Bulusan Volcano in Sorsogon was raised to Alert Level 1 on Sunday after a phreatic eruption that spewed a steam-rich grey plume, state volcanologists said.
The phreatic eruption, which was caused by boiling of water at the volcano's crater, was recorded at 10:37 a.m. and lasted for around 17 minutes, Phivolcs said.
A steam-rich plume of at least one kilometer drifting west was observed from Juban, Sorsogon, with ashfall reported in the same area and in Casiguran, Sorsogon.
Prior to the eruption, Phivolcs recorded a total of 77 volcanic earthquakes over the past 24 near Bulusan Volcano.
The Alert Level 1 status of Bulusan Volcano indicates that it is in an "abnormal condition", Phivolcs said as it advised local government units and the public to avoid entry into the four-kilometer radius Permanent Danger Zone.
Pilots must also avoid flying close to the volcano's summit as ash from sudden phreatic eruption can be hazardous to aircraft.
People living within valleys and along river or stream channels especially on the southeast, southwest and northwest sector are also advised to monitor stream flows and lahars in cases of heavy and prolonged rainfall.
Phivolcs director Renato Solidum said they would continue monitoring the situation at Bulusan Volcano.