PHIVOLCS reported that a phreatic explosion occurred at 1440 on 29 December from a vent on Bulusan's upper SE flank. The seismic network recorded the event as an explosion-type earthquake that lasted about 16 minutes. A grayish ash plume rose 2 km above the vent and drifted WSW, causing minor amounts of ash to fall in areas downwind including the barangays of Cogon, Tinampo, Bolos, Umagom, Gulang-gulang, and Monbon of Irosin, and Caladgao and Guruyan of Juban. Residents of Guruyan, Juban, Monbon, and Tinampo of Irosin noted a sulfur odor. 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)