Report on Bulusan (Philippines) — 15 March-21 March 2006
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 15 March-21 March 2006
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2006. Report on Bulusan (Philippines). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 15 March-21 March 2006. Smithsonian Institution and US Geological Survey.
12.769°N, 124.056°E; summit elev. 1535 m
All times are local (unless otherwise noted)
On 21 March at 2258, a modest ash explosion occurred at Bulusan's summit crater. The phreatic explosion produced an ash cloud that rose ~1.5 km above the volcano (or 10,050 ft a.s.l.). Based on interpretations of seismic data, the event lasted ~20 minutes. It was accompanied by lightning and rumbling sounds. Ash drifted N, W, and SW of the volcano, and ~1 hour after the explosion light ash fell (producing ~5-mm-thick deposits) in Barangays (neighborhoods) Cogon, Tinampo, Gulang-Gulang, and Bolos in the town of Irosin. Ash also fell in Barangays Puting Sapa and Bura-Buran of Juban town, and other neighboring barangays under the municipalities of Irosin and Juban, Sorsogon. Three explosion-type earthquakes were also recorded on the 21st, at 2330, 2332, and 2337, but the accompanying eruptive events were not observed because the summit was obscured.
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.