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VOLCANO WATCH: How C02 Provides Clues on Depth of Magma

February 26, 2019, 4:14 PM HST
* Updated February 26, 4:16 PM
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In the image, a USGS pilot and Hawaiian Volcano Observatory gas geochemist prepare to conduct a test flight of an unmanned aircraft systems (UAS) on Kīlauea Volcano in November 2018. This UAS was outfitted with a prototype miniaturized multi-gas sensor for the detection of volcanic gases emitted by Kīlauea, including sulfur dioxide and carbon dioxide. PC: USGS photo by Patricia Nadeau.

The latest edition of HVO’s Volcano Watch discusses carbon dioxide (CO2) and how it provides clues about the depth of magma.

To explain, we’ll use something familiar to everyone—a bottle of soda, which has only one gas (CO2) dissolved in it. This CO2 stays dissolved as long as the bottle is sealed, because the bottle creates enough pressure to keep the CO2 in the liquid. As soon as the bottle is opened, pressure on the liquid decreases and the CO2 creates bubbles that escape to the atmosphere.

Unlike soda, magma has many different gases dissolved in it, and they don’t all behave the same way. With soda, one pressure decrease is enough to release all the gas from it, but with magma, different degrees of pressure decrease result in the release different gases.

Even with no lava erupting at the surface, Kīlauea is currently producing significant amounts of CO2. (It’s important to note that these amounts are very small compared to anthropogenic, or human-caused, CO2 emissions.) It’s this CO2, in conjunction with the small amounts of sulfur gases still being emitted, that can provide clues to how deep Kīlauea’s magma is.

Because CO2 can escape even when magma is deep but sulfur dioxide (SO2) mostly stays dissolved until the magma is shallow, deeper magma produces a high ratio of CO2 to SO2. Geochemists typically use this CO2/SO2 ratio as an indication of magma depth.

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The catch is that we must be able to measure the CO2/SO2 ratio. However, accurately measuring the CO2/SO2 ratio in volcanic gas is tricky because of the large and variable amount of CO2 that already exists in the atmosphere. At Kīlauea, the situation is further complicated by collapse events that rearranged the summit caldera and damaged roads and other means of access to degassing areas.

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As this new phase of inactivity at Kīlauea continues, gas geochemistry scientists at the USGS Hawaiian Volcano Observatory will explore new avenues for measuring the helpful CO2/SO2 ratio, including the installation of multi-gas sensors at Kīlauea’s summit and the use of gas sensors mounted on unmanned aircraft systems (UAS). Our goal in collecting such gas data is to document changes that could eventually indicate an increased likelihood of renewed activity at Kīlauea.

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