Eruptive activity at the Tavurvur cone at Rabaul continued during September-December 2009. This caldera has been frequently active since September 1994, and most recently the activity has consisted of intermittent ash explosions and ejection of incandescent lava fragments (BGVN 34:08). The volcano is monitored by staff at the Rabaul Volcano Observatory (RVO). They work closely with the Darwin Volcanic Ash Advisory Center, who use their satellite imagery analysis capabilities to track plumes downwind and warn aviators of volcanic hazards.

Mild eruptive activity at Tavurvur in September was characterized by emissions of pale gray, and sometimes dark gray, ash plumes. Roaring and rumbling noises that were heard frequently during the first half of September gradually declined, and by the last week of the month had virtually ceased. The strong explosions that began about 20 August continued during the first two weeks of September. Occasional glow was visible at night during September with some small projections of incandescent lava fragments observed on some nights.

RVO reported that during 25 September-8 October gray ash plumes rose 2 km above the crater. Ashfall was reported in Rabaul town (3-5 km NW) and surrounding areas. Occasionally, incandescence from the summit crater was seen at night and incandescent lava fragments were ejected. During the second week of October ash emissions became more diffuse, with occasional long periods of white vapor. During October, occasional dull glow was visible during the first week only. Low roaring and rumbling noises were heard during the second and last weeks of October.

Based on analyses of satellite imagery, the Darwin VAAC reported that on 6 October an ash plume rose to an altitude of 3 km and drifted 55 km NW. A similar plume on 9 October rose to an altitude of 2.4 km and drifted 75 km NW. On 15, 18, and 20 October, ash plumes rose to altitudes of 2.7-3 km and drifted 25-85 km NW, W, and NE. Ash plumes during 28-29 October rose to an altitude of 2.4 km and drifted 35-45 km N, E, and SE.

Seismicity in September was dominated by high-frequency signals associated with roaring and rumbling noises, overshadowing low-frequency signals from ash emissions. Seismic activity during October was associated mainly with the ash emissions and rumbling noises. No high- frequency earthquakes were recorded during September and October. Ground deformation data from the GPS and tide gauge stations generally showed deflation during the first three weeks of September. In October the data showed a slow deflationary trend.

On 9 November an ash plume rose to an altitude of 1.8 km and drifted 110 km NW. During 11-14 November ash plumes rose to an altitude of 2.4 km and drifted 75-150 km NW and W. On 19 November ash plumes rose to an altitude of 2.1 km and drifted 35-90 km NW. During 20-26 November dense white plumes and gray ash plumes rose from Tavurvur. Strong explosions produced ash plumes that rose 1.5 km above the summit and showered the flanks with lava fragments that were incandescent at night. Shock waves rattled windows in the Kokopo area, ~ 20 km SE. Occasional incandescence from the summit crater was noted.

Dense white plumes and gray ash plumes rose above Tavurvur during 27 November-3 December. Strong explosions produced ash plumes that rose 1.5 km above the summit and showered the flanks with lava fragments that were incandescent at night. Shock waves rattled windows in the Kokopo area. Incandescence from the summit crater was occasionally noted. During 2-4 December, ash fell in Rabaul town (3-5 km NW) and other villages downwind. Activity consisted of a few strong explosions towards the beginning of the 4-10 December reporting period, and emissions of gray ash afterwards. Diffuse white vapor was emitted during quieter intervals. Ash plumes rose 1 km above the summit and drifted SE towards Tokua and the Tokua airport, causing suspension of some flights. Based on analyses of satellite imagery, the Darwin VAAC reported that during 5 and 7-8 December ash plumes rose to an altitude of 2.4 km and drifted 45-55 km E; on 14 December another ash plume drifted 35 km SE.

No emissions from the crater was observed on most days during 21-31 December, although there were occasional white steam plumes or blue plumes. On 23 December ash emissions rose less than 200 m above the cone. The Darwin VAAC reported that on 31 December an ash plume rose to an altitude of 2.4 km and drifted less than 30 km SE.

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.

Information Contacts: Ima Itikarai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/).