Large Magnitude Eruption Expected for Bali's Mount Agung

Mount Agung, seen on April 28, 2010 from Bali. Photo by Jesse Wagstaff via CC License.
Mount Agung, seen on April 28, 2010 from Bali. Photo by Jesse Wagstaff via CC License.
Monday, October 02, 2017 

Over 100,000 people have been evacuated from their homes in Bali, Indonesia, as the threat level for Mount Agung was raised to the highest level after hundreds of shallow volcanic earthquakes were recorded in recent days. If Mount Agung erupts, people living near danger zones could be exposed to deadly volcanic ash, rocks, and lava. Agung’s last eruption occurred in 1963 after over 100 years of dormancy, killing over 1,500 people.

More than 50 years later, Indonesian authorities are preparing for another large magnitude eruption. In Washington D.C., Carnegie volcanologist Hélène Le Mével explains what scientists know—and don’t know—about volcanoes, and how scientific observations contribute to risk assessments and alert systems.

Mount Agung, seen on Sep. 27, 2017 by Landsat 8. Image by Roberto Molar Candanosa, Department of Terrestrial Magnetism, using USGS data.

Will Mount Agung erupt?

Scientists cannot tell with certainty whether the volcano will erupt or not. The alert level for Mount Agung has been raised based on the increasing seismic activity recorded at the volcano. In particular, the frequency of the volcanic earthquakes and tremor have increased over the past weeks. There is also steam and gas emissions currently above the crater.

What do scientists and authorities know, and what can they do with that information?

Shallow volcano earthquakes occur due to the magma rising to the surface and making pathways through the crust. The way officials evaluate the alert level, which has been raised to the highest level, is based on the magnitude and number of these volcanic earthquakes.

The current hazard map of Mount Agung and the extent of the evacuation zones is mostly based on the most recent eruption that happened in 1963. If this scenario is repeated, then the eruption could be a Volcanic Explosivity Index 5 and lead to ash plume rising several kilometers in the atmosphere, as well as a fast-moving mix of hot volcanic gas, lava, and rocks traveling down the flanks of the volcano. Those flows are called pyroclastic flows, and they were responsible for killing about 1,500 people in 1963 and are the greatest volcanic hazard for the local population. However, scientists cannot predict whether the current unrest would lead to a similar type of eruption or whether it would be explosive or effusive.

Mount Agung Hazard Map, showing the location of Agung, disaster-prone areas in yellow and pink, and potentially affected villages in red and orange. (Credit: Indonesian National Board for Disaster Management)

What are the challenges for eruption forecasting? 

While I do not work directly on eruption forecasting or seismology, I can talk about ground deformation. So far, analysis of radar images has shown that there is no surface deformation observed on Mount Agung volcano. However, as of September 29, officials have indicated that ground-based instruments suggest there has been some inflation.

It is common for a volcano to inflate prior to an eruption, sometimes months before or right before, an eruptive event. This is because as the magma moves towards the surface, it pressurizes the chamber or volcanic conduit. However, all of the known pre-eruptive patterns of seismicity and deformation have been observed in different eruptions: No deformation and seismicity, no seismicity and no deformation, and others. This makes it hard to predict the occurrence of an eruption.

The official source of updated official information is MAGMA Indonesia, which now has Mount Agung activity updates in English, and a live seismogram:

Le Mével is a Staff Scientist at Carngie's Department of Terrestrial Magnetism. Her research focuses on surface deformation in volcanic areas to  better understand magmatic processes.