On 24 November, PHIVOLCS reported that an explosion from Bulusan, recorded for almost six minutes by seismographs, produced an ash plume that rose 1 km above the crater, drifted SW, and dissipated. Ashfall was not observed outside of the permanent danger zone, defined as a 4-km radius from the summit, suggesting that ashfall was confined to the upper flanks. On 26 November steam was emitted from known thermal vents and the crater. A steam plume rose 150 m above the NW vent and drifted SW. Later that night an explosion-type earthquake was recorded by the seismic network; cloud cover prevented visual observations of the crater. On 28 November steam rose from the SE and SW vents. During 29 November-1 December steam rose from both the NW vents and thermal vents.

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)