Volcano Watch: The nose knows — and so did HVO gas instrumentation … eventually

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

It was a dark and stormy night …

No, this isn’t a ghost story or horror movie — it’s a “Volcano Watch” about the eruption last fall near Nāpau Crater on the Big Island’s Kīlauea volcano’s East Rift Zone!

It was indeed a dark and stormy night when the eruption started Sept. 15, 2024. So much so that when we had conflicting geophysical data — tremor and increased infrasound, but no changes in tilt — our webcams were no help.

The poor weather meant cameras couldn’t see anything, and the southerly wind direction on that rainy night also meant none of the Hawaiian Volcano Observatory gas monitoring stations could detect whether there was eruptive degassing or not.

But you know who could tell there was degassing?

Residents of Volcano, a little less than 2 miles away. Community members in more than one part of Volcano took to social media to report sulfurous odors and burning smells.

Still, some Hawaiian Volcano Observatory staff members living in the area reported only smelling the burning, without sulfur. Their gas badges — used for situational awareness and safety, not precise volcanic gas measurements — didn’t register sulfur dioxide above background levels.

Many times, winds that blow from the East Rift Zone toward Volcano can bring residual hydrogen sulfide from the inactive Puʻuʻōʻō area, and hydrogen sulfide can be especially prevalent during rainy periods, such as that dark and stormy night in September.

So even amidst community reports of sulfur smells, we couldn’t be completely sure if there was an eruption.

Thankfully, the weather cleared the morning of Sept. 16 and Hawaiian Volcano Observatory confirmed there had been a small fissure eruption west of Puʻuʻōʻō — we were no longer restricted to people’s noses to indicate whether there was eruptive degassing or not.

The sulfur dioxide emission rate was measured to be only about 300 tonnes per day, which is consistent with the absence of eruptive activity.

It seemed like the eruption might be finished, but by the morning of Sept. 17, it was in full force again and sulfur dioxide emissions climbed to nearly 12,000 tonnes per day.

Winds had also switched to the right direction — from the north — for one of our East Rift Zone gas monitoring stations to detect a whiff of the sulfur dioxide as well.

Emissions then decreased to about 3,500 tonnes per day by that afternoon as the lava fountaining weakened. Emissions were similar, at about 2,000 tonnes per day, the morning of Sept. 18.

Again, activity seemed to be waning until later Sept. 18, when things escalated once more, which was reflected in increasing sulfur dioxide emissions.

That afternoon, Hawaiian Volcano Observatory scientists were measuring the plume with an ultraviolet camera that can see sulfur dioxide, when the imagery began to show a more intense plume.

Gas scientists recognized changes were occurring at that point and switched back to more reliable ultraviolet spectrometer measurements, which revealed a progressive increase in sulfur dioxide emission rate throughout the course of the afternoon.

In conjunction with the opening of new fissures and the development of “lava falls” cascading over the Nāpau Crater rim, emissions increased from 5,000 tonnes per day at about 3:30 p.m. to roughly 12,000 tonnes per day at 5 p.m., when it became too late to continue ultraviolet-based measurements.

With the fissures and lava falls still going strong, sulfur dioxide emissions were around 30,000 tonnes per day the morning of Sept. 19.

Just one day later, the eruption was finished, with sulfur dioxide emissions down to only 800 tonnes per day as of late the morning of Sept. 20.

Luckily, Hawaiian Volcano Observatory gas scientists were able to measure gases from the last gasp of lava earlier that morning using an infrared spectrometer, which measures the chemistry of erupted gas.

The gases were low in carbon dioxide, and therefore derived from magma that previously lost carbon dioxide while in the shallow magma plumbing system before eruption. This is very similar to other Kīlauea East Rift Zone eruptions and to recent Kīlauea summit eruptions.

A final sulfur dioxide emission rate measured Sept. 21, after the eruption ended, showed that just under 100 tonnes per day of sulfur dioxide were being emitted from the inactive fissures.

Sulfur dioxide emissions from the Nāpau fissures were undetectable by 2 days later on Chain of Craters Road.

Even though Hawaiian Volcano Observatory was ultimately able to track the variable gas emissions throughout the Nāpau eruption with our ultraviolet spectrometer, a ultraviolet camera, permanent stations and an infrared spectrometer, we know we weren’t the first to sniff the gases from the Nāpau eruption.

That honor still goes to the residents of Volcano.

Volcano Activity Updates

Kīlauea has erupted intermittently within the summit caldera since Dec. 23, 2024. Its U.S. Geological Survey Volcano Alert level remains at Watch.

The eruption is confined in Halemaʻumaʻu Crater within Hawaiʻi Volcanoes National Park and continued during the past week, with one eruptive episode.

Episode 11 was active from the night of Feb. 25 until the morning of Feb. 26. Kīlauea summit has been inflating since Episode 11 ended, suggesting another eruptive episode is possible.

Sulfur dioxide emission rates are elevated in the summit region during active eruption episodes.

No unusual activity has been noted along Kīlauea’s East Rift Zone or Southwest Rift Zone.

Mauna Loa is not erupting. Its U.S. Geological Survey Volcano Alert Level remains at Normal.

Three earthquakes were reported felt in the Hawaiian Islands during the past week:

• Magnitude-3.4 earthquake 8 miles south of Volcano at a depth of 0 miles at 3:33 a.m. Feb. 27.
• Magnitude-3.3 earthquake 9 miles west of Kailua-Kona at a depth of 8 miles at 9:31 p.m. Feb. 23.
• Magnitude-2.7 earthquake 8 miles north-northeast of Ocean View at a depth of 5 miles at 7:36 a.m. Feb. 20.

Hawaiian Volcano Observatory continues to closely monitor Kīlauea and Mauna Loa.

Visit the volcano observatory’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.