"After several weeks of intensified seismicity and occasional small phreatic explosions, a mild Strombolian eruption began on 4 September and ended on 11 September.

"Previous Ulawun eruptions developed rapidly (within hours) to full-scale eruptions, including lava effusion and paroxysmal explosive activity culminating in the formation of pyroclastic flows. However, the 1984 eruption developed slowly from the first appearance of summit glow on 4 September, and no effusion or paroxysmal activity took place. The eruption reached a peak 6-9 September when mild, rhythmic, almost ash-free, Strombolian explosions took place every few seconds.

"During an aerial inspection on 7 September, a pool of fluid lava was observed in the bottom of the summit crater, about 50-100 m below the rim; explosions took place from a small circular central vent. Most ejecta were contained by the crater walls, but occasional larger explosions showered the upper flanks of the volcano with incandescent tephra. Visible shock waves were generated by the stronger explosions. A white eruption plume several tens of kilometers long was formed at the peak of the eruption. Seismic amplitudes were 30-40 times normal on 7 and 8 September, and the seismicity was characterized by irregular tremor produced by the frequent explosions.

"Regular reports and advice were transmitted by RVO to provincial government authorities throughout, and reactions to the eruption were controlled and rational, resulting in minimal disturbance to daily life."

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.