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

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

Yuki Suzuki, Masashi Nagai, Fukashi Maeno, Atsushi Yasuda, Natsumi Hokanishi, Taketo Shimano, Mie Ichihara, Takayuki Kaneko, and Setsuya Nakada

Earth Planets Space, 65, 591–607, doi :10.5047/eps.2013.02.004, 2013


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.

  図1 火山灰から推定された2011年と先行的活動の特徴.各ステージの火山灰の特徴を四角枠に示している.写真のソースについては,論文参照(オープンアクセス). 図2 火山灰構成物の時間変化(主に250-500m径;オープンアクセスの論文参照).ローズカラーは全て2011年のマグマに由来するカテゴリーである.すなわち,噴火毎のマグマ物質(軽石とスコリア)や,2011年1月末に火口に定置した溶岩(図1)由来の粒子である.2011年6〜8月については,火口溶岩の変質が進んでいたため(図1),溶岩粒子の分類は試みていない.

図3 火山灰と岩石試料のバルクデータ.岩石について,2011年活動はSuzuki et al. (2013, JVGR), 過去噴火は田島•他(2013)のデータを引用.2011年噴火の岩石試料のSiO2平均量は,(白色軽石の噴出物に占める割合が5%未満であるので),57-58wt. %程度とみられる.一方,過去噴火については,59-63wt. %に集中する.K2O量が岩石試料に比べ火山灰で系統的に乏しいのは,SiO2に富みK2Oに乏しい変質物質を含有している影響である(論文参照;オープンアクセス)