This report documents ash plumes and explosions at Tavurvur, a cone on the NE flank of Rabaul caldera, from May through December 2008; our last report (BGVN 33:03) described activity from the end of July 2007 through early May 2008. Aviation notices were frequent from May 2008 through December 2008 (table 10); plume heights were typically to 3 km altitude or less, but they were visible considerable distances downwind, often over 100 km and in some cases during 22 July to 12 August, to 185 km. Additional details and ground observations were provided by the Rabaul Volcanological Observatory (RVO).

Table 10. Ash plumes from Rabaul's Tavuvur cone during 7 May-10 December 2008. The table distills ~ 150 reports, mostly from the Darwin Volcanic Ash Advisory Center (VAAC). Areas affected by ashfall can generally be found described in the text.

May 2008. Emissions were variable with some ejections being quite forceful; while at other times there were long periods of hazy emissions. Sub-continuous rumbling and roaring from the vent were reported, and was especially noticeable after heavy rain. The roaring was interpreted as a result of the interaction of water with hot rocks. Seismicity was dominated by low-frequency volcanic earthquakes, which increased from low levels at the beginning of the month to moderately high levels on and after the 15th. Ground deformation measurements continued to indicate slow subsidence. There were, however, two periods of significant uplift (1 cm each) on the 16th and 17th, each occurring over 4 hours. The speed of the uplift and then its return to 'normal' within 24 hours accompanied by increased roaring and occasional explosions, seemed to indicate this was a blockage in the vent that caused steam build up, followed by leakage and vent clearing.

June 2008. Continuous roaring and moderate ashfall ceased in June, but periodic (hours apart) explosions produced some large amounts of ash, followed by gentle steam emissions. The explosions were interpreted as the result of the vent becoming periodically blocked, due to a lessening of volume of the gas/steam emissions, enabling debris to accumulate rather than being instantly expelled. This process enabled the gas pressure to build until it was able to explosively clear the vent. The gas pressure then dropped and the debris began to accumulate again.

Although eruptions had ceased by the 10th, loud roaring continued and night glow was visible. On the 17th a small eruption occurred. Ground deformation measurements showed that during the latter half of June, the volcano was in a deflated state with the caldera stable. Seismicity was moderate.

July 2008. Tavurvur continued to generate occasional ash emissions accompanying white vapor in July (figure 49). The ash was mostly dark and gray throughout the month with some days being lighter. Ashfall was prevalent at Rabaul Town and other areas downwind, including Namanula Hill, Malaguna, Pilapila, Volavolo, Nonga, and Watom. Night glow was usually obscured by the ash cloud. Ground deformation measurements showed a 1 cm deflation since early June. Low-frequency earthquakes resulted from movement of steam or gas and ash in the conduit. Some of the earthquakes were associated with ash emissions.

Figure 49. MODIS satellite image of a Rabaul ash plume on 3 July 2008. In this image, the volcano's plume differs from the nearby clouds in both color and form. Whereas the clouds are bright white with distinct margins, the plume is more diffuse in shape with a dingy gray-beige color. The relatively dark color of the plume suggests volcanic ash mixed with water vapor. Courtesy of NASA Earth Observatory.

August 2008. Light to moderate winds continued to blow ash-rich plumes to the NW, while weak to moderate ash emissions continued. Rabaul Town has been the most affected, given its location along the main axis of the wind path and proximity to the volcano. The accumulated ash in Rabaul Town during the past 4-5 weeks has been very significant. Some visual change in the plume was evident shown by the slight increase in the number of light-colored ash emissions compared to previous weeks when dark emissions with more ash were dominant. Occasional roaring noises were heard, and weak to strong projections of incandescent lava fragments were present during moderate and strong explosions. Some small high-frequency earthquakes that began in late July appeared to be originating from NE of Rabaul Caldera, but exact locations could not be established.

September 2008. There was slight improvement in the conditions at Rabaul Town and the downwind areas during September, despite continuing ash emissions from Tavurvur. This resulted from decreased ashfall, changes in wind patterns, and light rains that inhibited ash re-suspension. The areas affected by the continuing ashfall included the villages between Rapolo and Raluan as well as Namanula Hill and Korere. On 24 September, flights to Tokua were cancelled due to ashfall.

October 2008. Tavurvur continued to emit occasional light to dark gray ash plumes in October. The changing wind directions continued to blow ash plumes to the W, NW, and E of Tavurvur. Loud roaring noises were heard on the 5th and a total of eight high-frequency earthquakes that originated NE of Rabaul were recorded that day. After a period of slight decrease in ash content, the eruptive activity began to change. On the 11th forceful emissions of pale gray to dark gray ash clouds occurred. Large explosions began on the 20th with emissions of ash-rich plumes. The ash plumes rose between 1-3 km above the summit before they were blown to the NW and W. A continuous red glow was visible at night, and roaring and rumbling noises continued. Ground deformation measurements from the water-tube tiltmeter continued to show the long-term deflationary trend at the central part of the caldera since July 2007. Minor inflation was noted during 7-11 October.

November 2008. Light to pale gray ash emissions continued in November, though the ash content was relatively low. Nearly continuous glow was visible at night. Occasional projections of incandescent lava fragments were produced by forceful emissions and weak explosions. No high-frequency volcano-tectonic earthquakes were recorded. The water-tube tiltmeter continued to show downtilt towards the center of the caldera, which was interpreted as a deflation of the central part of the caldera. Heavy rains washed excess ash from the upper slopes of the caldera wall down to low lying areas, causing flood deposition. An earthquake on 1 November in the Bismarck Sea did not result in a tsunami recorded by the RVO tide gauges.

1-12 December 2008. Activity continued in the first half of December, with steady emissions of ash-rich plumes. Some of the emissions were forceful and occasional roaring/rumbling noises were heard. Villages affected by the ash plumes included Rabaul Town, Malaguna, Rapolo, Raluan, Vulcan, Karavia, Davaon, Nguvalian, Raluana, Barovo, Butuwin and inland villages in the downwind paths. The down-tilt toward the center of the caldera, reported in previous months, stopped.

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: Steve Saunders and Herman Patia, Rabaul Volcanological Observatory (RVO), Department of Mining, Private Mail Bag, Port Moresby Post Office, National Capitol District, Papua New Guinea (URL: http://www.pngndc.gov.pg/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/).