Unrest at Agung Volcano, Bali

Unrest at Agung Volcano, Bali

  • EOS News
23 Sep 2017
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Agung is showing signs of increasing unrest (Source: CC-BY David Stanley/ Flickr)

According to the Center of Volcanology and Geological Hazard Mitigation (CVGHM) , Indonesia’s Geological Agency, Agung volcano in Bali has been showing elevated signs of unrest in the form of increasing volcanic seismicity over the past six weeks, starting from August 10th. CVGHM is closely monitoring the unrest, and putting appropriate exclusion zones in place. CVGHM raised the alert level from 1 (normal) to 2 (warning) on 14 September 2017, to 3 (eruption imminent) on 18 September 2017, and to the highest level 4 on 22 September 2017, at 8:30pm local time. 

Agung is a stratovolcano, a volcano made up of many layers of lava and ash. At 3,142 m above sea level, its is the highest peak in Bali, Indonesia, located almost 1,700 km from Singapore.  Agung has had many eruptions in the past, with an average of one explosive eruption every century. It last erupted in 1963, resulting in the deaths of over 1,000 people in its close vicinity. The current unrest of Agung is being closely monitored by CVGHM, and you can visit their website for accurate information on the current volcanic activity (in Bahasa Indonesia). 

A hazard map showing the areas that would be impacted if Agung volcano erupts (Source: CVGHM)

CVGHM has published a hazard map which, in the event of an eruption, outlines the areas that may be impacted by flow hazards such as lahars - volcanic mudflows - or pyroclastic flows - fast-paced currents of hot gas and volcanic matter. The map also outlines, with concentric circles, the areas where ejected rocks are expected to land - within 6 km of the vent - and where these rocks could reach during large a eruption - up to 9 km from the vent. 

The areas and circles provided on this map correspond to ‘Hazard Zones’ I, II and III which in turn correspond to specific actions that should be taken by people living in those areas. The text on the right side of the hazard map contains more details about these actions and how they vary according to the volcano’s current alert level.

It is a difficult task to predict the behaviour of a volcano like Agung. Decisions regarding alert levels and evacuation orders rely on numerous parameters currently monitored by CVGHM. 

Map showing volcanoes with Holocene eruptions - less than 10,000 years old - in Southeast Asia (Source: EOS, data from the Global Volcanism Program, Smithsonian Institution)

Agung is one of many volcanoes in Southeast Asia. A long arc of active volcanoes forms the backbone of the Indonesian Archipelago, from Sumatra and Java to Papua New Guinea, stretching over more than 4,000 km. About fifty of them have erupted during the last century. Such a concentration of very active volcanoes is not due to chance: the region’s tectonic settings account for it.

These volcanoes sit on top of subduction zones, where one plate moves toward and slides under another. This process causes material in the Earth's mantle and crust to melt; this molten magma, hotter and less dense than the surrounding rock, rises up towards the surface, where it can erupt to form volcanoes.

Ash column from the 1963 eruption (Source: K. Kusumadinata)

Previous eruptions at Agung have occurred throughout Balinese history, producing both smaller (VEI 2-3) and larger (VEI 5) events. Most recently, Agung erupted in 1963, and before that in 1843. Both eruptions produced pyroclastic density currents, lava flows, and tephra falls. The 1963 eruption also produced lahars.

The 1963 eruption, preceded by a few days of felt earthquakes, started with a lava flow and was followed by two explosive phases. The lava flow travelled 7.5 km over the first 26 days of the eruption. Each of the explosive phases peaked with eruptive columns at least 20 km high, each lasting for about four hours and producing fatal pyroclastic flows and lahars. Minor explosive activity and the generation of lahars by rainfall continued into early 1964. Ash from the 1963 eruption reached Jakarta,