Volcano Watch: Mapping Kīlauea’s Gas Emissions

Volcano Watch is a weekly article and activity update written by scientists and affiliated scientists at the US Geological Survey Hawaiian Volcano Observatory.

Volcanic eruptions release large amounts of volcanic gases, such as carbon dioxide (CO2), sulfur dioxide (SO2) and hydrogen sulfide (H2S) into the atmosphere. But even between eruptions, smaller amounts of the same gases continue to escape and can provide important clues about the current state of the volcano and the magma underneath. But to measure them, you first need to determine where the gas is coming from.

HVO scientists are hiking transects along the lowered portion of the Kīlauea caldera floor as part of a gas survey conducted on July 22. The multi-gas instruments carried on the scientists’ backs measure the amount of carbon dioxide (CO2), sulfur dioxide (SO2), water vapor (H2O) and hydrogen sulfide (H2S) through a supply hose pointed at the ground surface. These measurements will provide a map of areas of the caldera floor where specific volcanic gases are emitted and their concentrations. The light-colored soil the scientists walk on is evidence that such gases interact and alter the area’s dark volcanic rocks. USGS image by K. Mulliken.

Surveys of gas emissions from the Kīlauea caldera have been done in the past, but never from the entire caldera at once. And nothing was done after the 2018 eruption and collapse — at least not until this year.

In the summer of 2021, scientists at USGS Hawaiian Volcano Observatory (HVO) conducted a detailed gas survey of the caldera’s bottom and rim to understand the distribution of current emissions. The results are compared with previous studies; if any discrepancies are detected, they could indicate that the plumbing system of the top of Kīlauea has changed due to the 2018 collapses.

Measurements of volcanic gases can be made using a MultiGAS instrument, which pumps in air and then records the concentrations of CO2, SO2, and H2S plus water vapor in parts per million (ppm).


These MultiGAS instruments can be permanently stationed in an area of ​​interest, tethered to a backpack frame, or mounted on an Unposed Aircraft Systems (UAS), depending on the location and type of data required.


For the gas mapping that took place in the caldera of Kīlauea this summer, two MultiGAS instruments were mounted on backpack frames and HVO scientists walked transects in and around the caldera while continuously collecting data. These lines were spaced 25 to 50 meters (75-150 feet) apart, covering portions of the caldera rim, caldera floor, and lowered block that collapsed during the 2018 eruption.

Although the study covered the entire caldera floor, there was also evidence pointing to where the best place to focus would be. Often the gas emissions are concentrated along cracks or holes in the ground that provide the gas with an easy path to the surface. Visible plumes can be seen in various parts of the caldera floor and in the Haʻakulamanu (Sulfur Banks) and Steam Vents areas in Hawai’i Volcanoes National Park. Visible gas plumes like these are often good indicators of where gas concentrations may be elevated.

As gases rise to the surface from the magma below, they interact with and alter the rocks in the area, resulting in color changes. Searching for this altered rock is another way to identify areas with potentially increased gas emissions.


The scientists also collected gas samples from areas with elevated CO2 concentrations for later lab analyses. A large, plastic syringe was used to collect the sample which was then transferred to a foil bag designed to hold gas. Most of the samples were collected on the lowered block, because that area showed the highest concentrations of CO2.

Chemical analyzes of the different forms (isotopes) of carbon in the CO2 of these samples can provide information about where the magma is that releases these gases, and whether it is new, deep magma that has never been degassed before, or older magma that has had been stored in Kīlauea’s plumbing system for some time.

Although the mapping of the caldera floor is now complete, the walls and floor of Halema’uma’u crater have not yet been mapped and there are many visible gas vents. These areas are impossible to traverse on foot, so the next step is to use a UAS-mounted MultiGAS with permission from the National Park Service to measure gases there.

HVO scientists will create a new map of gas emissions in Kīlauea’s Caldera using data collected this summer. The map will be key to determining whether gas paths from deep magma to the surface have been altered by the 2018 collapses. An updated understanding of the locations of gas emissions in the caldera is essential for ongoing gas monitoring of HVO. This refined gas emissions map will also provide a critical benchmark of gas sources and concentrations that can be monitored for anomalous behavior. The ability to detect changes could also potentially enable HVO to better prepare for eruptions in the future.

Volcano activity updates

Kīlauea does not erupt. The USGS Volcano Alert level is at ADVISORY (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). Kīlauea updates are issued weekly.

The volcano Kīlauea does not erupt. Following the recent intrusion of subsurface magma in the area south of the Kīlauea caldera, which slowed significantly on August 30, earthquake rates and soil deformation in this area have remained near pre-intrusion levels. Other monitoring data streams, including sulfur dioxide emissions and webcam views, show no significant changes. For more information on the current monitoring of Kīlauea, see https://www.usgs.gov/volcanoes/kilauea/monitoring.

Mauna Loa does not erupt and remains at Volcano Alert Level ADVISORY. This level of alarm does not mean that an eruption is imminent or that progression to an eruption from the current level of unrest is certain. Mauna Loa updates are released weekly.

Over the past week, about 60 small-magnitude earthquakes were recorded under Mauna Loa’s summit and upper flanks. Measurements from the Global Positioning System (GPS) show no major distortion over the past week. Gas concentrations and fumarole temperatures at both the summit and Sulfur Cone in the Southwest Rift Zone remain stable. Webcams do not show changes in the landscape. For more information on Mauna Loa’s current monitoring, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring.

There were 3 events with 3 or more felt alerts in the Hawaiian Islands in the past week: a M3.5 earthquake 13 km (8 miles) E of Pāhala at 29 km (18 miles) depth on September 8 at 6:34 PM HST, a M3.6 earthquake 53 km (32 mi) SE of Nāʻālehu at 6 km (4 mi) depth on September 7 at 12:36 PM HST, and a M3.4 earthquake 3 km (1 mi) SSE of Pāhala at 34 km (21 miles) depth on Sept. 7 at 2:27 a.m. HST.

HVO continues to monitor both Kīlauea and Mauna Loa closely for signs of increased activity.

Visit HVO’s website for past Volcano Watch articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information, and more. Email inquiries to: [email protected].

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