TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 65 (No. 6), pp. 591-607, 2013

Precursory activity and evolution of the 2011 eruption of Shinmoe-dake in Kirishima volcano—insights from ash samples

Yuki Suzuki1, Masashi Nagai2, Fukashi Maeno1, Atsushi Yasuda1, Natsumi Hokanishi1, Taketo Shimano3, Mie Ichihara1, Takayuki Kaneko1, and Setsuya Nakada1

1Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
2National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, Ibaraki 305-0006, Japan
3Graduate School of Environment and Disaster Research, Fuji Tokoha University, 325, Obuchi, Fuji, Shizuoka 417-0801, Japan

(Received October 30, 2012; Revised February 13, 2013; Accepted February 17, 2013; Online published July 8, 2013)

Abstract: After a precursory phreatic stage (2008 to 2010), the 2011 Shinmoe-dake eruption entered a phreatomagmatic stage on January 19, a sub-Plinian and lava accumulation stage at the end of January, a vulcanian stage in February-April, and a second phreatomagmatic stage in June-August. Component ratio, bulk composition, and particle size of the samples helped us define the eruptive stages. The juvenile particles were first found in the January 19 sample as pumice (8 vol%) and were consistently present as scoria and pumice particles thereafter (generally ∼50 vol%, decreasing in weaker events). The January 19 pumice has water-quench texture. After the lava accumulation, particles of that lava origin came to account for 30∼70 vol% of the ash. The second phreatomagmatic stage is proposed because of fine ash and long eruption period. The SiO2 contents of bulk ash are lower in post-January 19, 2011 eruptions, reflecting lower average SiO2 contents in 2011 ejecta than in past ejecta. The free-crystal assemblages were two pyroxenes + plagioclase + Fe-Ti oxides until 2010; olivine joined the assemblage in 2011, when juvenile ash was erupted. This change is consistent with the absence or smaller sizes of olivine phenocrysts in past ejecta.
Key words: Volcanic ash, Shinmoe-dake, bulk ash composition, component ratio, particle size distribution.

Corresponding author E-mail: ysuzuki@eri.u-tokyo.ac.jp

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