by Rachel Crowell Thursday, November 8, 2018
Programs that manage volcanic hazards use a variety of tools and techniques to monitor impending eruptions. But researchers recently found evidence — in the form of gas bubbles bursting out of Hawaii’s Kilauea Volcano — suggesting that scientists should forgo one common method for assessing hazards from basaltic volcanoes: averaging gas composition measurements.
In March 2013, researchers positioned an infrared spectrometer along the rim of Kilauea’s Halema’uma’u Crater, pointing it toward the now-former lava lake about 200 meters away. (Eruptions lasting from May through September 2018 dramatically reshaped the landscape around Halema’uma’u, widening the crater and draining the lava lake.) The technique allowed them to monitor multiple gases in situ as they streamed out of the volcano. Spectra were collected every five seconds during two degassing events: one mild and one vigorous. The first event included occasional bubble bursts, while the second featured sustained spattering of lava along the lake’s perimeter, “driven by the ascent and rupture of bubbles up to several meters in diameter,” the team noted in a study published in Nature Geoscience.
The near-real-time nature of the technique allowed the team to collect numerous measurements each hour for later analysis offsite. This was crucial because “from one instance to the next, the activity would change,” says Clive Oppenheimer, a volcanologist at the University of Cambridge in England and lead author of the study. The rapidly changing activity means that, from one instance to the next, the hazard level could also change.
Counter to previous assumptions, the team found that volcanic gas bubbles — which are released when the pressure in magma decreases as it rises toward Earth’s surface — can cool as they rise, rather than remaining in thermal equilibrium with the melt. “We also see that the oxidation state [of the gas] is changing in close relationship with the temperature,” Oppenheimer says.
Together, this means that gases that undergo oxidation-reduction reactions in magma — such as carbonyl sulfide — may be present near volcanoes at levels different from what’s been previously thought, the researchers wrote. What’s more, the abundances of those gases can change from moment to moment, such that averaging observations of gas compositions emerging from a volcano over time may obscure a lot of variability in degassing levels.
Oppenheimer and his colleagues measured temperatures for gas released by Kilauea ranging from 900 to 1,150 degrees Celsius. At lower temperatures, they found, the gases were “significantly more oxidized than expected,” but it wasn’t clear why, Oppenheimer says.
The breakthrough clue “was in the size of the bubbles,” he says. Using thermodynamic modeling, the team studied the relationship between the cooling of rising bubbles, final bubble size, and melt viscosity for melts in three different temperature and viscosity scenarios, one each corresponding to magma in Kilauea’s lava lake, the lava lake of Erebus Volcano in Antarctica, and in a simulated case intended to reflect magma on early Earth. The effect of gas bubble cooling and increasing oxidation is strongest when larger bubbles rise out of lower-viscosity melts, Oppenheimer notes. The largest of these bubbles are several meters across. As each of these bubbles expands, less of their volume is in contact with the warm surrounding melt, resulting in a discrepancy between the temperature and oxidation state of the gas in the bubbles compared to the melt.
“This is an elegant study,” says Tobias Fischer, a volcanologist at the University of New Mexico who was not involved with the new work. It “illustrates that, in basaltic systems, the composition of emitted gas from the lava is determined by magma-gas exchanges during magma ascent and bubble growth,” Fischer says. “However, what’s striking … is that during times of vigorous and more explosive magma movement, the emitted gas composition diverges from what would be imposed by the ascending magma.” Altogether, he says, “these observations enable the authors to … predict magma viscosity or gas bubble radius from measured gas compositions, providing new and important real-time insights into the dynamics of the volcanic system.”
Oppenheimer says he and his colleagues plan to continue this work by analyzing gas data from Kilauea’s 2018 eruption, and to apply the knowledge gained in this study to data he previously collected at Erebus.
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