The increase in the level of venting activity . . . continued into May. Throughout the month the summit crater emitted moderate to thick white vapor, although there were occasional reports of gray and blue emissions on 17 and 18 May, and towards the end of the month. On 23 May, because of the ash cloud, pilots in the region were notified to "exercise caution and to report any increase in activity including height and movement of the ash cloud." In addition, during most nights in the first three weeks of the month the crater emitted a red glow that remained weak but steady.

May seismic activity underwent a slight progressive decrease: Daily earthquake totals early in the month were in the range 400-600; by month's end they had dropped to 400. Since the end of April earthquake amplitudes also decreased.

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: I. Itikarai, and C. McKee, RVO; BOM, Darwin.