地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
伊豆大島火山カルデラ形成以降の噴火史
小山 真人早川 由紀夫
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ジャーナル フリー

1996 年 105 巻 2 号 p. 133-162

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We reveal the detailed syn-and post-caldera eruptive history of Izu Oshima Volcano, Japan, by tephra and loess stratigraphy. Twenty-four tephra layers, which overlie the slope outside the caldera, show that 24 eruptions occurred since the formation of the caldera (about 1, 450 years ago). These eruptions are separated by 10-200 years clear dormant periods, which can be identified by eolian dust (loess) interbedded with tephra layers. The 24 eruptions can be classified into three types : 1) eruption with scoria and ash falls (12 eruptions), 2) eruption only with scoria falls (7 eruptions), and 3) eruption only with ash falls (5 eruptions). While tephra discharge mass of the type 1 is generally large (1.5×1010 to 7×1011 kg), that of the type 2 or 3 is small (0.6 ×109 to 1 ×1011 kg). The 1986 eruption is classified into the type 2. Debris avalanches, which occurred just before the caldera formation and covered almost all of the Izu Oshima island, demonstrate that the present caldera wall was formed by slope failure of an old edifice. The tephra-discharge stepdiagram, which shows a relationship between time and cumulative discharge volume / mass of magma, shows : 1) the average tephra-discharge rate is constant (92 kg/ s before the N1.0 eruption and 25 kg/s after the N1.0 eruption), showing an abrupt decrease of the rate at about the time of the N1.0 eruption, which occurred about 900 years ago and was the most voluminous eruption for the past 1, 450 years, 2) both before and after the N1.0 eruption, the type 1 eruption shows volume-predictability, that is, the discharge volume / mass of a next type 1 eruption can be predicted, 3) a type 1 eruption should occur sometime in the future again, and when it occurs, the discharge mass of tephra should attain to as much as 2×1011 kg or more.

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