By Hawaiian Volcano Observatory
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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 Napau Crater on Kilauea’s East Rift Zone!

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It was indeed a dark and stormy night when the eruption started on 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 that cameras couldn’t see anything, and the southerly wind direction on that rainy night also meant that none of the HVO gas monitoring stations could detect whether there was eruptive degassing or not.

But you know who could tell there was degassing? Residents of Volcano. Community members in more than one part of Volcano took to social media to report sulfurous odors and burning smells.

Still, some HVO 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 SO2 (sulfur dioxide) above background. Many times, winds that blow from the East Rift Zone towards Volcano may bring residual H2S (hydrogen sulfide) from the inactive Pu‘u‘o‘o area, and H2S can be especially prevalent during rainy periods, like 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 in the morning (Sept. 16). HVO confirmed that there had been a small fissure eruption west of Pu‘u‘o‘o, and we were no longer restricted to people’s noses to indicate whether there was eruptive degassing or not. The SO2 emission rate was measured to be only about 300 tonnes per day (t/d), which is consistent with the absence of eruptive activity.

It seemed like the eruption might be over, but by the next morning (Sept. 17), it was in full force again, and SO2 emissions had climbed to nearly 12,000 t/d. 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 SO2 as well. Emissions then decreased to about 3,500 t/d by that afternoon as the lava fountaining weakened. Emissions were similar, around 2,000 t/d, the next morning, Sept. 18.

Again, activity seemed to be waning until later on the 18th, when things escalated once more, which was reflected in increasing SO2 emissions. That afternoon, HVO scientists were measuring the plume with an ultraviolet (UV) camera that can see SO2 when the imagery began to show a more intense plume.

At that point, gas scientists recognized that changes were occurring and switched back to more reliable UV spectrometer measurements, which revealed a progressive increase in SO2 emission rate over the course of the afternoon. In conjunction with the opening of new fissures and the development of ‘lava falls’ cascading over Napau Crater rim, emissions increased from 5,000 t/d at about 3:30 p.m. to roughly 12,000 t/d at 5:00 p.m., when it became too late to continue UV-based measurements.

With the fissures and lava falls still going strong, SO2 emissions were around 30,000 t/d the morning of Sept. 19.

Yet just one day later, the eruption was over, with SO2 emissions down to only 800 t/d as of late morning on Sept. 20. Luckily, HVO 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 (CO2), and therefore derived from magma that previously lost CO2 while in the shallow magma plumbing system before eruption. This is very similar to other Kilauea East Rift Zone eruptions and to recent Kilauea summit eruptions.

A final SO2 emission rate measured on Sept. 21, after the eruption had ended, showed that just under 100 t/d of SO2 were being emitted from the inactive fissures. By two days later, SO2 emissions from the Napau fissures were undetectable on Chain of Craters Road.

Even though HVO was ultimately able to track the variable gas emissions throughout the Napau eruption with our UV spectrometer, a UV camera, permanent stations, and an infrared spectrometer, we know we weren’t the first to sniff the gases from the Napau eruption — that honor still goes to the residents of Volcano!

Volcano activity updates

Kilauea has been erupting intermittently within the summit caldera since Dec. 23, 2024. Its USGS Volcano Alert level is WATCH.

The summit eruption at Kilauea volcano that began in Halema‘uma‘u crater on Dec. 23 continued over the past week, with one eruptive episode. Episode 11 was active from the night of Feb. 25 until the morning of Feb. 26. Kilauea summit has been inflating since episode 11 ended, suggesting that 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 Kilauea’s East Rift Zone or Southwest Rift Zone.

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

Three earthquakes were reported felt in the Hawaiian Islands during the past week: a M3.4 earthquake 14 km (8 mi) S of Volcano at 0 km (0 mi) depth on Feb. 27 at 3:33 a.m. HST, a M3.3 earthquake 16 km (9 mi) W of Kailua-Kona at 14 km (8 mi) depth on Feb. 23 at 9:31 p.m. HST, and a M2.7 earthquake 13 km (8 mi) NNE of Hawaiian Ocean View at 9 km (5 mi) depth on Feb. 20 at 7:36 a.m. HST.

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.