Report on Bulusan (Philippines) — April 1981
Scientific Event Alert Network Bulletin, vol. 6, no. 4 (April 1981)
Managing Editor: Lindsay McClelland.
Bulusan (Philippines) Ash ejection and seismicity
Please cite this report as:
Global Volcanism Program, 1981. Report on Bulusan (Philippines) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 6:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198104-273010.
12.769°N, 124.056°E; summit elev. 1535 m
All times are local (unless otherwise noted)
Bulusan began to eject ash on 9 April at 1008, its first such activity since 28 September 1980. Eruption clouds reached 4 km height and deposited 4 mm of ash on villages on the W side of the volcano. AFP reports that the 9 April eruption lasted about 8 hours.
A second episode of ash ejection started 15 April at 1858. Lightning flashes were observed within the eruption clouds, which reached an approximate height of 8 km. Most of the ashfall was again on villages W of Bulusan, with accumulations of as much as 6 mm.
An earthquake swarm began 20 April. During the next seven days, 1,812 events were recorded, 64 of which were felt at MM I-V on the Modified Mercalli Scale and were accompanied by rumbling. On 27 April at 1745, renewed explosive activity produced ashfalls on villages SW of the crater. Clouds obscured the summit, preventing determination of the eruption column height. Seismicity has declined since then. No additional eruptive activity had occurred as of 11 May.
Geologic Background. Luzon's southernmost volcano, Bulusan, was constructed along the rim of the 11-km-diameter dacitic-to-rhyolitic Irosin caldera, which was formed about 36,000 years ago. It lies at the SE end of the Bicol volcanic arc occupying the peninsula of the same name that forms the elongated SE tip of Luzon. A broad, flat moat is located below the topographically prominent SW rim of Irosin caldera; the NE rim is buried by the andesitic complex. Bulusan is flanked by several other large intracaldera lava domes and cones, including the prominent Mount Jormajan lava dome on the SW flank and Sharp Peak to the NE. The summit is unvegetated and contains a 300-m-wide, 50-m-deep crater. Three small craters are located on the SE flank. Many moderate explosive eruptions have been recorded since the mid-19th century.
Information Contacts: O. Peña, COMVOL, Quezon City; AFP.