Rabaul Volcano Observatory (RVO) reported that the eruption at Ulawun continued to intensify in November and culminated in a larger event during 20-21 November. During 3-18 November white gas-and-steam plumes of variable densities rose from the summit crater. Low-level booming noises were reported on 9 November. Crater incandescence was observed nightly and fluctuated between dull and moderately bright; the most intense incandescence was observed during 10-11 November. Seismicity was at low-to-moderate levels and characterized by continuous, low-level volcanic tremors often punctuated by periods of small-to moderate discrete low-frequency volcanic earthquakes that evolved into sub-continuous volcanic tremors.

Small ash emissions were observed on 19 November. Seismicity began to intensify at around 0200 on 20 November and then again at 0930. The ash emissions increased to moderate levels and eruption noises were heard between 1430-1500. The Alert Level was raised to Stage 3 (on the four-level scale). The ash plumes drifted W and NW, causing ashfall in Navo. The eruption significantly intensified during 1530-1600 and continued at high levels. According to the Darwin VAAC ash plumes rose as high as 9.1 km (30,000 ft) a.s.l. at 1600 and drifted SW. Ash plumes had risen to 15 km (50,000 ft) a.s.l. by 1630 and spread almost 65 km W by 1720.

The Alert Level was raised to Stage 4. Continuous ash plumes obscured the summit area and by 1800 ashfall was reported in the Ulamona Mission area. A photo taken at 1807 showed incandescent material being ejected above the summit and a dark, dense ash plume rising from the crater. Ashfall was significant in areas to the N and NW and absent in areas to the N and E. Roaring and booming noises continued. Residents of Ubili and Ulamona Mission Station moved to Kabaya and Koasa, and Noau and Voluvolu residents moved to Bakada. Video and photos posted on social media showed tall lava fountaining at the summit and a pyroclastic flow descending the NW flank.

Ash plumes continued to obscure the summit along with darkness from 2200 on 20 November to 0200 on 21 November, though the intensity of the eruption had declined. The VAAC continued to report a sustained ash plume rising to 15 km; by 0320 on 21 November ash had drifted as far as 520 km W. By 0450 the VAAC noted that ash had reached the stratosphere, rising to 18.2 km (60,000 ft) a.s.l. and drifting 75 km SE; ash at 15 km continued to drift W. Dark, dense ash emissions rose from the summit crater during 0200-1300 but were less intense. The Alert Level was lowered to Stage 3, but RVO also noted that at 0700 one of the two seismic stations had stopped working, making monitoring even more difficult. Low roaring and booming continued, and ash continued to fall mainly to the W and NW, affecting infrastructure and crops. By sunrise the view from the observation post to the summit was blanketed by ash; dense ash obscured views of the summit and flanks. RVO also noted that pyroclastic flows had likely descended the N, and possibly the SE, flanks during the early part of the eruption the day before. The VAAC reported that sustained ash plumes continued, though by 1630 they were rising to 12.2 km (40,000 ft) a.s.l. and drifting W while the ash at 18.2 km continued to drift E.

During 1300-1400 ash emissions significantly decreased, allowing confirmation of a fissure vent on the SW flank at around 1,000-1,400 m elevation about 1 km SE of the 2019 fissure vent. Webcam images suggested that the lava effusion rate from the fissure was high, and that flows had descended possibly to 400 m elevation. Minor incandescent at the summit was visible as well as on the NW flank, possibly from a pyroclastic flow. Several centimeters of ash and scoria had accumulated on rooftops in areas to the N and NW. Minor ashfall was reported in Kimbe. By 2240 the VAAC stated that the ash at 18.2 km was no longer discernable in satellite images.

RVO reported that during 2100 on 21 November to 0600 on 22 November the eruption was at low levels with only minor amounts of ash being emitted, though lava continued to effuse from the fissure. Summit incandescence was no longer visible by 0319. A few ash puffs were visible during 0600-0800 and then the summit was obscured by weather clouds. The Alert Level was lowered to Stage 2.

Geological Summary. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Sources: Rabaul Volcano Observatory (RVO), Darwin Volcanic Ash Advisory Centre (VAAC), Reuters, Lekei Kilala