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Eclogite
2017-Nov-26, 08:12 PM
Explosive activity at Mount Agung has led to a Red Warning (http://www.bbc.co.uk/news/world-asia-42126284) for aircraft in relation to ash clouds. A larger eruption is possible in the near future. Here is some background information on the volcano.

It is located on the north east side of the island of Bali, in Idonesia, and is a product of melting arising from the subduction of the Indian-Australian plate beneath that of the Eurasian plate, along the Sunda arc. It is one of a hundred and fifty volcanoes in the Indonesian archipelago, including Krakatoa, famed for its 1883 eruption; Mount Tambora, which produced the largest eruption globally in historic times (1815); and Toba, which was responsible for a super-eruption some 70,000 years ago.

Agung's last major eruption was in 1963, summarised in this abstract from a 1964 report in the Bulletin of Vulcanology (https://link.springer.com/article/10.1007/BF02597526).

Mt. Agung in Bali which has been dormant for about hundred and twenty years showed increased activity on February 18 this year culminating with a paroxysmal eruption early in the morning on March 17; the second paroxysmal eruption occurred on May 16. The activity started with minor explosions in the main crater with the production of pyroclastics followed by the effusion of lavas which flowed over the lowest northern crater rim and the formation of nuées ardentes d’explosion which came down into the northwestern sector of the volcano. Successive nuées ardentes d’explosion which accompanied the paroxysmal cruption on March 17 and on May 16, came down along the southern, southeastern and northern slopes, devastating many villages. The first cycle of activity killed about 1700 people of which 1500 died from the nuées ardentes. Cold lahars, caused by heavy rainfall immediately after the eruption destroyed villages and constructions on the southern slope and killed about 200 more people. The nuées ardentes from the second paroxysmal eruption killed also about 200 more people who were all caught in the « Closed Zone ».

This 2012 Bulletin article, "The 1963-1964 eruption of Agung volcano (Bali, Indonesia) (https://link.springer.com/content/pdf/10.1007%2Fs00445-012-0615-z.pdf)" provides a detailed and updated account of the eruption.

This excellent paper, "A 5000-year record of multiple highly explosive mafic eruptions from Gunung Agung (Bali, Indonesia): implications for eruption frequency and volcanic hazards. (https://www.researchgate.net/profile/Karen_Fontijn/publication/279225389_A_5000-year_record_of_multiple_highly_explosive_mafic_eru ptions_from_Gunung_Agung_Bali_Indonesia_implicatio ns_for_eruption_frequency_and_volcanic_hazards/links/59bf7559aca272aff2e13ec0/A-5000-year-record-of-multiple-highly-explosive-mafic-eruptions-from-Gunung-Agung-Bali-Indonesia-implications-for-eruption-frequency-and-volcanic-hazards.pdf)" explores the eruptive history of the volcano. One of the interesting features of eruptions from Agung is the presence of two magma types in the same eruption. This paper, with much data and a well reasoned argument, attributes this to ". . . repeated intrusions of basaltic magmas into basaltic andesitic to andesitic reservoirs producing a hybrid of bulk basaltic andesitic composition with limited compositional variations".

Background note: in simplistic terms there is direct correlation between silica content, magma viscosity and the tendency to produce explosive eruptions. Basalts have a relatively low silica content and tend to produce quiescent eruptions, such as those on Kilauea, Hawaii. Rhyolites (the effusive equivalent of granite) have a high silica content and tend to produce explosive eruptions, such as those at Yellowstone. Andesites are intermediate in composition. Superimposed on these trends is the extent to which degassing is inhibited during upward migration of the magma, increasing the risk of explosive eruptions.

Squink
2017-Nov-26, 11:56 PM
"nuées ardentes" seems to tanslate as pyroclastic clouds. (English and Latin).
That bit about multiple paroxysmal eruptions does not sound good.

Eclogite
2017-Nov-27, 12:59 AM
"nuées ardentes" seems to tanslate as pyroclastic clouds. (English and Latin).
That bit about multiple paroxysmal eruptions does not sound good.Correct on the translation (literally, I think, glowing cloud). The term came to prominence when Mont Pele,on Martinique erupted in 1902 and all but two of the inhabitants of Saint Pierre, some 20,000 IIRC, were wiped out by a pyroclastic flow.


Explosive eruptions are classified on the Volcanic Explosivity Index (https://en.wikipedia.org/wiki/Volcanic_Explosivity_Index) - a logarithmic scale of intensity. Paroxysmic eruptions are ranked at 5 on a scale of 8. The descriptors run thus:

0-Effusive; 1-gentle; 2-explosive; 3-catastrophic; 4-cataclysmic; 5-paroxysmic; 6-collosal; 7-super collosal; 8; mega collosal


News update from the BBC (http://www.bbc.co.uk/news/world-asia-42132912), @ 2:30 am GMT, with the authorities warning of an imminent eruption.

BigDon
2017-Nov-27, 04:05 PM
More lahars reported. And a nice picture of two different colored smoke and ash columns coming out of the volcano. I assume the white column is steam.

http://abcnews.go.com/Technology/wireStory/erupting-bali-volcano-dusts-resorts-ash-flights-continue-51387606

Also, they have airport closures concurrent with evacuation orders. (Why does that arouse my inner Terry Pratchett?)

Swift
2017-Nov-27, 06:14 PM
And a nice picture of two different colored smoke and ash columns coming out of the volcano. I assume the white column is steam.

That would be my assumption.

geonuc
2017-Nov-27, 10:16 PM
This excellent paper, ...

Yes, at least as far as my fading geology memory can discern.

Nice post, thanks.

Eclogite
2017-Nov-28, 02:19 PM
This is a live video feed (https://coconuts.co/bali/news/live-watch-balis-mount-agung-volcano-livestream/) set up by the Indonesian Volcanology and Geological Disaster Mitigation Center. It was down for most, or all of yesterday, but is functioning as of now.

It's currently night in Bali, but the glow from the summit and a portion of the ash cloud are discernible. (As is the occasional flying insect.)

Edit: Camera 1 is due west of the volcano. Based on that the ash cloud appears to be drifting roughly north.

And a live seismogram is available here (https://magma.vsi.esdm.go.id/live/seismogram/).

Copernicus
2017-Nov-30, 08:46 AM
Watching the news on this is scary. So many people aren't taking it serious enough to evacuate.