Mayon was elevated to Alert Level 1 on Sunday, indicating low-level unrest in one of the Philippines' most active volcanoes, Phivolcs said.
State vulcanologists noted a short-term swelling of Mayon's cone and a slight extrusion of "remnant lava" from its 2018 eruption.
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Though the volcano's average sulfur dioxide (SO2) emission in August was near baseline levels, a spike in low-frequency volcanic earthquakes was recorded in May and June even as sulfur emissions stayed at baseline levels.
This indicates gas-driven pressure from inside the volcano, Phivolcs said.
"In view of the above, PHIVOLCS-DOST is now raising the alert status of Mayon from Alert Level 0 to Alert Level 1. This means that the volcano is exhibiting abnormal conditions and has entered a period of unrest," Phivolcs said.
Entry into the six-kilometer permanent danger zone is prohibited due to the increased possibility of phreatic or steam-driven eruption, as well as rockfalls, avalanches and ash bursts at the summit area, Phivolcs said.
Phivolcs cautioned people living in valleys and river channels to be vigilant against sediment-laden streamflows and lahar or mudflows that may occur during prolonged and heavy rainfall, which Albay is known to experience during the rainy season.
"PHIVOLCS-DOST is closely monitoring Mayon Volcano’s activity and any new development will be immediately communicated to all concerned," it said.