Report on Bagana (Papua New Guinea) — September 1987
Scientific Event Alert Network Bulletin, vol. 12, no. 9 (September 1987)
Managing Editor: Lindsay McClelland.
Bagana (Papua New Guinea) Pyroclastic avalanches end 12-year lava flow
Please cite this report as:
Global Volcanism Program, 1987. Report on Bagana (Papua New Guinea). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 12:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198709-255020.
Papua New Guinea
6.137°S, 155.196°E; summit elev. 1855 m
All times are local (unless otherwise noted)
"On 8 September between 0600 and 0800, a series of pyroclastic avalanches descended the E flank. There were no reports of any explosive activity at the time, but low-level ash clouds were seen on the flanks. Later in the day, a new lava flow was reported on the E side of the summit.
"The cumulative deposit of these avalanches covered an area of ~1 km2. Its outline was complex because about half of the material formed a fan-shaped deposit on a gently sloping bench near the E foot of Bagana. Most of the other material moved 2.5 km from the summit, filling an arcuate depression between Bagana and neighboring Reini volcano. The deposit had a maximum thickness of 10-20 m and is poorly sorted; clast size ranged from fine ash to several-meter-wide blocks. The clasts were andesitic and mostly dense, although some show a slight degree of vesiculation.
"The deposit was still somewhat inflated when inspected on 15 September and numerous rootless fumaroles were active in the central part. The fumaroles were releasing sulphurous gases and depositing sulphur at some vents. The ground around many fumaroles had collapsed, producing craters as large as 1 m across and 20 cm deep. The craters indicated that a considerable amount of gas release had already taken place by 15 September. Crater density was >50% in the central part of the deposit. Near-surface temperatures were measured using a 30-cm-long mercury-in-glass thermometer; the highest temperature recorded was 460°C.
"Eight days before this activity, an increase in high-frequency earthquakes began. By about 5 September, seismicity had built up to several hundred of these events/day. During this time some long-duration (1-5 minute) seismic events occurred that were interpreted as probable rockslides. At the presumed time of the pyroclastic avalanches, a high-frequency tremor was recorded that lasted about 2 hours. Another event of this type, though considerably smaller in amplitude and lasting only half an hour, occurred on 11 September.
"The pyroclastic avalanches may have been caused by mechanical failure of the summit crater wall, which in turn destabilized the active lava dome. The dome then began shedding material in a series of hot avalanches down the E flank . . . . The lack of accompanying explosive activity, the predominantly unvesiculated nature of the clasts, and the lava dome source, suggest that the deposit is of the non-explosive block and ashflow type. These were the first pyroclastic flows recorded at Bagana since the ones that accompanied explosive activity in 1966.
"Within a few days after its appearance the new lava flow showed little activity. At the end of the month, it was ~100-200 m long, 50-100 m wide, and perhaps 10-20 m thick. Activity of the lava flow on the N flank of Bagana has probably ended, 12 years after its commencement."
Geologic Background. Bagana volcano, occupying a remote portion of central Bougainville Island, is one of Melanesia's youngest and most active volcanoes. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is frequent and characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although explosive activity occasionally producing pyroclastic flows also occurs. Lava flows form dramatic, freshly preserved tongue-shaped lobes up to 50 m thick with prominent levees that descend the flanks on all sides.
Information Contacts: J. Mori, C. McKee, and P. Lowenstein, RVO.