Volcano Watch: Sniffing out stealthy gas escape between Kīlauea’s eruptions

“Volcano Watch” is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

Kīlauea has erupted three times in 2023 — January–March, June and September — and has also experienced significant intrusive activity to the southwest of the summit since the beginning of October.

During eruptions, the Hawaiian Volcano Observatory frequently reports sulfur dioxide (SO2) emission rates as a means of tracking the progression of eruptive activity. But for the periods before eruptions, or when there is an ongoing intrusion with no eruption, most of the data HVO relies on is geophysical data, such as deformation or seismicity, rather than geochemical data such as SO2 emissions.

However, as “Volcano Watch” has discussed before, there is another type of gas that can be important during non-eruptive periods — carbon dioxide (CO2).

CO2 behaves very differently from SO2 in Kīlauea’s magmatic system, and these differences can be exploited to help better understand processes occurring beneath the ground surface.

For example, CO2 can begin to escape from Kīlauea’s magma when it is still many miles beneath the surface even though SO2 is largely released when magma is just a few tens or hundreds of yards beneath the surface. In a practical sense, this often means we don’t see much SO2 being emitted until lava begins erupting at the surface.

Because CO2 escapes the magma from deeper, we should be able to see changes in the amount of CO2 coming from Kīlauea as magma gets shallower, even if it’s not shallow enough to erupt yet.

The tricky thing about CO2 though is that it is already present — and highly variable — in the atmosphere. This is different from SO2, which is not normally present in background atmosphere so it’s easy to pick out a volcanic SO2 signal in ambient air measurements. But atmospheric CO2 can vary throughout the course of a day, as well as with the seasons.

So, picking out a small volcanic CO2 signal from variable amounts in the background atmosphere can be tough, and it has indeed proved difficult through the years at Kīlauea and Mauna Loa.

Recently, however, in cooperation with colleagues at the U.S. Geological Survey’s Cascades Volcano Observatory, we have been looking a little closer at CO2 data from Kīlauea. We have a multi-GAS station just to the southwest of Halemaʻumaʻu that measures four volcanic gases (CO2, SO2, hydrogen sulfide and water vapor), as well as meteorological data such as wind speed and wind direction.

Instead of using all the CO2 data from the multi-GAS, which can be noisy because of background variations in CO2, we separate out CO2 data that reaches the station from certain directions at certain wind speeds. This allows us to try to isolate the volcanic CO2 signal.

What have we seen?

Well, the data are still noisy, so instead of looking at individual data points (up to eight per day), we calculate weekly averages of the CO2 concentration. Once we do that, if we look only at data coming from two portions of Halemaʻumaʻu (roughly the western part of the crater and the southeastern part of the crater) at moderate wind speeds, we see patterns in the CO2 concentration relative to the recent summit eruptions.

For both wind directions we look at, we can see that CO2 coming from those directions appeared to increase — slowly and slightly — before the June and September Kīlauea summit eruptions. Once the eruptions occurred, CO2 concentrations dropped back down.

Now, since the September eruption, those CO2 concentrations are increasing again, and the increase is likely related to the intrusion of magma into the shallow storage regions beneath the summit and south caldera regions.

Often when Kīlauea erupts, HVO uses the low ratio of eruptive CO2 to SO2 to be able to say that the magma feeding the eruption was stored very shallow because that low ratio tells us the magma already degassed most of its CO2 before eruption. What we’re seeing right now is that pre-eruptive CO2 loss in the form of these CO2 increases before eruptions as magma gets closer to the surface.

The next step with this new data analysis method is to try to turn the CO2 concentration data into emission rates of CO2, which could then perhaps tell us not just that magma is rising to shallow depths beneath Kīlauea, but how much magma is rising.

Volcano Activity Updates

Kīlauea is not erupting. Its Volcano Alert level is Advisory.

The unrest associated with the intrusion that began in early October southwest of Kīlauea’s summit continues. Elevated earthquake activity has continued in the Southwest Rift Zone, summit and upper East Rift Zone during the past week, with earthquake swarms Dec. 8, 9 and 13-14.

The Uēkahuna tiltmeter—located northwest of the caldera shows little net tilt during the past week. The Sand Hill tiltmeter located southwest of the caldera shows inflation to the south of the caldera since Dec. 11.

Unrest could continue to wax and wane with changes to the input of magma into the area and eruptive activity could occur in the near future with little or no warning. The most recent sulfur dioxide emission rate for the summit of approximately 70 tonnes per day was measured Dec. 5.

Mauna Loa is not erupting. Its Volcano Alert Level is at Normal.

Webcams show no signs of activity on Mauna Loa. Summit seismicity has remained at low levels during the past month. Ground deformation indicates continuing slow inflation as magma replenishes the reservoir system following the 2022 eruption. SO2 emission rates are at background levels.

Six earthquakes were reported felt in the Hawaiian Islands during the past week: a magnitude-3.1 earthquake 13 miles southwest of Laupāhoehoe 19 miles deep Dec. 11 at 3:08 p.m., a magnitude-2.3 earthquake 5 miles south-southwest of Leilani Estates at at depth of 3 miles Dec. 11 at 12:02 a.m., a magnitude-2.2 earthquake 8 miles southeast of Waimea 16 miles deep Dec. 9 at 6:54 a.m., a magnitude-3.4 earthquake 11 miles north of Pāhala at a depth of 4 miles Dec. 8 at 9:22 p.m., a magnitude-3.6 earthquake 1 mile west-southwest of Hōlualoa 8 miles deep Dec. 8 at 9:21 p.m. and a magnitude-3.6 earthquake 6 miles east-northeast of Pāhala at a depth of 20 miles Dec. 7 at 7:13 a.m.

HVO continues to closely monitor Kīlauea and Mauna Loa.

Visit HVO’s website for past “Volcano Watch” articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Email questions to askHVO@usgs.gov.