Mt. Bulusan
Mt. Bulusan
Restive Mt. Bulusan in Sorsogon spewed ash last Monday evening, generating sulfuric odor and rumbling sound, according to the Philippine Institute of Volcanology and Seismology (Phivolcs).

Based on Phivolcs' seismic record, the minor phreatic eruption, which was observed at 10:29 p.m., lasted for approximately 12 minutes.

Phivolcs explained that phreatic eruption is driven by steam produced by heat and expansion of ground water due to an underlying hot source.

The eruption last Monday could not be observed visually due to thick clouds covering the summit, but traces of ash, sulfuric odor and rumbling sound were reported in Barangays Monbon and Cogon in Irosin.

Meanwhile, sulfuric odor was observed at Barangay Bolos, also in Irosin.

In the past 24 hours, Mt. Bulusan generated three volcanic earthquakes.

Weak to moderate emission of white steam plumes that rose up to 50 meters before drifting northeast was observed coming from the active vents.

Sulfur dioxide emission was measured at an average of 8 tonnes/day, as of April 29. Sulfur dioxide emission tends to increase through time as magma degasses with increasing rates as it moves up from great depths beneath the volcano, Phivolcs explained.

The precise leveling data obtained from January 29 to February 3, 2017 indicated deflationary changes since October 2016.

"This is consistent with data from continuous GPS measurements that indicated short-term deflation of the edifice since January 2017," Phivolcs said.

Many volcanoes are known to have inflated before eruptions.

"However, the edifice remains generally inflated with respect to July 2016 based on continuous GPS data," it added.

Alert Level 1 remains in effect over Mt. Bulusan, which means that the volcano continues to exhibit "abnormal" activities.

Phivolcs said hydrothermal processes are underway beneath the volcano, which may lead to more steam-driven eruptions.

The local government units and the public were reminded that entry to the four-kilometer radius permanent danger zone is strictly prohibited due to the possibility of sudden and hazardous steam-driven or phreatic eruptions.