A few days ago Spica wrote a post asking us for our most dangerous volcanoes since she is going to give a speech at Ars Electronica in Linz. Normally I try to stay away from discussing megadeath volcanoes since the subject is fairly gruesome. But, I got intrigued by the notion and started thinking about what would be the most dangerous volcano on the planet.
First I had to think up a set of requirements to judge it by. The premiere one would have to be a lot of nearby residents. The second requirement would be that it has a history of frequent medium sized eruptions (VEI-3 to VEI-5). The third requirement is that it should have an eruptive style that poses a large threat to the residents. The fourth requirement is that it should be a known killer volcano, and the fifth one is that it should have the capacity for a large scale eruption (VEI-6 to VEI-7). For those who know the history of volcanic eruptions will know that this is a true recipe for disaster. With this list of requirements in the back of my head I compared it to the list of volcanoes known to me. It was surprising how many well known bad boys in the world that did not meet these requirements. In the end it was though one that climbed out on top of all the others by virtue of being in the top 3 in all of the categories.
Mount Mayon was the top contender, locally known as Bulkang Mayon after the heroine Daragang Magayon (Beautiful Lady in the local language). For those who love stories of love that end badly, read this one.
First, let us go through the list of requirements and see how Mayon fares. The volcano is encircled by eight cities and numerous villages. The combined population of the cities are 736 000 people, with an additional half a million living in the villages. All of these people are within a striking distance for a VEI-4 sized eruption. Almost all parts of this area has either been previously covered in up to 15 meters of ash, and/or been overrun by pyroclastic surges or lahars.
According to the Global Volcanism Program Mount Mayon has suffered 14 VEI-3 eruptions and 2 VEI-4 eruptions since 1616. That gives an average of one medium sized eruption every 25 years. This is a very high number if one sets it into relation with the high number of potential victims.
Mount Mayon has a very perfect and steep cone built out of alternating layers of ash, lava and pyroclastic deposits. This makes it into a very unstable volcano with a highly varied arsenal of deadly expressions. The number one historical killer has been lahars. Secondly comes the pyroclastic flows. The rest of the list covers suffocation due to ash and gas, rockslides, and incandescent bombs. The volcano also frequently suffers from unexpected phreatic detonations that occur without warning. Mount Mayon last killed people in May when climbers that had sneaked inside the 6km permanent danger zone got blasted by a phreatic detonation.
A realistic death toll for Mayon is about 50 000 people since 1616. This number has been achieved without any episode where a city has been hit directly. The highest known death toll for one single eruption is 2 200 people that died from lahars, suffocation, and ash fall. If there had been a lahar or pyroclastic surge hitting one of the cities the number would have been much higher.
Due to the shape and height of Mayon the aperture of the volcanic crater has grown increasingly narrow. This increases the pressure inside the system during an eruption. One could compare this with a rifle that has a barrel that is getting narrower over time. As one fires the rifle the bullet will be squeezed harder and harder for every shot with an increase in gas pressure behind the bullet. One day the rifle barrel will rupture, and the same goes for Mayon.
Basically this gives the same basic setting as for Pinatubo, as a large eruption happens the volcanic aperture cannot withstand the pressure and an explosive widening of the crater occurs. The other thing that could happen is that Mount Mayon will suffer a flank collapse like the one that happened at Mount St Helens. This is though unlikely since Mount Mayon erupts so frequently that the volcanic tubing is in a state of being semi-open all the time. A flank collapse requires a lot of pressure building up prior to an eruption, but in the case of Mount Mayon that would cause a normal eruption to occur at an earlier stage.
Due to the mountains height it exerts a tremendous downward strain on the underlying bedrock. At the same time magmatic intrusions lift the mountain upwards as the volcano inflates. The highest downward pressure is in the center of the volcano, and the maximum upwards push is at the outer edges of the volcanic system. This over time creates a ring fault running around the volcano. This is a permanent weakness and as the systemic pressure increases over time as the crater aperture diminishes more and more stress will happen at the ring fault line. And when the pressure becomes too high a flank vent eruption will occur somewhere along the ring fault line.
I think I should explain how high the volcano is compared to its prominence. The volcano is 2 463 meters high above oceanic level. That does not sound much compared to some Andean giants. But here is the thing. If you measure from the bottom of the volcano to the top (prominence) it is 2 447 meters high. Or in other words, the volcano rises from a height of only 16 meters above sea level. This gives the volcano an ample supply of water to drive explosive eruptions.
Sooner or later the volcano will start to have predominantly flank vent eruptions until the ring fault gives way and the volcano goes down in the middle and the entire ring fault line erupts and the ring fault is almost down at sea level giving the flank eruptions a higher explosive force due to available water. This is your basic somma caldera formation. Mount Mayon is quite near this stage, but near means within geological time. Due to the number of flank vent eruptions we do know that Mayon is about as close to this happening as is possible, but it will most likely take a long time from a human perspective. It could though occur if a larger than normal eruption took place, say for instance during a Pinatubo style widening of the crater aperture.
Mount Mayon is my candidate for deadliest volcano around since it could quite easily kill more than 100 000 people during a medium sized eruption that takes a bad route and also since it could have a VEI-6 to VEI-7 eruption that would directly kill between 1 000 000 to 5 000 000 people in the initial stages. She is equally beautiful as deadly.