Phivolcs describes deadly Mayon episode as ‘small phreatic event’

MANILA, Philippines—Steam trapped in the crater of Mayon Volcano triggered a series of explosions on Tuesday morning, dislodging rocks and emitting gray to brown clouds that rose half a kilometer above the summit, seismologists said. Five trekkers were reported killed.

The Philippine Institute of Volcanology and Seismology (Phivolcs) described the episode as “a small phreatic event” characterized by steam and ash ejections that lasted 73 seconds. But the agency dispelled fears of any imminent magmatic eruption.

Phivolcs director Renato Solidum Jr. said the institute would maintain its Alert Level 0 status, indicating “quiet” volcanic activity, and all monitored parameters “within background levels.”

Even so, Phivolcs issued an advisory strongly advising the public to refrain from entering the six-kilometer  Permanent Danger Zone “due to the threat of stream-driven eruptions and rock falls from the upper and middle slopes of the volcano.”

“No volcanic earthquake was detected within the past 24-hour observation period. Seismic and gas emission parameters remain within background levels and indicate no intensification of volcanic activity,” Phivolcs said in a bulletin.

It said the phreatic event occurred at about 8 a.m. with the gray to brown clouds reaching 500 meters above the summit and drifting in a west southwest direction.

“With the above observations, Phivolcs is maintaining an Alert Level 0 status, which means that no magmatic eruption is imminent. However, small phreatic explosions, including small steam and ash ejections, may occur frequently with little or no warning,” it said.

In a press briefing, Solidum said there were no precursors to Tuesday phreatic episode, although steam-driven eruptions were within expectations under the current alert level.

Describing what could have caused the incident, he said water entered the crater and made contact with hot rocks below, producing steam.

“The steam was trapped. Normally, the steam would just leak out, but there was an obstruction. There are many possible causes of the obstruction, such as rocks that fell in  or precipitate that solidified, blocking  the steam,” Solidum said.

He said it was like a pressure cooker with boiling water that could no longer contain the steam inside and blew up.

Located in Albay province, Mayon Volcano is known for its almost perfect cone shape. It has an elevation of 2.46 kilometers, and a base diameter of 20 kilometers. Historically, it has erupted 49 times, the latest of which occurred in December 2009.