Report on Bulusan (Philippines) — March 1980
Scientific Event Alert Network Bulletin, vol. 5, no. 3 (March 1980)
Managing Editor: David Squires.
Bulusan (Philippines) Occasional steam and ash explosions continue
Please cite this report as:
Global Volcanism Program, 1980. Report on Bulusan (Philippines). In: Squires, D. (ed.), Scientific Event Alert Network Bulletin, 5:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198003-273010.
12.769°N, 124.056°E; summit elev. 1535 m
All times are local (unless otherwise noted)
Occasional explosions continued through March. The 7 February explosion began at 0215, after an increase in the number of volcanic earthquakes. Cauliflower-shaped, ash-laden clouds rose about 6 km above the crater rim before drifting SSW. Rumbling and hissing sounds accompanied the activity. Milder ejection of ash-laden steam clouds started at 1102 the next day, with light ashfall continuing until late afternoon on the volcano's SW sector. Another weak ash ejection occurred 9 February.
Clouds and rain made observation of the summit and collection of ashfall data difficult in March. A strong odor of H2S was detected at San Benon station, 6 km SSW of the crater, at 0945-0950 and 1030-1200 on 10 March, but no ashfall was reported. On 22 March, light ashfall occurred from 1120 to 1135.
Three weak ejections of ash-laden steam clouds took place on 26 March. At 0449, a cloud rose to approximately 400 m above the crater rim before winds blew it NW. A second steam and ash column reached 500-700 m above the rim about 1200, and a third was ejected to 300 m height two hours later. About 2 mm of ash fell on a settlement 8 km NNW of the crater.
At 1525 the next day, a volcanic earthquake of intensity I on the Modified Rossi-Forel (MRF) scale was felt at Lake Bulusan station, 4.5 km SE of the crater. Rumbling and hissing sounds accompanied the earthquake. Ejection of ash-laden clouds was visible through weather clouds at the summit.
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.