"A mild eruption took place between 1 and 10 January. Frequent ejections of dark grey ash were observed rising as much as 2 km above the summit. These produced light ashfalls on the SE side of the volcano. Bright red glow from the summit crater was observed during the ejections. During the eruption, visible activity often coincided with regular periods of low-amplitude, high-frequency tremor. Included in the tremor were discrete B-type events. This pattern of seismicity had begun on 26 October 1988. There was a slow, irregular increase in amplitude through November and early December, but a significant increase took place in mid-late December. Seismicity declined during the eruption."

"Dry tilt and EDM measurements were carried out on 2 and 12 January. Compared with the previous tilt measurements in June 1988 (SEAN 13:06), inflationary tilts of as much as 19 µrad had accumulated by 2 January. Further inflation of up to 16 µrad took place between 2 and 12 January. These tilt changes were recorded at stations within a few km of the summit. EDM measurements were inconsistent. From 11 January, mainly white vapour emissions from the summit crater were observed. On some days, emissions appeared to contain ash. The pattern of regular periods of seismic tremor disappeared and was replaced by occasional small B-type events.

"There are some indications that the eruption was phreatomagmatic. Microscopic examination of ash samples indicates that the ash is very fine grained with a large proportion of accessory material. However, some fresh dark glass is also present. Secondly, the significant increase in seismicity in mid-late December coincided with a period of heavy rainfall that may have triggered the eruption. On the basis of tilt measurements, it is speculated that a magmatic eruption could still take place, although it is not considered to be imminent."

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, H. Patia, and P. de Saint-Ours, RVO.