Report on Bulusan (Philippines) — 16 February-22 February 2011
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 16 February-22 February 2011
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2011. Report on Bulusan (Philippines). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 16 February-22 February 2011. Smithsonian Institution and US Geological Survey.
12.769°N, 124.056°E; summit elev. 1535 m
All times are local (unless otherwise noted)
During 17-20 February, PHIVOLCS reported that up to three daily volcanic earthquakes at Bulusan were detected by the seismic network. Cloud cover mostly prevented observations of the summit area, although steam rose from NW thermal vents on 19 February. An explosion on 21 February produced a gray ash plume that rose 3 km above the crater and drifted SW. Rumbling sounds were heard up to 10 km away in the town of Juban (NW). The event was recorded by the seismic network as an explosion-type earthquake lasting for about 19 minutes. Field investigations confirmed ash deposits in the towns of Irosin (up to 5 mm), 8 km SSW, and Bulan (up to 3 mm), 22 km SW. Traces of ashfall were also reported in the municipalities of Juban and Magallanes (24 km WNW), and in Masbate City (70 km SW), Masbate. According to news articles, about 2,000 people evacuated. During 21-22 February 16 volcanic earthquakes were detected by the seismic network. The Alert Level remained at 1 (on a scale of 0-5).
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.