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.

Simulations of large, wet volcanic eruptions

Citation: Van Eaton, A. R., M. Herzog, C. J. N. Wilson, and J. McGregor (2012), Ascent dynamics of large phreatomagmatic eruption clouds: The role of microphysics, J. Geophys. Res., 117, B03203, doi:10.1029/2011JB008892.

Lake Taupo, New Zealand. The eruptions from this volcano were so large that the erupted material left behind a huge space below the surface that caused the surface to subside and form a caldera, or a collapsed volcano. Afterwards, the lake filled in the hole created by the subsidence. Image source

Interactions between magma and surface water can play a significant role in the development of ash clouds in explosive volcanic eruptions. The authors of this study looked at such interactions by simulating volcanic eruptions with the computer model ATHAM (Active Tracer High-resolution Atmospheric Model). 2-dimensional simulations in ATHAM were run for a variety of different surface water contents, (between 0 and 40%) such as erupting into a lake. Their results indicate that increased water content can significantly alter the size and shape of the ash cloud. Previous studies have found that more surface water can shorten the height of the eruption cloud. However, this work by Van Eaton and others shows that that statement isn’t always true. For example, with the addition of more water (greater than 24%), simulations showed an eruption column partially collapsing and then dispersing into a high ash cloud. This shows that even smaller eruptions with column collapses can reach the same altitudes as larger eruptions, all thanks to the availability of moist air (see Figure 1). 

As part of this study, the authors used field data from a past eruption of Taupo volcano in New Zealand. The eruption occurred about 27,000 years ago and is known as the Oruanui supereruption. This particular eruption was chosen because it was a very large, wet eruption that dispersed ash far away from the volcano despite not having a very stable eruption column. We know that it was a wet eruption because of the abundance of ash aggregates in the deposits around the volcano. Ash aggregates are formed when wet ash collides with other ash particles in the air and then combine by sticking together, forming a larger particle. The authors used information about this eruption in ATHAM to determine how water content can impact an eruption and produce such widely dispersed ash patterns seen at Taupo volcano.

Figure 1: Cross-section of two eruptions into different atmospheres. With the availability of more water in a wet eruption, there is more exchange of heat and energy when water cools and becomes ice. These heat exchanges from phase changes impacts the size and shape of the eruption column by feeding or removing energy and giving the ash column the ability to rise higher. The grey box represents the troposphere, the atmospheric layer in which we all live. The black lines indicate different ash concentrations, increasing by an order of magnitude from an initial 0.01 g/m³ at the outer edge. 

Why do we want to know how water can impact an eruption? Well, as it’s been shown by the authors, water can play a significant role in plume development and therefore ash dispersal, which has implications for air traffic safety and accurately forecasting where the ash will go. For their simulations with water content above 10%, the ash column becomes increasingly unstable (or not buoyant) and some of the rising ash collapses back down toward the earth’s surface. Some of the collapsed ash rises back up into the atmosphere, similar to ash in a forest fire, and continues to disperse far away from the volcano. This type of eruptive style leads to more ash emplaced into the troposphere than an identical, stable eruption in which more ash is injected into the stratosphere. This leads to greater difficulty in forecasting ash dispersal, especially in an emergency context, where there is concern for aircraft safety or ground population.

Another important finding in this study is the effect of the tropopause on the maximum column height. The tropopause is a thermal boundary where eruption columns tend to stop rising and its height varies between 7 and 15 km high, depending on your location on earth. Ash often rises to the tropopause, at which point the ash levels out (think: smoke from a fire rising up and spreading out on the ceiling in a room). The fraction of ash that successfully penetrates through the tropopause can disperse in the stratosphere or fall back down. Large modeled eruptions in this study, regardless of their water content, were able to easily reach the tropopause, indicating that 1) the eruption output rate is a major controller on volcanic emissions and 2) the tropopause can play a significant role on the maximum column height reached by volcanic eruptions.