Sakurajima
Sakurajima is Japans most active volcano, with 8000 vulcanian eruptions in the past fifty years (Aizawa 2010). There are two patterns of activity, intermittent vulcanian explosive eruptions, and extended low energy release of volcanic gas and ash (Yokoyama 1979). This regular activity makes Sakurajima an ideal study example for the study of volcanic plumes (Lane 1992), and is already continuously monitored by Sakurajima Volcano Research Centre.
Sakurajima is situated on the South Cost of Kyushu Island, Japan, in Kagoshima Bay, and was separate from the mainland up until 1914, where one of the Taisho lava flows connected the volcano to the mainland (Yanagi 1991). It started erupting 13 kyr ago, within the Southern rim of Aira caldera as a post caldera volcano. It is made up of two main strato volcanoes, Kitadake, and Minamidake, which started up 13 kyr and 5 kyr ago respectively (Tamegueri 2009). Minamidake has been erupting since 2006 after fifty years of dormancy (Aizawa 2010). The activity has gradually shifted from Minamidake crater to Showa crater (Aizawa 2010).
Landsat image of Sakurajima. The volcanoes plume is clearly visible, along with the 1914 Taisho lava flow connecting the volcano to the mainland. http://en.wikipedia.org/wiki/File:Sakurajima_Landsat_image.jpg
There have been four gigantic eruptions producing nine lava flows and three layers of pumice (Yanagi 1991). The first was Bunmei eruption from 1471- 1476 after 700 years of dormancy, this lead to the deposition of a pumice layer and two lava flows (Kobayashi 1982). The second was An-ei in 1779 leading to two lava flows and a pumice unit being deposited (Kobayashi 1982). The third gigantic eruption was the aforementioned Taisho 1914 eruption that produced three lava flows (Kobayashi 1982), one that connected the volcano to the mainland, and a pumice layer. After this eruption a marked concentric depression of about a meter (Omori 1916) was observed due to a pressure drop in the magma chamber ~10km below the crater (Mogi 1958). The land gradually elevated until the final eruption, known as the Showa eruption in 1946, which produced two lava flows (Ishihara 1981).
Sakurajima is a major SO2 emitter, and contributes significantly to the local SO2 budget (Kawaratani 1990). Anthropogenic emissions are around 0.6 Tg/a, but Sakurjima emits more that three times this at 2 Tg/a (Kawaratani 1991). It was also noted that Sakurajima emits significant amounts of halogen oxides such as ClO and BrO (Lee 2005), where Cl and Br catalyse reactions that breakdown ozone into oxygen. The surrounding topography generally has a lower relief than Sakurajima. The highest peak of the surrounding Takahuna Mountains is 1237m, but Sakurajimas aerosols are usually emitted from a height of 1040m, close to the volcanoes peak at 1117m. This means that the volcanic plume can freely disperse into the atmosphere (Kinoshita 1996).
The magmas produced by the volcano have gradually become increasingly andesitic (more mafic) (Yanagi 1991). Petrographical analysis has lead to the discovery of binary mixing curves, and bimodal phenocrysts of plagioclase and pyroxene (Yanagi 1991). It is thought that there are two magma chambers underlying Sakurajima with a plug between them (Yanagi 1991). The upper chamber is predicted to contain dacitic magma and the lower high-alumina basalt. As the plug continuously sinks into the basalt, more basalt is displaced into the upper magma chamber. This is then mixed, and when eruptions occur, magmas of discrete compositions are produced (Yanagi 1991).
Showa crater vulcanian eruption 01/01/2010 at 11:25pm http://www.photovolcanica.com/VolcanoInfo/Sakurajima/Sakurajima.html
Minami 2010 studied deformation patterns associated with the eruption of Showa crater, and explained the deformation of two magma chambers one at 0.1km and the other at 4km. Similar work has also been carried out by Eto 1997, but covering the whole of Aira Caldera. They predicted a main magma reservoir 5km beneath Minamidake, similar to the 4km reservoir of Minami 2010, but also a large magma reservoir at 10km deep beneath the Aira caldera. These two chambers are linked by a NE-SW striking tensile fault identified by Hidayati 2007. So it appears that an upper dacite chamber ~0.1 km beneath Sakurajima is being supplied by basaltic magma from a lower chamber (4-5km) due to a sinking plug (Yanagi 1991). When a H2O threshold is reached in the upper chamber, an explosive eruption occurs (Yanagi 1991). The lower, basaltic chamber (4-5 km) is fed along a fault by a magma reservoir 10km below the Aira caldera (Eto 1997).
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References
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Bouquet T, Kinoshita K, Watson M. Observing SO2 emissions at Japenese volcanoes using an ultraviolet imaging camera. Proc. 14th CEReS International Symposium and SKYNET workshop on “Remote Sensing of the Atmosphere for Better Understanding of Climate Change”, Chiba Univ., Nov. 13-14, 2008, pp.173-176
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Hidayate, Ishihara, Iguchi. Volcano-tectonic Earthquakes during the Stage of Magma Accumulation at the Aira Caldera, Southern Kyushu, Japan Bull. Volcanol. Soc. Japan Article 2007;52:289-309
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Kasasaku K, Minari T, Mukai H, Murano K. Stable Sulfur Isotope Ratios of the Gases from Mt. Sakurajima and Satsuma-Iwojima Volcanoes. Assessment of Volcanic Sulfur on Rainfall Sulfate in Kagoshima Prefecture. Nippo Kagaku Kaishi, 1999,7, 479 – 486
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Lee C, Kim Y J, Tanimoto H, Bobrowski N, Platt U, Mori T, Yamamoto K, Hong C S. High ClO and ozone depletion observed in the plume of Sakurajima volcano, Japan. Geophysical Research Letters, 2005;32:21809.
Minami S, Iguchi M, Mikada H, Goto T. Takekawa, J. Numerical simulation of magma plumbing system associated with the eruption at the Showa crater of Sakurajima inferred from ground deformation American Geophysical Union, Fall Meeting 2010, abstract #V33C-2386
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Mori T, Ishiharq K, Hirabayashi J, Kazahaya K, Mori T. SO2 gas monitoring by DOAS at Sakurajima and Suwanosejima volcanoes. Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No.47 C, 2004
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Tameguri T, Iguchi M Shallow three-dimensional P-wave velocity structure revealed by seismic experiment Sakurajima volcano, Japan American Geophysical Union, Fall Meeting 2009, abstract #V23D-2104
Yanagi T, Ichimaru Y, Hirahara S. Petrochemical evidence for coupled magma chambers beneath the Sakurajima volcano, Kyushu, Japan. Geochemical Journal, 1991;25, 17-30.
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