A long history of volcanic eruptions at Gunung Agung, Bali

A long history of volcanic eruptions at Gunung Agung, Bali

Oxford Researcher Karen Fontijn writes about her work on the volcano that has affected so many over the past week.

Recent signs of activity at Mt Agung, a volcano in Eastern Bali, are causing lots of concern to local populations and authorities, as well as tourists visiting the Indonesian island. This briefing note is intended to provide an objective summary of what we know about the volcano’s past history, and is based on scientific studies that have been published in international scientific journals.

Up until September 2017, Agung volcano was most famously known in the volcanology community for its 1963 eruption. This was a large explosive event, which caused more than 1100 fatalities among the communities living on the lower slopes of the volcano. It was also the first eruption to have a documented short-lived climatic impact. The large amounts of SO2 gas that were released formed an aerosol blanket in the higher atmosphere that resulted in an estimated decrease of 0.1-0.4 °C of the global average temperature in the months to years after the eruption.

The 1963 eruption was preceded by at least a few days of felt earthquakes before a lava flow was erupted from the summit crater, flowing down the northern flank of the volcano. This was then followed by a few weeks of minor explosive activity in the crater, after which an explosive eruption column sent large amounts of volcanic ash (small particles of rock debris) and gases more than 15 km up into the atmosphere. Another similar large explosive eruption followed two months later. Some of the volcanic ash fell back down to the northwest of the volcano and buried crops and roofs under a layer up to 50 cm thick. In addition to eruption columns and ash fall, the eruption also caused hot mixtures of volcanic gases and rock debris (pyroclastic flows) to travel down the slopes of the volcano. Some of these pyroclastic flows turned into devastating mudflows (lahars) further down. It is these pyroclastic flows and lahars which caused most of the fatalities in 1963. The entire sequency of events, from the first earthquakes that were felt and the lava flow to small-scale explosions and the two big explosions, lasted more than 4 months. After that, minor explosive activity continued and lahars were still generated by rainfall onto the fresh volcanic debris, until early 1964. This really highlights the complexity and possible long duration of volcanic unrest and eruptive events at Agung volcano.

There was only little or even no instrumental monitoring done in 1963 – in those days the instruments and methods were not yet developed as they are today, and even volcanology as a science hardly existed. In the last few decades we have learnt a lot about how volcanoes work and our capacity to monitor volcanoes using a range of instruments has improved significantly. Today Indonesian and international volcanologists are using all this combined knowledge that we have acquired from studying volcanoes around the world to interpret the signs of unrest at Agung, and inform emergency managers. There is a variety of instruments in place both on the ground and in space to keep an eye on the volcano. The hazard maps and evacuation zones that are currently in place around the volcano are largely based on what we know from Agung’s past eruptions, including the 1963 event.

The eruptive history of Agung was pieced together by studying the volcanic rocks that are found all around the volcano. Together with colleagues at the Indonesian Volcanological Survey (CVGHM) and the Earth Observatory of Singapore, we have been able to go 5000 years back in time and identify the typical style of activity at the volcano. Underneath the volcanic ash layer from the 1963 eruption, we find another 51 such ash layers from older volcanic eruptions. The oldest ash layer we found was about 5000 years old – we determined this age using radiocarbon dating. So that basically tells us that the volcano – on average – erupts about once every century. By comparing the thickness of the ash layers from the older eruptions with that of the 1963 eruption, we find that some eruptions were probably a bit smaller than 1963, others were of similar size. Everywhere in the valleys to the north-northeast and southwest-southeast we find rocks that were emplaced by pyroclastic flows and lahars, including in 1963, but also before that. This means that these same valleys are prone to similar phenomena in the future. Finally we also find a lot of lava flows on the upper slopes of the volcano. All this information together tells us that Agung has a repeated history of volcanic eruptions that are very similar in style to the 1963 eruption. We cannot know for sure whether every Agung eruption is always associated with volcanic ash fallout and pyroclastic flows and lahars and lava flows, but it is likely.

A large team of volcanologists will continue monitoring the situation and provide official updates on the activity on http://magma.vsi.esdm.go.id.

For further information on this research or how to get in touch, see Karen’s research profile, or follow her on Twitter: @VolcKaren.

(Last updated 04/10/2017, 16:58 GMT)