RVO reported an eruption from Rabaul caldera's Tavurvur cone on 23 July, after increased seismicity likely beneath Tavurvur cone was detected the previous day. The eruption was preceded by a few small hybrid earthquakes at 1034 followed by small low-frequency earthquakes and later continuous volcanic tremor. Diffuse white plumes were initially emitted at 1320, and then pink-gray fumes with low ash content were seen. A strong odor of hydrogen sulfide was noted, and a diffuse cloud rose 1 km and drifted NW. Billowing gray clouds a few hours later (at 1600) indicated a higher ash content and increased activity. They were also accompanied by roaring and rumbling noises. Discrete explosions commenced at 1730. Ashfall was reported in areas to the NW and NNW.

During 23-25 July seismicity was variable. Ash emissions and ashfall in areas to the NW continued. Visibility was poor in Rabaul town (3-5 km NW) due to re-suspended ash from moving vehicles. Ash emissions stopped at about 1430 on 25 July. Later that day and into 26 July only diffuse brown-tinted vapor plumes were emitted and seismicity was very low.

Geological Summary. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the asymmetrical shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Source: Rabaul Volcano Observatory (RVO)