Report on Bulusan (Philippines) — 29 April-5 May 2015
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 29 April-5 May 2015
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2015. Report on Bulusan (Philippines). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 29 April-5 May 2015. Smithsonian Institution and US Geological Survey.
12.769°N, 124.056°E; summit elev. 1535 m
All times are local (unless otherwise noted)
PHIVOLCS reported that at 0809 on 1 May a steam-and-ash explosion from Bulusan was detected for five minutes by the seismic network. Dense rain clouds at the summit prevented visual observations at the time, but during a clear period around 1030 gray-white steam plumes were observed rising 200 m above the NW vent and drifting WNW. Minor ashfall affected areas to the W and NW, including Bolos, Cogon, Gulang-Gulang, Sangkayon, Tinampo, and Umagom in Irosin, Sorsogon, and Puting Sapa in Juban, Sorsogon.
Only five volcanic earthquakes had been recorded during the past week prior to the event; after the event the network detected 62 volcanic earthquakes within an eight-hour period. Alert Level 0 and the 4-km restricted zone, the Permanent Danger Zone (PDZ), were maintained due to the possibility of sudden and hazardous steam-driven or phreatic eruptions.
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.