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Report on Manam (Papua New Guinea) — August 1990


Bulletin of the Global Volcanism Network, vol. 15, no. 8 (August 1990)
Managing Editor: Lindsay McClelland.

Manam (Papua New Guinea) Small ash eruption follows inflation and seismic changes

Please cite this report as:

Global Volcanism Program, 1990. Report on Manam (Papua New Guinea) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 15:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199008-251020


Papua New Guinea

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

All times are local (unless otherwise noted)

"A relative build-up of activity resulted in a small ash eruption from Southern Crater on 28 August. Prior to the 28th, both craters emitted weak white vapour but on 15-17 and 20 August, Southern Crater released emissions of blue vapour, low in volume. Weak low rumbling noises were heard from Southern Crater on the 17th. From 11 August, small-amplitude harmonic tremor was recorded with occasional large B-type earthquakes occurring after the 16th. At 1300 on the 28th, Southern Crater started to forcefully expel a dark column of ash to 500-600 m above the crater, accompanied by a sub-continuous weak rumbling sound with associated strong harmonic tremor. The emission decreased in vigour after 1345 and ended by 1415. A light ashfall occurred on the SW part of the island. Interestingly, a week before the ash eruption, the daily number of microearthquakes dropped from an average of 1000 to 500, and the seismic amplitude dropped by almost half. A 'normal' level of activity returned rapidly after this short eruptive phase and both Southern and Main Craters were again releasing very weak plumes of thin white vapours with a weak blue vapour plume from Southern Crater.

"Tilt measurements at Tabele Observatory showed a definite inflationary trend since late June and had accumulated up to 6 µrad by the end of August (and thus had more than recovered the deflation of April-May; 15:4-5)."

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: I. Itikarai and C. McKee, RVO.