PHIVOLCS reported that during 0900-0918 on 23 June Bulusan's NW summit vent vigorously emitted dirty-white steam plumes that drifted down the WNW flank. The color of the plume suggested low-energy ash emissions; no seismic signals surrounding the event were detected, indicating a shallow source. Minor amounts of ash fell in the neighborhoods of Poblacion (11 km NW), Añog (12 km NW), and Bacolod (13 km NW) in the Municipality of Juban, and Mabini (12 km NNW) in the Municipality of Casiguran. A sulfur odor was detected in the neighborhoods of Mabini, Bacolod, Añog, and Puting Sapa (Juban). Steam plumes rose as high as 200 m during 25-26 June and drifted NW; steam plumes drifted down the flank on 28 June. The Alert Level remained at 1, indicating abnormal conditions and a 4-km-radius Permanent Danger Zone (PDZ).

Geological Summary. 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.

Source: Philippine Institute of Volcanology and Seismology (PHIVOLCS)