by EUNADICS-AV consortium, 5 December 2017
On November 21, the Indonesian volcano Mount Agung on the island of Bali produced its first small eruption after three months of geological unrest starting in early August. The recent magmatic eruption and emissions of volcanic ash eventually led to a temporarily close down of the Bali airport, causing thousands of tourists to be stranded on the tourist island. Since then, the airport has opened and closed depending on the prevailing winds and the direction of the volcanic ash cloud. The eruption could eventually also cause larger scale disruptions in international aviation, especially in the Southern Hemisphere, reaching up to Australia.
The eruption was a so-called phreatic eruption, caused by overheated steam due to water that has been in contact with magma. A few days later on 25 November a magmatic eruption started, releasing volcanic ash clouds up to altitudes of 9 km, as well as significant amounts of sulphur dioxide (SO2). This eruption has since then been ongoing.
The activity of Agung has the potential of a considerable disaster. This is because many people (4 million people live within 100 km from the volcano) are exposed to the hazards, and people living in the area do not have enough experience to face the eruption of Agung given that the last eruption occurred in 1963. Authorities authorized the evacuation of about one hundred thousand people living in the direct vicinity of the volcano.
Agung is a stratovolcano capable of large magnitude explosions. Due to the chemistry of the erupted magma, the eruptions may also be highly gas-rich. The primary volcanic hazards are likely to be pyroclastic flows (generated by the collapse of an explosive eruption column), lava flows, lahars (mudflows with volcanic ash), ash fall, volcanic ash clouds and volcanic gases (and thus consequently air pollution).
The last two historical eruptions were 1963-64 and 1843 and both included explosive phases of moderate to large magnitude as well as pyroclastic flows, lava flows, ash fall and lahars.
In 1963, the eruption began with a lava flow (occurring over 26 days) then a moderate explosive phase (ash column to 19-26 km a. s. l.) for one day, then two months later a 4 hour explosive phase (ash column to 20 km a. s. l.). There was minor volcanic activity over the course of about one year.
The current Pusat Vulkanologi Mitigasi Bencana dan Geologi (PVMBG) hazard map for Mount Agung shows where most of the pyroclastic flows, lava flows and lahars occurred (see Fig. 1) during the last two eruptions. The distribution of ash deposits from past eruptions is predominantly to the west of the island, due to easterly winds.
Figure 1: Map with high (red), middle (pink) and minor risk (yellow) for pyroclastic flows, lava flows, lava avalanches, lahars and hazardous gases. Source: PVMBG.
Satellite based SO2 column observations provided by the OMI and TROPOMI instruments are shown in Fig. 2 for the 27th November 2017 and in Fig. 3 for the 28th by GOME-2B instrument.
It has to be acknowledged that this is one of the first official TROPOMI images, the instrument was which was launched very recently in October 2017. A more detailed description of TROPOMI as well as other examples will be published soon on the EUNADICS-AV webpage in a separate news item, as it is expected that this new sensor will be a very important data source for the project.
Figure 2: Measurements of sulphur dioxide total columns over Indonesia on 27 November 2017 from the OMI (left, source: SACS/temis [BIRA/ KNMI/DLR/ESA]) and TROPOMI (right, source: DLR/BIRA/ESA) satellite instruments. Click here to see the high quality images on the ESA website.
Figure 3: SO2 total columns as observed by the GOME 2-B instrument on board the METOP-B satellite on Nov., 28th. The red circle indicates the location of Mt. Agung. Source: SACS (DLR/BIRA/EUMETSAT).
Examples of volcanic ash observed by GOME-2 and MODIS are depicted in Fig. 4 and Fig. 5. The slightly brownish clouds in Fig. 5 indicate the presence of volcanic ash in fresh volcanic plumes, which are located at different heights which have different wind directions.
Figure 4: Absorbing aerosol index (see red dot to the east of Bali) as observed by GOME-2 on Nov., 26th. Source: temis (KNMI/ACSAF/EUMETSAT).
Figure 5: MODIS (terra) visible image over Bali on 29th November 2017. Source: NASA WorldView.
Information provided by British Geological Survey (BGS), using the following resources:
http://www.vsi.esdm.go.id/index.php/gunungapi/liputan-khusus/g-agung/1818-peningkatan-status-gunungapi-agung-bali-dari-level-iii-siaga-ke-level-iv-awas and http://volcano.si.edu/volcano.cfm?vn=264020.
Fontijn K., Costa F., Sutawidjaja I., Newhall C. G., Herrin J. S., 2015. A 5000-year record of multiple highly explosive mafic eruptions from Gunung Agung (Bali, Indonesia): implications for eruption frequency and volcanic hazards. Bulletin of Volcanology, 77:59. http://dx.doi.org/10.1007/s00445-015-0943-x.
Self, S., and Rampino, M. R., 2012. The 1963–1964 eruption of Agung volcano (Bali, Indonesia). Bulletin of Volcanology, 74(6), pp. 1521-1536.