The nose knows (and so did HVO gas instrumentation…eventually) — USGS Volcano Watch

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 Kīlauea’s East Rift Zone!

A USGS scientist aims the viewfinder of an infrared spectrometer to measure the chemistry of volcanic gas on the last day of the Nāpau eruption, September 20, 2024. USGS photo by P. Nadeau.

It was indeed a dark and stormy night when the eruption started on September 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 Puʻuʻōʻō 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 (September 16). HVO confirmed that there had been a small fissure eruption west of Puʻuʻōʻō, 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 (September 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, September 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 Nāpau 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 September 19.

Yet just one day later, the eruption was over, with SO2 emissions down to only 800 t/d as of late morning on September 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 Kīlauea East Rift Zone eruptions and to recent Kīlauea summit eruptions.

A final SO2 emission rate measured on September 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 Nāpau fissures were undetectable on Chain of Craters Road.

Even though HVO was ultimately able to track the variable gas emissions throughout the Nāpau 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 Nāpau eruption – that honor still goes to the residents of Volcano!

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