Mount Mayon, the Philippines’ most active volcano, was erupting like a fountain as molten lava poured down the side, according to experts from the Philippine Institute of Volcanology and Seismology (Phivolcs).
Lava had advanced almost a mile and a half from the volcano’s crater and ash reached up to 2km as it fell on nearby villages and communities.
Several small pyroclastic flows were generated by fragments in the lava streams and not by an explosion from the crater vent, like occurred with Mount Pinatubo, according to Phivolcs chief Renato Solidum.
Pyroclastic flows are superheated gas and volcanic debris that can race down slopes and incinerate everything in their path, and are feared in a major eruption.
Mr Renato Solidum said: “The pyroclastic flows, there were several, were not generated by an explosion from the crater with lava, molten rocks and steam, shooting up the volcano then rolling down.
“These were generated by lava fragments breaking off from the lava flow in the upper slopes.”
The potentially deadly lava flows have prompted the provincial government to shut more schools after Phivolcs recorded nine episodes of tremors, four of which accompanied lava fountains.
It also noted 75 lava collapse events as pressure builds up leading to lava flows and ash plumes, reiterating its warning that a hazardous eruption could happen any time.
Mount Mayon, a volcano in the coconut-growing central Bicol region that draws tourists because of its near-perfect cone shape, has shown increased restiveness since Saturday, displacing thousands of residents.
Nearly 15,000 people have fled the danger zone within four miles of Mayon, and the institute strongly advised people not to re-enter the area.
Phivolcs said the advancing lava and pyroclastic flows had reached the six-kilometre radius no-go zone, from which some residents fled.
“Alert level 3 remains in effect over Mayon Volcano, which means that it is currently in a relatively high level of unrest as magma is at the crater and hazardous eruption is possible within weeks or even days,” it said.
Alert level 4 means an eruption is possible “within days” while level 5 is when a hazardous eruption is under way.
The Albay provincial government has expanded its class suspension order to include more towns around the 2,462-metre (8,077-foot) volcano, and advised travellers to avoid ashfall-hit villages amid poor road visibility.
Class suspensions have allowed the government to use schools as temporary shelters for displaced people.
The Philippines is part of the Pacific “Ring of Fire” of islands that were formed by volcanic activity, and is perennially under threat from 22 active volcanoes.
The most powerful explosion in recent years was the 1991 eruption of Mount Pinatubo, about 100 kilometres northwest of Manila, which killed more than 800 people.
David Rothery, a geosciences professor at The Open University said it was highly unlikely Mayon would have a similar eruption, adding: “It erupts quite often, and volcanoes that erupt frequently tend to have smaller eruptions than those that erupt less frequently.”