Eruptive history of Azuma volcano, NE Japan, during last 7,000 years: Stratigraphy and magma-plumbing system of the Azuma-Jododaira products.

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  • Yamamoto Takahiro
    Deep Geological Environments Research Center, Gological Survey of Japan, AIST

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  • 福島県，吾妻火山の最近7千年間の噴火史：吾妻－浄土平火山噴出物の層序とマグマ供給系
  • フクシマケン アズマ カザン ノ サイキン 7センネンカン ノ フンカシ アズマ ジョウド ダイラ カザン フンシュツブツ ノ ソウ ジョ ト マグマ キョウキュウケイ

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Azuma volcano is one of the active volcanoes at the volcanic front of the Northeast Japan arc. This volcano consists of several andesitic edifices, which are older than 0.30 Ma, and recent products erupted from fresh craters on the eastern part of the volcano; the latter is newly named as the Azuma-Jododaira products. In this paper, the eruptive history and magma plumbing system of the Azuma-Jododaira products are clarified by tephrostratigraphic study, new 14C ages, and whole rock chemistry. The recent craters are concentrated within a NW-SE trending, 3-km-long, and 1-km-wide zone through Jododaira; Goshikinuma, Issaikyo, Issaikyo-minami, Oana, Tsubakurosawa, Iwotaira-minami, Kofuji, and Okenuma craters from north to south. The Azuma-Jododaira products consist of five magmatic units and seven phreatic ones. The magmatic eruptions involving Vulcanian explosions occurred at 6.7 ka from Okenuma, at 6.3 ka from Goshikinuma, since 5.9 ka to 4.8 ka from Kofuji with lava flow extrusion, at 4.3 ka from Issaikyo, and at 0.6 ka (ca. 1331 AD) from Oana craters, respectively. The volumes of erupted magma are about 5×10 -4 DRE km3, 3×10-4 DRE km3, 4×10-1 DRE km 3, 2×10-4 DRE km3, and 8×10-5 DRE km3, respectively. So, more than 99 % magma had erupted from Kofuji crater. The essential magmas in each units are made up largely of andesite with 56.8~59.7 wt% in SiO2 content, with minor amount of dacite with 60.7~62.8 wt% SiO2. Because the same magmas erupted from different craters, the Azuma-Jododaira products were fed by dike-like intrusion from the unchanged magma plumbing system involving distinct magma pockets. Presumably, this system has been controlled by local NE-SW extensional stress field around the volcano. The five phreatic fall units occur between the Issaikyo and Oana units. There is the ca. 1711 phreatic fall unit ejected from Oana above the Oana units. Immediately after the ca. 1711 eruption, gas emission and plume activity started around Oana and Iwodaira. Small phreatic explosions occurred within the geothermal area around Oana in 1893~1895, 1914, 1950, 1952, 1966, 1977 AD, respectively. <br>

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