this discovery isn’t that surprising given how dynamic volcanoes are. “It is an important step forward relating eruption dynamics and gas compositions together through measurements and models,” he says. To get an even better idea of what happens in volcano degassing, Rubin says, this work should be expanded to include other volcanoes and magma types, he suggests.