Sangeang Api erupted at 3:55pm local time on 30 May 2014 with a large explosive eruption.
Initial reports suggest that the ash cloud reached 65,000 ft above sea level . The island is uninhabited but still used for farming. The local authorities ordered the evacuation of local farmers. At the time of writing, flights in the area are disrupted; two airlines have cancelled flight to and from Darwin Airport . The latest VAA from Darwin VAAC  estimates the ash cloud at 50,000 ft above sea level.
About Sangeang Api
Sangeang Api is one of the most active volcanoes in the Lesser Sunda Islands, located 7 km off the NE coast of Sumbawa. She is a complex volcano with two overlapping cones, the active cone Doro Api (1949 m) and the older cone and Doro Mantoi (1795 m); and, she lies in an older caldera. Sangeang Api is next to and east of Tambora which famously erupted in April 1815 with an estimated size of VEI – 7. Note that Sangeang Api’s recent eruption have been smaller than Tambora’s.
The Lesser Sunda Islands are part of the Sunda Arc, a volcanic island arc which formed in the Miocene 15 million years ago when the Australian and Asian plates met.
Sangeang Api’s earliest recorded historic eruptions were in 1512 and she has erupted frequently since with approximately 20 eruptions ranging in size from VEI-1 to VEI-3 [1, 4] . (GVP’s site is down at the moment so I cannot give the most recent number of eruptions).
Her eruptions are often effusive with lava flows accompanying strombolian activity following an initial explosive phase. Her eruptive products are: potassic, volatile enriched silica-undersaturated lavas abundant in clinopyroxene-rich, mafic and ultramafic xenoliths. The xenoliths are believed have formed from fragments of cumulate magmatic bodies in the arc crust by progressive crystal liquid differentiation of her parental magmas.
Her lavas are potassic trachybasalt to trachyandesite. She has lavas typical of a MORB (mid ocean ridge basalt) source – clinopyroxene olivine; and, lavas more typical of relatively evolved magmas – plagioclase clinopyroxene magnetite and plagioclase amphibole clinopyroxene magnetite. Her more evolved magmas are similar to potassic Pleistocene volcanoes Soromundi and Sangenges to the west and Batu Tara to the north east. 
Sangeang Api is close to the junction of the Sunda Plate, Timor Plate and Banda Sea Plate, north of the Australian Plate in a very lively tectonic setting.
She is located 190km above the Benioff zone. The crust is made up of oceanic crust thickened by both volcanic and tectonic activity. The Moho is around 14km to 17km depth. Looking at the most recent earthquakes (4,621) we can see the descending Australian Plate, although the picture is complicated by the motion of the other plates.
Latitude v Longitude as shown on IRIS Earthquake Browser:
Depth v Longitude (looking north):
reproduced here with her kind permission.
Depth v Latitude (looking west):
Hope you enjoyed reading this. The usual caveats apply: “Not an expert, etc..”
KarenZ, 30 May 2014.
- Daily Mail: http://www.dailymail.co.uk/news/article-2644253/Incredible-moment-huge-volcano-erupts-Indonesia-sending-ash-spewing-thousands-feet-sky.html
- SBS: http://www.sbs.com.au/news/article/2014/05/31/darwin-airport-shut-volcanic-ash-looms
- Darwin VAAC: http://www.bom.gov.au/products/IDD65290.shtml
- Volcanoes of the World Third Edition Lee Siebert, Tom Simkin, and Paul Kimberly, Smithsonian Institution, University of California Press.
- Volcano Discovery: http://www.volcanodiscovery.com/sangeang_api.html
- “Rates and Process of Potassic Magma Evolution Beneath Sangean Api Volcano, East Sunda Arc, Indonesia” S Turner, J Foden, R Geaorge, P Evans, R varne, M Elburg and G Jenner, Journal of Petrology, Volume 44, Number 3, Pages 491 to 515, 2003. http://petrology.oxfordjournals.org/content/44/3/491.full.pdf
Editors note: THANK you KarenZ for such a lovely post on such a short notice. Easy to edit in as usually with your articles!
More images found on Twitter:
Erik Klemettis blog Eruptions covers Sangeang Api too: http://www.wired.com/2014/05/explosive-eruption-at-sangeang-api-in-indonesia/
Regarding KarenZ´s reference No. 6 commenter Bruce Stout summarized: “So what we have here is a close relative of tambora with evidence of recent magma injections and fractionation occurring at least at two levels, one below the crust in the Moho and the other in a crustal chamber. The lava is silica under-enriched (trachy-basalt) but high in potassium and volatiles making it fairly explosive stuff.”
One series of satellite images is embedded on http://laculturevolcan.blogspot.co.at/2014/05/activite-explosive-courte-mais.html#more
An article with IR images and satellite images on the event.http://cimss.ssec.wisc.edu/goes/blog/archives/15798
and some daily fail images 😉 http://www.dailymail.co.uk/news/article-2644253/Incredible-moment-huge-volcano-erupts-Indonesia-sending-ash-spewing-thousands-feet-sky.html
Ash Advisory http://www.bom.gov.au/aviation/warnings/volcanic-ash/
and a wind map which shows that the ash should be blown away from Australia ( if the map is correct, i have never tried to verify it for that part of the world.) http://earth.nullschool.net/#current/wind/isobaric/850hPa/orthographic=-4.50,51.20,821
The remaining two riddles of the weekend shall also not be forgotten:
4. At the end of a crack, my lava will flow-right! Clue: We could discuss this tête-à-tête. Answer: Monte Muambe, Mozambique. It’s at the south end of the great rift, and has carbonatite lava with rich fluorite inclusions. It’s in the province of Tete, near the town of Tete. No points.
5. Sea dogs and monkey suits? I am the newest. Clue: My magma is not very evolved, in theory. Answer: Fernandina Island. The sea dog is the HMS Beagle, and the monkey suit refers to the Scopes “Monkey Trial” over evolution leading to Darwin, and the Galapagos. Fernandina is the youngest island there. Double clue – again leading us to Darwin, plus the lava is basalt, which is not very evolved. Evan Chugg, 1 point.