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

By US 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 USGS Hawaiian Volcano Observatory (HVO) 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 that HVO relies on is geophysical data, like deformation or seismicity, rather than geochemical data like 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 kilometers (miles) beneath the surface even though SO₂ is largely released when magma is just a few tens or hundreds of meters (yards) beneath the surface. In a practical sense, this often means that 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—SO₂ 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 over 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 over the years at both Kīlauea and Mauna Loa.

Recently, however, in cooperation with colleagues at the USGS 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 like 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 USGS 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 over the last week, with earthquakes swarms on December 8, 9, and 13-14. The Uēkahuna tiltmeter—located northwest of the caldera—shows little net tilt over the past week. The Sand Hill tiltmeter—located southwest of the caldera—shows inflation to the south of the caldera since Monday, December 11. Unrest may 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 (SO2) emission rate for the summit—approximately 70 tonnes per day—was measured on Dec. 5.

Mauna Loa is not erupting. Its USGS Volcano Alert Level is at NORMAL.

Webcams show no signs of activity on Mauna Loa. Summit seismicity has remained at low levels over 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:

• M3.1 earthquake 13 mi SW of Laupāhoehoe at 31 km (19 mi) depth on Dec. 11 at 3:08 p.m.
• M2.3 earthquake 5 mi SSW of Leilani Estates at 5 km (3 mi) depth on Dec. 11 at 12:02 a.m.
• M2.2 earthquake 8 mi SE of Waimea at 26 km (16 mi) depth on Dec. 9 at 6:54 a.m.
• M3.4 earthquake 11 mi N of Pāhala at 7 km (4 mi) depth on Dec. 8 at 9:22 p.m.
• M3.6 earthquake 1 mi WSW of Hōlualoa at 12 km (8 mi) depth on Dec. 8 at 9:21 p.m.
• M3.6 earthquake 6 mi ENE of Pāhala at 32 km (20 mi) depth on 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.