Report on Rabaul (Papua New Guinea) — January 1997
Bulletin of the Global Volcanism Network, vol. 22, no. 1 (January 1997)
Managing Editor: Richard Wunderman.
Rabaul (Papua New Guinea) Large gas fireball, Strombolian eruptions, and lava flows
Please cite this report as:
Global Volcanism Program, 1997. Report on Rabaul (Papua New Guinea) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 22:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199701-252140
Papua New Guinea
4.271°S, 152.203°E; summit elev. 688 m
All times are local (unless otherwise noted)
A strong Strombolian and lava-producing eruption occurred at Tavurvur, the small active crater on the E side of Rabaul Caldera, on 9 January. The eruption, very similar to the one on 4 October last year, marked the start of a new eruptive phase in the caldera.
Since early December, a reversal in trend at the Sulphur Creek (SCK) water-tube tiltmeter (3 km NW of Tavurvur) suggested a slight deflation of the central magma reservoir. From mid-December, SO2 emissions measured by COSPEC dropped to very low levels and emissions from Tavurvur were reduced to only white vapor. From 25 December onward, low-frequency earthquakes started to occur under Tavurvur and became increasingly common. In early January, emissions from Tavurvur contained more blue vapor and only occasional ash. A hissing sound could be heard from nearby Matupit Island accompanied by red glow at night.
At 0053 on 9 January, a powerful explosion woke residents of the caldera and Kokopo, 1 km S. It was reported to have produced a dark ash cloud that rose 3,000-4,000 m and blocks could be heard falling around the Tavurvur cone.
No further unusual activity was noticed until 0120, when a "large fireball" rose silently above the crater to a height of ~1,500 m while expanding to a ~500 m radius before falling onto the cone where it ignited bush fires. Witnesses say that it produced very intense light for a few seconds before dying off instantaneously and that there were no incandescent projections. It is presumed that this phenomenon corresponded to the ignition of a large bubble of methane, a gas that is relatively abundant in Tavurvur emissions.
Following the gas fireball, background RSAM rose (from <50 to >100 scale units) and weak Strombolian activity started within ten minutes. The strength of the Strombolian activity increased rapidly as did the RSAM levels (~200 by 0200, ~500 by 0245, and ~700 by 0300). The activity culminated between 0400 and 1200, with sub-continuous fountaining of pasty lumps of lava to ~1,000 m above the crater rim and sub-continuous detonations that broke glassware at Kokopo and were heard as far away as Bialla and Buka Islands (200 km SW and 300 km SE, respectively). The detonations were quieter on the N side of Rabaul (presumably due to the shape of Tavurvur crater, now open to the S). A column containing ash and scoria rose to ~4,000 m; it blew SE, leaving >1 cm thickness of coarse ash at Talwat village (1.5 km SE) and a few millimeters over the Tokua-Kokopo area.
Starting at about 0315 on 9 January, a chunky lava flow (estimated volume 4 x 106 m3) poured over Tavurvur's S rim, over and onto either side of the 4 October lava flow, until it reached the sea. By 10 January the coastline had been extended ~100 m offshore. Throughout the main phase of the Strombolian activity and lava flows, RSAM levels were steady at ~850, similar to the October 1996 eruption.
Coconut and cocoa groves, the road leading to Talwat, and a few houses (deserted since the 1994 eruption) were ignited and buried by the lava flow. During the night, inhabitants of Matupit Island (~2 km W of Tavurvur) began to evacuate on their own accord. Upon encountering RVO staff at about 0300 and 0400 on 9 January, they learned that the eruption was probably not a threat to life or property and many returned to their homes. That day, many businesses stayed closed in the Kokopo area, due to the continuous, unnerving detonations.
Activity started to decline after noon, and decreased rapidly after 1400. By 1800, Strombolian explosions were discontinuous and weak. RSAM levels dropped to pre-eruption background levels (~40).
The SCK water tube tiltmeter showed a radial deflation of ~10 µrad in response to the eruption (compared to ~12 µrad in response to the October eruption). Some GPS measurements along a 3 km radius to Tavurvur showed significant (centimeter scale) horizontal changes. Although some motion occurred away from the volcano, the horizontal movement appeared complex and irregular.
Inspection of the S flank of Tavurvur revealed that, at an unknown time during the eruption, part of the crater wall slumped half way down the cone, now forming a hump partly covered and partly surrounded by the lava flow.
On the following days, activity was low. Pale gray ash clouds discharged at irregular intervals with load roaring or explosion sounds. For nearly a week afterward, bands of harmonic tremor occurred at intervals of a few hours, sometimes energetic enough to be recorded by the whole seismic network. RSAM values remained at background levels, with discrete spikes in response to explosions. SO2 levels also remained low (300-800 tons/day) throughout.
From 15 January, ash emissions became darker. By 21 January, these Vulcanian explosions were occurring at 1-5 minute intervals, rising to 3,000 m and resulting in light, damp ashfalls to the SE. However, past the third week in January, activity was back to a moderate level of weaker, grayish Vulcanian explosions, while SCK tiltmeter showed a slow recovery trend.
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 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.
Information Contacts: B. Talai, D. Lolok, P. de Saint-Ours, and C. McKee, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.