During the early weeks of Mayon’s unrest, some netizens, whose only evidence were the photos their o online friends were sharing, criticized the Philippine Institute of Volcanology and Seismology (Phivolcs) for not raising the alert level of Mayon Volcano to the highest 5 especially after seeing photos of its eruption columns online.
But Phivolcs Director Renato Solidum said that netizens must let the volcanologist at Phivolcs interpret Mayon’s activities and declare its alert level.
“If you look at those eruptions, those are not very tall columns, at most five kilometers. And this essentially would mean that the lava flow or the ‘uson’ (pyroclastic density currents) are confined within the 6 kilometer danger zone. Essentially, the activities are Strombolian activities. That is not the most explosive eruption,” Solidum explained.
Strombolian eruptions, which are eruptions wherein lava fountaining and lava effusion can be observed, are minor eruptions compared to much explosive eruptions like Vulcanian, Peléean, and Plinial eruptions.
The 1968 and the second phase of the 1984 eruptions are Vulcanian in nature, while the 1814 eruption that killed 1,200 Albayanos was a Plinian eruption. In a Vulcanian eruption, magma and magmatic gas are released high into the atmosphere by a great force resulting to large eruption clouds. In a Peléean eruption, glowing avalanches made of dense, hot gases are present along with the large amount of gas from the magma, which is now lava, that causes pyroclastic materials. In the violent Plinian eruption, there is a larger quantity of pyroclastic flow with pumice and the eruption collapses the top of the cone resulting to a caldera.
While the criteria for every alert level varies in different volcanos, Solidum said that in Mayon Volcano’s Alert Level 5, where eruptions are sustained for even hours, the eruptions are typically Vulcanian or Plinian.
“When you look at the alert level description, you need to be looking at all the criteria and what the volcano can show in the very near future,” Solidum said.
He said that when Mayon’s alert level was raised to 4, the height of the ash column was not because of the powerful nature of the explosion. The columns looked tall because the lighter ash and steam were lofted to a higher elevation due only to buoyancy.
“In very explosive eruptions, the force will really be maximized to very tall heights. [Mayon’s] eruption was not sustained. What you are seeing is the typical Strombolian phase of the activity. There is regularity, but Alert Level 5 will not be like that. In fact, last night, the activity was characterized not by explosions but just simply continuous flow of lava,” he said last January 26.
He said that the scenarios that they are looking at still stand. One scenario is that the Strombolian activity will transform into a Vulcanian activity just like in the 1968, 1984, 2000, and 2001 eruptions after Mayon lulled visually.
“The climactic part of those eruptions are Alert Level 5. Those are very tall columns and extensive ash fall and pyroclastic flows due to column collapse,” he said. Ash columns may collapse if they become dense in the air.
He also said that in an Alert Level 5 scenario the sustained eruption produces pyroclastic density currents that reaches farther down and so far, Mayon has only generated short and short-lived pyroclastic flows.
“The ideal thing is that people should be evacuated before the occurrence of Alert Level 5,” Solidum said.
The provincial government of Albay has recommended the evacuation more than 85,000 residents inside the 9-kilometer radius of Mayon. This includes a kilometer buffer from Phivolcs’ recommended 8-kilometer radius.
Solidum explained that in the past, Mayon’s pyroclastic density currents and lava are confined well within the 7-kilometer radius of Mayon Volcano.
“At Alert Level 5, once you see the hazardous eruption occur and it is sustained, we have to evaluate the danger zone we declared at 4 if it will be maintained or increased depending on the volcano’s activity,” he said.
In some volcanos, Solidum said that pyroclastic density currents occur even at Alert Level 1.
“In some cases, PDCs are too damp. When it has a lot of water in it that you can actually see grayish and whitish colors in different parts of the column and it can help in the slow collapse of the mass,” Solidum said. “It does not mean that when you have a PDC and it can reach farther, that it is already dangerous.”
Mr. Solidum then showed me this photo of Kanlaon erupting and asked me what alert level this photo was taken from. You’d be surprised: Alert Level 1.
UPDATE: While Phivolcs has said again and again that visually, Mayon may look like its activities has weakened, but it may still be capable of a major Vulcanian eruption, the provincial government of Albay has decamped some of the evacuees from the 8- to 9-kilometer danger zone. This is still well in compliance with Phivolcs’ recommendations.
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