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

Kilauea 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 Kilauea’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 Kilauea’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 Kilauea 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 Kilauea 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 Kilauea. We have a multi-GAS station just to the southwest of Halema‘uma‘u 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 Halema‘uma‘u (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 Kilauea 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 Kilauea 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 Kilauea, but how much magma is rising.

Volcano Activity Updates

Kilauea is not erupting. Its USGS Volcano Alert level is ADVISORY.

The unrest associated with the intrusion that began in early October southwest of Kilauea’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 Dec. 8, 9, and 13-14. The Uekahuna 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, Dec. 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: a M3.1 earthquake 21 km (13 mi) SW of Laupahoehoe at 31 km (19 mi) depth on Dec. 11 at 3:08 p.m. HST, a M2.3 earthquake 9 km (5 mi) SSW of Leilani Estates at 5 km (3 mi) depth on Dec. 11 at 12:02 a.m. HST, a M2.2 earthquake 13 km (8 mi) SE of Waimea at 26 km (16 mi) depth on Dec. 9 at 6:54 a.m. HST, a M3.4 earthquake 19 km (11 mi) N of Pahala at 7 km (4 mi) depth on Dec. 8 at 9:22 p.m. HST, a M3.6 earthquake 3 km (1 mi) WSW of Holualoa at 12 km (8 mi) depth on Dec. 8 at 9:21 p.m. HST, and a M3.6 earthquake 10 km (6 mi) ENE of Pahala at 32 km (20 mi) depth on Dec. 7 at 7:13 a.m. HST.

HVO continues to closely monitor Kilauea and Mauna Loa.

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

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