Report on Bulusan (Philippines) — June 1983
Scientific Event Alert Network Bulletin, vol. 8, no. 6 (June 1983)
Managing Editor: Lindsay McClelland.
Bulusan (Philippines) Two small phreatic explosions from summit crater
Please cite this report as:
Global Volcanism Program, 1983. Report on Bulusan (Philippines) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 8:6. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198306-273010.
12.769°N, 124.056°E; summit elev. 1535 m
All times are local (unless otherwise noted)
Mild phreatic explosions from the summit crater occurred at 0515 on 25 June and 0515 on 29 June. Eruption clouds reached heights of 1,000 and 1,200 m. Between 1 and 2 mm of ash fell on the NW quadrant of the volcano but no damage was reported. An intermittent increase in steaming from the summit crater and vents on the upper W flank was noted about a month before the eruption. A week before the first explosion, the temperature of hot springs 4 km S of the summit crater increased by 2°C and their discharge rate increased by 4 l/s. No increase in seismicity was noted.
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, PHIVOLCS, Quezon City.