Bulusan is classified by volcanologists as a stratovolcano (or a composite cone) and covers the northeast rim of Irosin caldera that was formed about 40,000 years ago.

The volcano's hot springs are: Adjacent volcanic edifices are: Bulusan is generally known for its sudden, steam-driven or phreatic explosions.

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) declared alert level 1 on March 19, after it recorded increased seismic unrest.

A cloud of steam and ash shot into the air from Mount Bulusan to a height of 5 kilometres (3.1 mi), drifting and blanketing the surrounding countryside.

State volcanologists noted that at least 10 volcanic earthquakes and four explosion-type events were documented after the emission of ash and steam at 8 a.m. on November 5.

A PHIVOLCS bulletin on November 9 stated that eight volcanic earthquakes were recorded from the volcano during the past 24 hours and weak to moderate emission of ash explosion was observed.

[8] After months of little activity, the volcano suddenly erupted and released a plume of ash 2 kilometres (1.2 mi) high, after water reacted with the hot magma.

The incident was classified as a "minor explosion event" by PHIVOLCS leading to the raising of alert level 1 (abnormal activity) for the surrounding area.

Further advisories were given to low-flying aircraft against the dangers of sudden phreatic eruptions, and to local residents near rivers and streams in case of lahar flows.

[16] On June 16, Mount Bulusan generated two volcanic eruptions with the first one recorded at 11:02 am which lasted for 10 minutes producing a one kilometer high grayish steam and ash plume and accompanied by rumbling sounds.

[19] According to PHIVOLCS, the eruption was phreatic in nature and had a shallow source, with the ash drifting northwest to the direction of Juban town.

[21] On December 29, Mount Bulusan had a phreatic eruption, shooting a plume of ash about two kilometers high.

The eruption prompted PHIVOLCS to raise Alert level 1 for the surrounding areas and remind the public not to enter the 4-kilometer radius permanent danger zone.