"A brief but powerful eruption took place at Ulawun between 6 and 7 October. The visible commencement of the eruption was late at night on 6 October. At about 2300, faint glow was seen at the summit, intensifying at intervals of 1-2 minutes, presumably signifying weak ejections of incandescent lava fragments. At 0400 on the 7th the lava ejections were more frequent, occurring at intervals of 1 minute or less. No sounds of the eruption had been heard up until this time by observers at Ulamona Catholic Mission, and no significant ash emission had been observed.

"After 0600, weak emissions of dark ash were seen from Ulamona. Observers in aircraft approaching from the NE noted that at about 0640 the emission cloud above Ulawun was slightly more voluminous than normal, and was reported as pale to dark. At about 0700, a series of strong explosions commenced, heard as deep rumbling at Ulamona. Within 10 minutes the top of the eruption column had reached about 4 km above sea level. Red incandescence was seen at the base of the column. By about 0715 the eruption column had grown to about 7-10 km in height. It was vertical and straight-sided right up to its top, where slight lateral expansion had begun.

"Small pyroclastic avalanches were seen shortly after the beginning of the strong explosive activity. At about 0720 they were reportedly much larger and descended all flanks of the volcano, particularly the N and SW flanks. All observers reported that these avalanches originated directly from the crater, and were not formed by collapse of the eruption column. They were described as not moving quickly down the volcano's flanks. Between 0720 and 0730 several particularly strong explosions occurred, accompanied by visible shaking of the volcano, likened to the initial shaking seen in quarry blasts. The eruption column was reinforced by these explosions, and the reports of directly associated pyroclastic avalanches may be interpreted as base surges. However, pyroclastic avalanching was also reported to have been more or less continuous during this period.

"By about 0735 the upper part of the eruption column was spreading out more noticeably and the clouds on the volcano's flanks had become more voluminous. The diameter of the `mushroom' top of the cloud was estimated at 50-60 km. The eruption column continued to be fed by apparently frequent explosions in the crater but only slight upward growth was evident. The volcano gradually became obscured as the eruption cloud began to dissipate. By 1000 the volcano was totally obscured down to its base, and at Ulamona the darkness was total for several hours. The ash cloud drifted slowly towards the SW and was thick enough to cause 1 hour of total darkness in mid-afternoon at Bialla, 45 km from the crater.

"Observations from the flank of a neighbouring volcano indicated that strong explosive activity continued until about 1215. Until this time a violent electrical storm had prevailed in the eruption cloud. The cessation of electrical discharges coincided with the cessation of strong explosions.

"At about 1800 the ash had cleared sufficiently to allow observations of the summit from Ulamona. No ash emission was seen, but through the night a faint glow was present above the crater. Bursts of glow on the upper NE flank and associated clouds of ash on the same flank were observed from Ulamona.

"No further ash emissions from the summit crater were seen. However, ash clouds were seen occasionally for several days on the upper N flanks, and at night spots of incandescence were seen in the same place on the volcano. The ash clouds may have originated as slides of unstable parts of the cone, and in one case a large ash cloud may have been produced by explosive interaction of meteoric water and hot parts of the cone.

"No lava flows were produced during the eruption. Estimation of the total volume of fragmental flows awaits receipt and analysis of aerial photographs, but the volume is probably greater than that of the 1978 pyroclastic avalanche deposits (about 17 x 10⁶ m³ of juvenile material). A preliminary estimate of the volume of airfall ash is 10-20 x 10⁶ m³. The lava appears similar to the basaltic material produced in previous eruptions.

"Apart from the addition of a veneer of deposits from the pyroclastic avalanches and airfall ash, the topographic changes to the volcano brought about by the eruption include the formation of a series of gouges on the upper NE flank and the reaming out of an enlarged summit crater. The crater is now slightly elliptical with its larger diameter, estimated at 100-150 m, oriented E-W. The inner walls of the crater are steep, and the deepest part of the crater floor is about 60-100 m below the rim.

"Since the installation of a short-period vertical component seismograph at Ulamona in December 1976, B-type volcanic earthquakes have been common, sometimes occurring in swarms. During 1980 these events often occurred at intervals of less than 2 minutes, but during the night of 6 October they became more frequent, resembling patchy volcanic tremor. Several strong local earthquakes, probably A-type volcanic events, were recorded on 3, 5, and 6 October.

"At the commencement of the strong, visible activity, the seismic activity intensified dramatically, becoming continuous tremor, which persisted until about 1215 on 7 October. After that time, tremor ceased altogether, signifying cessation of the eruption."

Geological Summary. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: C. McKee, RVO.