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Aso is situated in central Kyushu, SE Japan. It is one of five large Quaternary caldera-forming volcanoes along with 23 smaller volcanoes that make up a volcanic chain known collectively as the Hoki Volcanic Zone (Hunter 1998). It lies 20 km behind the volcanic front, and 275km from the Ryukyu trench (Hunter 1998), where the Philippian plate is subducting beneath the Eurasian plate, one of many plate contacts within Japan.
Figure showing areal view of Aso caldera, central peaks can also be seen. From http://www.trekearth.com/gallery/Asia/Japan/Kyushu/Kumamoto/photo42042.htm
Aso volcano has a typical composite structure mainly made up of andesitic rocks (Ono and Watanabe 1985). It is made up of a large elliptical caldera 17km E-W, and 25km N-S (Tanaka 1993), and within the caldera there are several cones but the only active one is Mount Naka-Dake (Tanaka 1993). The crater of Mt Naka-Dake is made up of 7 craterlets resulting in a rather elongate crater. The only currently active craterlet is the First crater, which is also the most northerly; other craterlets were active before the 1933 eruption (Ono 1995).
Volcanic activity has been occurring for about 3Ma, and three different stages of activity have been defined: Pre-Caldera activity (3-0.4Ma); Caldera activity (370-70Ka); and Post Caldera activity (Hunter 1998). The Caldera was formed by 4 large eruptions, and their respective eruptive units have been named accordingly from 1-4 (Hunter 1998). A dominant trend has been noted during the caldera activity, which is a switch from dominantly tholeiitic to calc-alkaline, however it is a feature that both magmas are present constantly throughout the volcanoes lifetime (Hunter 1998). This is unusual as volcanoes normally follow one trend, or may even switch trend, it is unique that the volcano displays both throughout its history. The main cause for the switch is thought to be due to fractional crystallisation where Wo is continually removed due to augite and hypersthene extraction, resulting in Fe3+ enrichment. This results in the crystallisation of titanomagnetite, and orthopyroxene crystallises instead of augite (Hunter 1998). Fractional crystallisation is also probably the main cause of the different eruptive compositions, but some evidence of crustal assimilation is found, which generally decreases with time (Hunter 1998).
Figure of Naka-Dake crater from Tanaka 1993.
Recent activity has followed a broadly similar cycle (Tanaka 1993, Ono 1995). Fumarole activity is continuous in the southern part of First crater, and during periods of quiescence there is an acidic crater lake (pH 0.81 (Onda 2003)). As activity increases, the water evaporates, and increased lake stirring can be seen with some fountaining and mudflows as well. Eventually the lake dries up and parts of the crater floor become incandescently red. Volcanic ash is then ejected as the vent opens, with occasional strombolian or phreatic events. Peles hair have been found suggesting a body of hot magma is close to the surface. Eventually the energy of the eruptions decline and the vent is closed off by landslides, a new crater lake is then formed. Recent eruptions occurred in 1979, 1984-1985, 1989-1990, 1994, 1995, and 2003.
Attempts at measuring gas compositions have been made by Shinohara, Ono and Mori using FT-IR. Direct sampling has been impossible due to the steepness of the inner walls of the crater; where the drop is almost shear. SO2 emissions during quiescent periods are around 200t/d, but fluxes rise to 2000 t/d during eruptive phases (Institue of Seiesmology and Volcanology 2004, Mori 2008). CO2 fluxes also increase prior to eruptive cycles (Word organisation of volcano observatories). Shimabara Earthquake and Volcano Observatory continuously monitor gaseous emissions, which is part of Kyushu University. There are two sites of degassing within the volcano, from the lake, and from the fumaroles located in the south of First Crater. The lake gasses have lower CO2/SO2, and HCl/SO2, but higher SO2/H2S than the fumaroles (Shinohara 2010). The lower HCl can be explained by scrubbing, when the lake was at its highest level, HCl emissions were at their lowest (Mori 2008). But SO2 should be higher in the fumaroles than in the lake, but CO2/SO2 show the opposite, as it is higher in the fumarole emissions (Shinohara 2010). This may show a hydrothermal input into the fumaroles, which result in SO2 scrubbing, but the bulk of the fumarole gases come from a magmatic source as RH (log H2/H2O) of -2 to -3 and SO2/H2S of 10 to 30 indicate equilibrium temperatures of 750- 950 °C (Shinohara 2010).
Hunter A G. Intracrustal controls on the coexistence of tholeiitic and calc-alkaline magma series at Aso Volcano, SW Japan. J. Petrol. 1998;39:7:1255–1284.
Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu Universityhttp://www.sevo.kyushu-u.ac.jp/index-e.html
Mori T, Notsu K. Remote CO, COS, CO2, SO2, HCl detection and temperature estimation of volcanic gas. Geophys. Res. Lett., 1997;24:2047–2050.
Mori T, Notsu K. Temporal variation in chemical composition of the volcanic plume from Aso volcano, Japan, measured by remote FT-IR spectroscopy. Geochem J 2008;42:133-140
Onda Y, Ohsawa S, Takamatsu N. A colorimetric and geochemical study of the coloration factor of hyper-acid active crater lakes (in Japanese). Jpn J Limnol 2003;64:1–10
Ono K, Watanabe K. Geological map of Aso volcano Geol. Surv. Japan. (1985).
Ono K, Watanabe K, Hoshizumi H, Ikebe S. Ash eruption of the Naka-dake crater, Aso volcano, southwestern Japan. J. Volcanol. Geotherm. Res, 1995;66:137–1148.
Shinohara H, Yoshikawa S, Miyabuchi Y. Degas- sing of Aso Volcano, Japan through an acid crater lake: differentiation of volcanic gas-hydrothermal fluids deduced from volcanic plume chemistry, Abstract V23A- 2387, AGU Fall Meeting. 2010
Tanaka, Y., Eruption mechanism as inferred from geomagnetic changes with special attention to the 1989–1990 activity of Aso Volcano, J. Volcanol. Geotherm. Res.1993;56:319–338.
Word organisation of volcano observatories http://www.wovo.org/0802_10.html