April 2015 will mark the 200 year anniversary of the largest recorded eruption in historic times. Yes, Tambora.
Tambora’s sulphate dust clouds are now attributed with disrupting major weather systems for more than three years, causing net global cooling, global crop failures in 1816, famine and disease, including a cholera pandemic. Yet, despite being one the largest observed eruption in recorded history, at the time it passed relatively unnoticed outside the region. In Europe, Napoleon had returned to power in March 1815; and, armies were being mobilised to defeat him. Sir Stamford Raffles, the British governor of the region based in Batavia, was in the process of building a local system of free-trade. However, eye witness accounts from trading vessels off the cost of Sumbawa and resident administrators obtained by Raffles have survived.
On 10 April 1815, an eruption with an estimated size of VEI-7 destroyed the summit of Tambora, removing the top third of the edifice, leaving a 6 to 7km in diameter, 600 to 700m deep caldera. The immediate death toll on the island of Sumbawa is estimated at around 10,000 to 11,000 people from the eruption, itself, with a further 60,000 to 110,000 in the region from starvation or disease. An administrator sent to investigate the earlier 5 April 1815 eruption was among the first victims.
Fig 1: Mount Tambora. Image by Jialiang Gao (peace-on-earth.org). Published under Creative Commons Attribution-Share Alike 3.0 Unported license. http://commons.wikimedia.org/wiki/File:Caldera_Mt_Tambora_Sumbawa_Indonesia.jpg
Before the April 1815 eruption, Tambora had been dormant for over a thousand years, awakening in 1812. She was between 4,000m and 4,300m high and clearly visible from Bali. On 5 April 1815, she erupted with what is described by local witnesses as a loud thunderclap, tremors and huge plumes of flames – the first of two plinian eruptions. This settled down after a few hours and, from then to 10 April, there were smaller emissions. At around 19:00 hrs on 10 April 1815 the main eruption started – the second plinian eruption. It is described as three merging columns of flames, a flowing mass of liquid fire, heavy pumice and ash fall, and pyroclastic flows. The eruption lasted approximately two days. The ash cloud reached west Java, southern Borneo and south Sulawesi. Villages on the north, west, south and east slopes of the volcano were destroyed by pyroclastic flows and lava. A 4m high tsunami caused by pyroclastic material entering the sea devastated the Sanggar coast. Between 95 and 175 km3 of tephra (30-33km3 DRE) are estimated to have been erupted.
Analysis of the deposits show that the eruption occurred in two phases: the first phase was at least four tephra fall episodes; and, the second comprised at least seven pyroclastic flows. It is during the second phase that the summit is believed to have collapsed and the caldera created.
So what caused the eruption?
Tambora is an active shield like alkaline volcano located on the Sanggar Peninsular on Sumbawa Island, Indonesia, in the eastern sector of the Sunda arc. Volcanic activity on Sumbawa have occurred from the early Miocene through to the Holocene. Tambora, herself, is younger than 200 ka, overlying the 410ka Kawinda Toi volcano.
Tambora is not Indonesia’s most active volcano. GVP lists only seven confirmed eruptions in the Holocene: 3910 BCE ± 200 yrs, 3050 BCE ?, 740 ± 150 yrs, 1812 to 1815 VEI-7, 1819 VEI-2, 1880 ± 30 yrs VEI-2, and 1967 ± 20 yrs VEI-0.
Fig 2: Sumbawa Island, Indonesia from Google Satellite.
Tambora is located 340km north of the Sunda Trench at the back of the main volcanic arc 180 to 190 km above the Benioff Zone. The crust is young and between 14 to 17 km thick. The upper part of the crust is made up of Cenozoic siliciclastic marine sediments and limestones with volcanic and intrusive rock. Here Indo-Australian Plate subducts beneath the Sunda Plate. To the east, the subducting plate changes nature from oceanic to continental. Back arc thrusting has been reported from Bali to Flores Islands.
Fig 3: Sunda Plate. Image by Sting and Rémih, published under Creative Commons Attribution-Share Alike 2.5 Generic license. http://upload.wikimedia.org/wikipedia/commons/e/e6/Sunda_Plate_map-fr.png
Looking at the latest 5,000 earthquakes published by IRIS for the area between Bali and the western end of East Nusa Tenggara we can clearly see the subduction zone:
Fig 4: Subduction zone. Copyright rests with the author. Reproduced here with her kind permission.
Tambora’s lavas are potassic, silica-under saturated lavas and pyroclastic rock ranging from nepheline-normative alkali basalt to trachyandesite. Her lavas are consistent with other Sunda arc quaternary volcanoes: Muriah, Ringgit Beser, Sangeang Api and Batu Tara.
The rocks now exposed in the caldera wall show four pre 1815 volcanic formations. Lava flows of 43ka which fill an earlier caldera, which are overlain by the Black Sands pyroclastic unit and the Brown Tuff formation. Radio-carbon dating of the Brown Tuff formation indicated that it formed between 5900 and 1210 14C years BP. This was the last known activity before the 1815 eruption. Check out the photos in Tambora’s caldera for an idea of the strata (not reproduced here as copyright is not certain). The lavas of the 1815 eruption were almost exclusively trachyandesite (latite) – tephriphonolite, which is relatively rare in volcanic arc settings. They are believed to be some of the most evolved products of the volcano.
Cooling of a hydrous magma in the magma chamber led to the exsolution of fluid magma and crystallisation of the magma during the period of dormancy. Overpressure in the magma chamber led to the eruption. The mechanics of the eruption were similar to that seen more recently in the VEI-6 eruption of Pinatubo in June 1991. SiO2-undersaturated potassic trachybasalt formed from the partial melting of a garnet-free I-MORB like mantle source fluxed with fluid from the subducting slab and small amounts of subducting sedimentary material. This trachybasalt differentiated to trachyandesite in two stages: around the Moho at 14 -17 km; and, in and a shallow crustal reservoir at 2 to 3 km. In a deep magma reservoir near the Moho, aluminium oxide rich trachybasalt and basaltic trachyandesite evolved from the mantle sourced aluminium oxide poor trachybasalt. In the shallow crustal reservoir, trachyandesite and phonolite evolved from the aluminium oxide rich basaltic trachyandesite.
Fig 5: 12 June 1991 eruption of Pinatubo (three days before the climactic eruption). Image by Richard P. Hoblitt, USGS, http://www.usgs.gov/
Fig 6: Gas emissions on the caldera floor of Tambora. Image by Georesearch_Volcanedo_Germany, published under Creative Commons Attribution-Share Alike 3.0 Unported license. http://upload.wikimedia.org/wikipedia/commons/e/e5/TAM_NE-Gas-4966.jpg
Hope you enjoyed reading this. The usual caveats apply “not an expert, etc.”
KarenZ, March 2015.
- Wikipedia: http://en.wikipedia.org/wiki/Mount_Tambora ,
- Wikipedia: http://en.wikipedia.org/wiki/1815_eruption_of_Mount_Tambora ,
- GVP: http://www.volcano.si.edu/volcano.cfm?vn=264040 ,
- IRIS: http://ds.iris.edu/ieb
- Igan Supriatman Sutawidjaja, Haraldur Sigurdsson, Lewis Abrams, “Characterization of volcanic deposits and geoarchaeological studies from the 1815 eruption of Tambora volcano”, Indonesian Journal on Geoscience Volume 1 Number 1 2006: 49 – 57 http://jgi.bgl.esdm.go.id/index.php/IJOG/article/view/8
- Ralf Gertisser, Stephen Self, Louise E. Thomas, Heather K. Handley, Peter Van Calsteren, and John A. Wolff, “Process and timescales of magma genesis and differentiation leading to the great Tambora Eruption in 1815”, Journal of Petrology Volume 53 Number 2 2012: 271 – 297 http://petrology.oxfordjournals.org/content/53/2/271.short
- Gillen D’Arcy Wood, “Tambora: The Eruption That Changed the World”, Princeton, NJ: Princeton University Press, 2014. ISBN 978-0-691-15054-3
- Clive Oppenheimer, “Eruptions That Shook the World”, Cambridge: Cambridge University Press, 2011. ISBN 978-0-521-64112-8