Report on Rabaul (Papua New Guinea) — 17 August-23 August 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 17 August-23 August 2005
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2005. Report on Rabaul (Papua New Guinea). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 17 August-23 August 2005. Smithsonian Institution and US Geological Survey.
Papua New Guinea
4.271°S, 152.203°E; summit elev. 688 m
All times are local (unless otherwise noted)
Rabaul caldera's active Tavurvur cone continued to emit ash during 15-21 August, although there was a slight decline in the frequency of emissions in comparison to earlier weeks. Ash plumes rose 800-1,500 m above the volcano (or 4,900-7,200 ft a.s.l.) and drifted N and NW, occasionally depositing ash on the E part of Rabaul Town and in areas farther downwind. Projected incandescent lava fragments were visible at night. Seismicity was at moderate-to-high levels, with most earthquakes associated with ash emissions and explosions. Ground-deformation measurements from GPS and tide-gauge instruments fluctuated, however the general trend showed a slow rate of uplift. As a safety precaution, people were discouraged from venturing within 1 km of the erupting vent.
Geologic Background. 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 688-m-high asymmetrical pyroclastic 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 1400 years ago. An earlier caldera-forming eruption about 7100 years ago is now considered to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the northern and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and western 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.