Report on Bagana (Papua New Guinea) — July 1986
Scientific Event Alert Network Bulletin, vol. 11, no. 7 (July 1986)
Managing Editor: Lindsay McClelland.
Bagana (Papua New Guinea) Ash emission declines, lava flow still active
Please cite this report as:
Global Volcanism Program, 1986. Report on Bagana (Papua New Guinea). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 11:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198607-255020.
Papua New Guinea
6.137°S, 155.196°E; summit elev. 1855 m
All times are local (unless otherwise noted)
"The phase of stronger activity weakened during the second half of July. Activity during the first half of the month was similar to that of the latter part of June, with 20-40 seismic events/day (probably rockfalls off the active lava flow). Occasional summit incandescence was reported. Emissions consisted of moderate to strong white vapours and moderate brown ash clouds. For the second half of the month, the activity appeared to be declining as the seismicity decreased to 10-20 events/day.
"During an aerial inspection on the 29th, moderate emissions of off-white vapour were observed. Although voluminous, the emissions were being released gently and no emission column was formed. A faint brown tint in the emission plume suggested that it contained some ash. This ash was not being produced by explosive shattering and fragmentation of lava, but is believed to result from fine comminution of fragments broken off relatively cool blocks on the surface of the active lava mound in the summit crater.
"In contrast to previous reports, no true lava dome was observed in the crater. Several rockfalls from the margins of the active lava flow were observed during the inspection flight. The lava channel on the upper flank was full, and several terraces were observed in the distal parts of the flow where new lava units had overridden older units. Arcuate pressure ridges were common on the distal part of the lava flow.
"During the 1986 phase of stronger activity, spillover of lava from the summit crater has been occurring on the upper E flanks. Two adjacent scree deposits were observed there. This observation tends to confirm occasional reports of incandescence on this part of the volcano."
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 and C. McKee, RVO.