Frequency of Large Volcanic Eruption in Southeast Asia

Southeast Asia is home to 50% of the world’s active and dormant (not erupting but is expected to erupt again) volcanoes. These volcanoes have produced some of the largest eruptions on record.  The 1991 eruption of Pinatubo was the 3^rd largest eruption of the 20^th century. The eruption of Krakatoa in 1893 resulted in the deaths of over 36,000 people. The eruption of Toba in 1815 released enough volcanic gas to cover the entire earth, leading to massive crop failures and killing more up to 100,000 people globally. Despite large hazard risk, the area has not been rigorously evaluated for eruption frequency due to difficult terrain and dense vegetation. In comparison, these sorts of analyses have existed for Europe and other parts of the world since the 1980’s.

A new study published in the Bulletin of Volcanology uses satellite imagery to classify volcano morphology to determine the decadal chance of different eruption sizes, or VEIs, for the region. The VEI, or Volcanic Explosively Index, ranks the explosively of an eruption based on volume of ejected material. The VEI scale ranges from 1-8, with larger numbers associated with larger eruptions; a VEI 4 or greater will disrupt air traffic. They found that there is a 1% chance of a VEI 4, 0.6% of a VEI 5, 0.15% of a VEI 6, 0.012% of a VEI 7, and a 0.001% of a VEI 8 eruption for the region every decade.

These probabilities were calculated by determining the number of volcanoes in the region possible of producing each VEI and knowing the frequency of an eruption of each VEI. The scientists were able to determine the number of volcanoes of each VEI based on volcano shape using satellite imagery. Each of the 748 volcanoes were sorted into one of five categories: distributed cones and fields, open vent stratocones, semi-plugged stratocones, well plugged stratocones, and large calderas.  To classify these different morphologies, they measured the longest axis of crater in the satellite image and identify any lava flow deposits, pyroclastic deposits, debris avalanche deposits, cones, calderas, collapse scarps, and domes. Areas less than 5 km wide with volcanic centers and lava flows were classified as distributed cones and fields. If the volcano was actively releasing gas, it was classified as an open vent stratocone.

Figure: Map of volcano locations and classifications in Southeast Asia (modified slightly from Whelley et al., 2015)

Each volcano was assigned a maximum VEI, based on these descriptive classification and some simple interpretations: a) more explosive volcanoes produce larger craters; b) small volcanoes and volcanoes with open vents are unable to produce the pressure needed to produce large scale explosions.   Distributed cones and fields were designated VEIs VEI 1-2, open vent stratocones VEIs 3-4, semi-plugged stratocones VEI 5, well plugged stratocones VEI 6, and large calderas VEI 7-8. Smaller eruptions occur more frequency, explaining why there is a much higher chance of a VEI 4 eruption over the next decade in Southeast Asia as opposed to a VEI 8.

Southeast Asia has one of the most rapidly growing airline traffic in the world, with a projected 6.6% annual growth over the next 20 years. There are already increasing concerns about aviation safety in the region, including mechanical safety and weather related incidents. With better long term forecasting, aviation safety boards will be able to better prepare for the next volcanic eruption, both with regard to size and possible location, and will therefore be able to make more timely decisions to ensure passenger safety.

For more information, check out the original article, “The frequency of explosive volcanic eruptions in Southeast Asia,” by Patrick L. Whelley, Christopher G. Newhall, and Kyle E. Bradley, here.