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Report on Manam (Papua New Guinea) — June 1987


Scientific Event Alert Network Bulletin, vol. 12, no. 6 (June 1987)
Managing Editor: Lindsay McClelland.

Manam (Papua New Guinea) Violent Strombolian eruption; pyroclastic flows

Please cite this report as:

Global Volcanism Program, 1987. Report on Manam (Papua New Guinea) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 12:6. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198706-251020


Papua New Guinea

4.08°S, 145.037°E; summit elev. 1807 m

All times are local (unless otherwise noted)

"A short-lived phase of violent Strombolian activity occurred at Southern Crater on 30 June, producing pyroclastic flows, minor lava flows, and moderate scoria and ashfalls. This activity followed moderate Strombolian activity that had peaked between 10 and 24 June.

"Weak to moderate Strombolian activity similar to that of late May occurred at Southern Crater in early June. Incandescent lava was ejected several times/minute and ejecta reached maximum heights of ~200 m. The ash content of emissions was low. Activity began to increase about 3 June and reached a peak on 10-12 June when lava was ejected to ~800 m and blue vapour was strongly emitted. A temporary increase in ash emission on 15 June fed a 800-m-high curved plume that extended ~1 km. Between 10 and 24 June, seismicity fluctuated at a moderate-high level before declining. During the latter part of this period, lava ejections rose a few hundred meters and an ash column up to 500 m high crowned the summit. A small scoria cone (~20 m) formed around the active vent in Southern Crater. Small debris avalanches from the cone's flanks flowed into the SW and SE valleys.

"Although there were visible and seismic indications of declining activity in late June, a sharp increase in activity took place on 30 June. Seismicity began increasing in the morning and Strombolian activity strengthened progressively. At 2140, activity suddenly culminated in 2 exceptionally large ejections of lava that reached over 500 m height. A continuous eruption of a >2-km-high ash column followed.

"Eyewitnesses on mainland New Guinea saw cascades of glowing lava fragments descending into the SW valley during the 30 June eruption. Vivid lightning was observed, first only at the base of the eruption column, then after a few minutes throughout the dense eruption cloud. The cloud was blown ~20 km W in the following hour and lightning discharges continued in and around the cloud every few seconds.

"A mild roaring accompanied the eruption which generated several pyroclastic flows that descended ~2.5 km into the SW valley and ~3.5 km into the SE valley. The flows were 20-30 m wide and overlapped near their source, but with distance flow width narrowed to 10-20 m. Most flows were 1-2 m thick. The clasts in the flow deposits were generally small, the largest ~30 cm. However, large accidental blocks up to 5 m in diameter were picked up by a stronger flow in the SE valley.

"Airfall scoria was concentrated in a narrow sector downwind of the eruption column. The deposit was ~2 km wide on the W coast, ~5 km from the source. The maximum scoria thickness was ~30 mm and the largest scoria clasts were ~60 mm, with an average of ~10 mm. The scoria clasts were highly vesicular and contained small olivine phenocrysts, appearing to be basaltic in composition. Small rafts of scoria were observed offshore on the morning of 1 July, but most had sunk by later that day. Ashfall continued after the scoria fall had ended. The dispersal axis during ashfall migrated N (~1 km at the W coast), relative to the dispersal axis of the scoria fall, indicating a slight wind shift. The maximum ash thickness was ~5 mm. Very light ashfall took place on Boisa Island ~12 km NW of the summit, and near Awan on the mainland ~20 km W of Manam. No human injuries resulted from the scoria fall but some birds were reported to have been killed and some food gardens were damaged.

"Two minor lava flow sources were identified at the scoria cone at the top of Southern Crater. One flowed through a chute into the SW valley, where the lava flow broke up as it cascaded down the valley headwall. The other flow appeared to breach the scoria cone's E flank but may have been partially welded scoria. A lobate flow was emplaced on the upper part of the talus fan heading into the SE valley. The flows may only have been active for a short time.

"Southern Crater activity decayed rapidly following the end of the violent Strombolian phase. No incandescent lava ejections were seen after 2300 on 30 June, but thick ash clouds continued to be released throughout the following day. Seismicity had returned to the levels of late May by about 7 July. Main Crater emitted vapour and possibly ash and showed weak fluctuating glow throughout June. Main Crater glow appeared brighter after activity at Southern Crater declined at about 2300 on the 30th. No significant changes in tilt were observed in June, although readings tended to be somewhat erratic.

"Aerial Inspections in early July indicated that there had been no rise of magma in Main Crater, which was at least 100 m wide, 70 m deep, and funnel-shaped. White and blue vapours were being emitted from an incandescent vent of ~10x3 m at the base of the crater. Southern Crater activity at this time consisted of 3-6 weak Strombolian ejections/minute, rising to the level of the rim from the 60-m-deep base of the funnel-shaped crater. Thin orange-brown ash emissions from Southern Crater and the blue and white vapours from Main Crater rose gently to ~200 m above the summit before being blown W and dispersing ~2-3 km downwind."

Geological Summary. The 10-km-wide island of Manam, lying 13 km off the northern coast of mainland Papua New Guinea, is one of the country's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical basaltic-andesitic stratovolcano to its lower flanks. These valleys channel lava flows and pyroclastic avalanches that have sometimes reached the coast. Five small satellitic centers are located near the island's shoreline on the northern, southern, and western sides. Two summit craters are present; both are active, although most observed eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent eruptions, typically of mild-to-moderate scale, have been recorded since 1616. Occasional larger eruptions have produced pyroclastic flows and lava flows that reached flat-lying coastal areas and entered the sea, sometimes impacting populated areas.

Information Contacts: C. McKee and P. Lowenstein, RVO.