"A new eruptive phase started in October 1984. The extrusion of a fresh batch of andesitic magma into the summit crater was accompanied by a marked increase in the volume of vapour rising above the crater, an increase in the area of fumarolic activity in and around the crater, night glows, and incandescent material tumbling down the flanks. Simultaneously, volcanic seismicity increased from fewer than 10 B-type events per day to over 100 per day by 19 October; harmonic tremor appeared on the 12th and became sub-continuous after the 15th. A relative drop in volcano seismicity (24-27 October) was followed by re-intensification. The daily frequency of events was about 1000 by 11 November, and consistently above this after 15 November. Strong tremor was recorded for periods of several hours on 4, 5, 9, 13, 18, 20, and 22 November. Explosion earthquakes were occasionally recorded.

"An aerial inspection by Bougainville Island Copper Ltd. geologists revealed that the dome of viscous andesite had bulged to about 15 m above the crater rim and lava was spilling over the N, E, and W parts of the rim. Debris on these three flanks corroborated the observations of incandescent material avalanching down the sides of the volcano, presumably from collapse of parts of the dome. Paradoxically, the long-established lava flow channel on the N flank of the volcano seems to have been drained, leaving an empty lava channel from the crater rim down to about 1,100 m altitude."

Geological Summary. Bagana volcano, in a remote portion of central Bougainville Island, is frequently active. 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 characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although occasional explosive activity produces pyroclastic flows. Lava flows with tongue-shaped lobes up to 50 m thick and prominent levees descend the flanks on all sides.

Information Contacts: P. de Saint-Ours, RVO; K. McCue, Bougainville Island Copper Ltd.