On 23 December, PHIVOLCS reported that an explosion from Bulusan's summit crater was recorded by seismographs as an explosion-type earthquake following tremor that lasted for almost 31 minutes. A grayish steam-and-ash plume rose to about 500 m above the crater rim and drifted S-SW. Based on information from PHIVOLCS, the Tokyo VAAC reported that on 24 December an ash plume rose to an altitude of 2.1 km (7,000 ft) a.s.l. and drifted SW.

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.

Sources: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Tokyo Volcanic Ash Advisory Center (VAAC)