It also noted that a "faint crater glow" was observed on Saturday evening and Mayon has been "slightly swelling" since October and November a year ago.
It was followed by other phreatic eruptions early morning on Sunday.
"This means that current unrest is probably of magmatic origin, which could lead to more phreatic eruptions or eventually to hazardous magmatic eruptions", the Phivolcs said in its latest bulletin released on 10:50 a.m. local time.
He said not all residents evacuated live inside the 6-km danger zone.
Traces of ash fell on Barangays Anoling, Sua, Quirangay, Tumpa, Ilawod and Salugan of Camalig and in Barangays Tandarora, Maninila and Travesia in Guinobatan. In case of ash fall events, which are most likely to affect communities on the western and southwestern flanks downwind of Mayon's crater, people should cover their nose and mouth with damp, clean cloth or dust mask.
Aircraft are also advised not to fly too close to the summit.
Photo of Mayon Volcano covered with thick clouds on the right side and grayish ashes at the left side.
Trump denies vulgar remarks about Haiti, African countries
And Haiti's USA ambassador told the BBC that Haitians are not "simply immigrants who come here to take advantage of the U.S". The Haitian government has summoned a US official to explain the comment, which Trump on Friday said he did not make.
Government seismologists said ashfall was experienced in Camalig town in Albay, which has already been affected by the two earlier eruptions.
People living outside the danger zone but on the slopes of volcano were warned to take precautions against potential roof collapses due to the weight of ash and rainfall.
"We have not reached the critical level", said the head of the Albay Provincial Disaster Risk Reduction and Management Office, Cedric Daep, as cited by Reuters.
"Alert level 3 is what we considered critical, 4 is when eruption is imminent, and 5 is eruption in progress", he said, adding that people in evacuation centers may be allowed to return homes if Phivolcs does not raise the level.
Mayon's most violent eruption was in 1814, when more than 1,200 people were killed and a town was buried in volcanic mud.
Prior to this phreatic eruption, Mayon's edifice has undergone inflationary changes or a slight swelling of the edifice as indicated by ground deformation data recorded by continuous Global Positioning System and tilt since October and November 2017, respectively.
Its last eruption came in 2014 when thousands were forced to evacuate.
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