支笏降下軽石堆積物について : 特に支笏カルデラ形成直前の活動について

• 勝井 義雄

  北海道大学理学部地質学鉱物学教室

書誌事項

タイトル別名

• On the Shikotsu Pumice-fall Deposit : Special Reference to the Activity just before the Depression of the Shikotsu Caldera

抄録

A number of pyroclastic deposits of the late Pleistocene to Recent are widely developed around the Shikotsu caldera, southwestern Hokkaido (Table 1, Photo. 1 & Figs. 1～4). A detailed volcano-stratigraphical studies on these pyroclastic deposits showed that the Shikotsu volcano began the eruption of mafic scoria followed by repeated ejections of intermediate to felsic pumice; then, after a short period of quiescence, a rhyolitic pumice-fall (Spfa 1) followed close by a tremendous amount of dacitic pumice-flow (Spfl) were erupted. The last violent activity caused a depression of the Shikotsu caldera (Krakatoan type of H. WILLIAMS) which measured 13 × 15km in diameter. After the depression, the alluvial volcanoes of Fuppushi, Eniwa and Tarumai were formed along the fissure which trends N 30°W across the caldera. The Shikotsu pumice-fall deposit (Spfa 1) was distributed in a fan-shaped area widening out southeastward from the caldera, decreasing in thickness as well as in grain size with the increase of distance from the source; it is actually traceable beyond the Hidaka mountain range more than 200km from the caldera (Fig. 3). The volume of the deposit is calculated as 2.5 × 10¹⁰m³ which is nearly comparable with that of the great eruption of Krakatau in 1883. Adding to this, the volume of the Shikotsu pumice-flow deposit (Spfl, 6～9 × 10¹⁰m³), the total volume of the pumiceous deposits erupted just before the depression of the caldera, amounted to as much as 1 × 11¹¹m³ (Table 5). The thermal energy released in this activity is estimated as 1 × 10²⁷ergs. However, the actual volume of liquid magma discharged by this eruption was considerably less than that of the vanished material. The Shikotsu pumice-fall deposit is covered by stream terrace deposits at some places and cut by neolithic beach line; it covers the Mammonteus bed correlated with the lower terrace deposit at Cape Erimo, and embeds a fossil forest composed of a number of erect truncks of Picea jezoensis at a level only a few meters above sea level in the Sapporo-Tomakomai low-land. According to the evidence supplied by these observations, the pumice-fall must have been deposited in the later Wurm ice age. Consequently, the Shikotsu pumice-fall deposit as well as the pumice-flow deposit rise in importance in Pleistocene chronology.

収録刊行物

• 火山.第２集

  火山.第２集 4 (1), 33-48, 1959

  特定非営利活動法人 日本火山学会

詳細情報

• CRID: 1390282681100161536
• NII論文ID: 110002993073
• NII論文ID: 10003669167
• NII論文ID: 10007431433
• DOI: 10.18940/kazanc.4.1_33
• ISSN: 24330590
• 本文言語コード: ja
• データソース種別:
  • JaLC
  • CiNii Articles
• 抄録ライセンスフラグ: 使用不可