LEGAZPI CITY — Mount Bulusan in Sorsogon spewed a 500-meter-high grayish ash column, which drifted towards villages in the town of Juban and Irosin, at about 5:01 p.m. Monday, the Philippine Institute of Volcanolgy and Seismology (Phivolcs) said.

Ed Laguerta, Phivolcs resident volcanologist, said the volcano exhibited two successive minor explosions that produced a grayish ash plume, followed by a steam and ash plume, which was 500 meters high, that drifted towards the west side of the volcano.

Laguerta said the eruption was a “phreatic” type as indicated by the two steam-driven explosions that took place.

The eruption consisted of two earthquakes, followed by rock-fracturing events a few second later by an explosion-type earthquake that lasted for about four minutes and 20 seconds.

Prior to the low-energy ash and steam ejection on Monday afternoon, Phivolcs seismic instruments detected no volcanic earthquake, although increased seismicity was recorded over the weekend.

In a phone interview, Laguerta said the easterly wind that prevailed triggered the ash fall to drift to the villages of Cogon and Bulos in Irosin town and Puting Sapa and Sangkayon in Juban town — areas located at the west side of the volcano.

The Phivolcs will examine on Tuesday the extent of ashfall that drifted towards the villages in Juban and Irosin towns.

Laguerta said the phreatic explosion might have been produced by hydrothermal fluid or steam.

He said the steam pressurization underneath the volcano edifice might have triggered the movements in the local earthquake faults, saying further that “it needs to be thoroughly evaluated.”

The Phivolcs official said it needs further close monitoring because the volcano, although still in restive state, did not show signs of magma movements signified by volcanic tremors or ground deformation.

Laguerta said Mt. Bulusan remained in a restive mode following series of eruptions that have taken place since June last year.

Alert Level 1 remains in effect and the Phivolcs warned the villagers not to venture in the 4-km Permanent Danger Zone due to sudden phreatic explosions.

It also told civil aviation authorities to advise pilots to avoid flying close to the volcano’s summit as ash from any sudden phreatic eruption can be hazardous to an aircraft and its passengers. PNA/