著者

南 裕介 中川 光弘 佐藤 鋭一 和田 恵治 石塚 吉浩

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.66, no.3, pp.211-227, 2021-09-30 (Released:2021-10-29)

参考文献数

30

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Meakandake Volcano is a post-caldera active stratovolcano located on the south-eastern rim of Akan Caldera, eastern Hokkaido, Japan. Recent eruptive activity has occurred in 1955-1960, 1988, 1996, 1998, 2006, and 2008 at Ponmachineshiri, which is one of several volcanic bodies that form the stratovolcano. These events indicate that Ponmachineshiri has a high potential for future eruptions. In order to better understand the hazards posed by Meakandake Volcano, this study focused on the modern eruptive activity of Ponmachineshiri during the last 1,000 years. The authors conducted field observations at outcrops in the summit area, excavation surveys on the volcanic flanks, component analysis for pyroclastic deposits, and radiocarbon dating for intercalated soil layers. As a result, at least four layers of pyroclastic fall deposits derived from Ponmachineshiri during the last 1,000 years were recognized, ranging from Volcanic Explosivity Index (VEI) levels of 1 to 2. In chronological order, the major pyroclastic fall deposits consist of Pon-1 (10th to 12th century; VEI 2), Pon-2 (13th to 14th century; VEI 2), Pon-3 (15th to 17th century; VEI 1), and Pon-4 (after AD 1739; VEI 1), with small-scale (VEI<1) phreatic and phreatomagmatic eruption deposits intercalated within Pon-1, Pon-2, and Pon-3 pyroclastic fall deposits. The presence of scoria and minor pumice in the Pon-1, Pon-2, and Pon-3 pyroclastic fall deposits suggests that these eruptions were phreatomagmatic events. On the other hand, the absence of juvenile materials in the Pon-4 pyroclastic fall deposits suggests that the activity was a phreatic eruption. The decreasing proportion of juvenile materials in eruptive deposits over the last 1,000 years is consistent with a reduced magma contribution and indicates that the development of the hydrothermal system is likely to play an important role in future eruption scenarios for Meakandake Volcano.

著者

和田 恵治 弦巻 峻哉 池谷内 諒 佐野 恭平 佐藤 鋭一

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2016年大会

巻号頁・発行日

2016-03-10

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黒曜石はほとんどガラスからなりガラス構造中にH2O成分を含む。高温で加熱するとH2O成分が発泡し,軟化した緻密な黒曜石ガラス中で気泡が膨張して内部が多孔質な軽量物質(パーライトと呼ばれる)ができる。筆者らはこれまで北海道産黒曜石を電気炉中で加熱して発泡させる実験を行ってきたが,電気炉の温度設定や加熱時間,発泡開始の定義が確立されていなかった。またパーライトの組織観察を十分に行っていなかった。今回,(1)各産地における黒曜石の発泡開始温度とパーライト形成温度の測定,(2)パーライトの組織観察による分類,(3)天然の多孔体試料との組織比較を行ったので報告する。黒曜石11試料の発泡開始温度の測定においては,径2.5~4mmの黒曜石片10個を磁製皿に入れ,設定温度に昇温させた電気炉中で30分間保持した後に取り出して気泡の有無を実体顕微鏡下で確認して10個すべてが発泡した場合にその黒曜石試料の発泡開始温度(Tf)とした。パーライト形成温度についても同様の実験方法で計測し,10個すべてが完全に発泡してパーライトとなった温度をパーライト形成温度(Tp)とした。これらの加熱実験の結果,赤井川産Tf =780℃;Tp =830℃,奥尻産Tf =790℃;Tp =850℃,神津島産Tf =890℃;Tp =950℃,白滝産(IK露頭)Tf =900℃;Tp =1030℃,十勝三股産Tf =930℃;Tp =1060℃,置戸産(所山)Tf =990℃;Tp =1100℃,置戸産(北所山)Tf =1010℃;Tp =1090℃,白滝産(十勝石沢露頭)Tf =1030℃;Tp =1160℃,白滝産(球顆沢露頭)Tf =1060℃;Tp =1150℃,白滝産(西アトリエ)Tf =1070℃;Tp =1190℃,白滝産(あじさいの滝露頭)Tf =1070℃;Tp =1190℃であった。パーライト組織の観察では各産地の黒曜石を1cmキューブ状にしたものをパーライトに作成した。気孔の大きさや形態・数密度から3つのタイプ(A~C)に分類した。Aタイプは気孔の大きさが約1mmであり,1つ1つが独立して球形をなす。表面・断面共に光沢がある。これらは発泡開始温度が990℃以上,及びパーライト形成温度が1060℃以上の6試料である。Bタイプは気孔の大きさが1.5mm〜5mmで1つ1つ独立している。気孔は球形〜不規則形で歪んだ形状を示す。表面は白灰色だが断面は光沢を示す。これらはTfが900℃〜930℃,Tpが1030℃〜1060℃の2試料(白滝IK露頭・十勝三股)である。Cタイプは気孔の大きさがパーライトの気孔組織は,加熱温度や加熱時間・黒曜石の水分量が深く関係し,ガラス構造に基づく物性(ガラス粘度など)も気泡の形状に関係するかもしれない。天然の多孔体(軽石や発泡した黒曜石)の気孔組織と比較すると,気孔の形状や数密度が天然多孔体と異なる。これは,(1)黒曜石がすでに脱ガスした試料で水分量が少ないこと,(2)天然多孔体がマグマ流体の動きの中で気泡が生成し移動や引き延ばしによってできた形状なのに比して,パーライトは静的な条件のもと,軟化した黒曜石壁を気泡が等方状に膨張したことに起因すると考えられる。

著者

佐藤 鋭一 和田 恵治 野口 昌宏

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.3, pp.255-271, 2022-09-30 (Released:2022-10-27)

参考文献数

37

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Kurodake volcano in the Taisetsu volcano group was formed approximately 0.2 Ma, producing andesitic lava flows and a dome. The lavas contain numerous mafic inclusions (<20 vol.%) ranging from approximately 1 cm to about 30 cm in diameter. The mafic inclusions exhibit typically rounded to ellipsoidal shapes and have smooth contacts with the host lavas. The mafic inclusions are classified into two types, fine and coarse, based on the size of the groundmass crystals. The groundmass crystals of the fine-type inclusions are composed of acicular minerals (0.1-0.3 mm in length). On the other hand, the groundmass of the coarse-type inclusions is primarily composed of tabular minerals (>85 vol.% and 0.2-0.5 mm in length). The plagioclase core compositions of the host lavas and two types of mafic inclusions vary substantially from An38 to An90. The plagioclase phenocrysts are classified into three groups based on their core compositions: An-rich (type A: An>80), An-poor (type B: An<60), and intermediate (type C: 60<An<80). Type A and type B plagioclases were derived from mafic and silicic magmas, respectively, and type C was derived from a hybrid magma formed by the mixing of the mafic and silicic magmas. The host lavas predominantly contain type B plagioclase phenocrysts, with infrequent types A and C, and most of the plagioclase microphenocrysts and groundmass crystals are type C. In the fine-type inclusions, type A and type B plagioclase phenocrysts coexist, and most of plagioclase microphenocrysts and groundmass crystals are classified into the type C, similar to the host lavas. In the coarse-type inclusions, most of the plagioclase phenocrysts, microphenocrysts, and groundmass crystals are classified as type B. These assemblages in the host lavas and fine-type inclusions can be explained by the mixing of the magmas, whereas the coarse-type inclusions were formed in the silicic end-member magma. Initially, mafic magma containing type A plagioclase was injected into bottom of the silicic magma chamber containing type B. A small amount of mafic magma was mixed with silicic magma to form the host magma. Subsequently, mixing occurred near the boundary between the mafic and silicic magmas, producing the fine-type inclusion forming magma. We presume that the margin of the silicic magma chamber was highly crystalline and the coarse-type inclusions were derived from the margin of the silicic magma chamber.

著者

和田 恵治 佐野 恭平

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.60, no.2, pp.151-158, 2015-06-30 (Released:2017-03-20)

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The Shirataki obsidian-rhyolite field (Shirataki Geopark, Hokkaido) contains many outcrops of densely compact obsidian layers of excellent quality. The Shirataki obsidian lavas (SiO2=76.7-77.4 wt.%) were erupted at ca. 2.2 Ma and formed a monogenetic volcano comprising 10 obsidian-rhyolite lava units. The thickness of the units ranges from 50 to > 150m, and each unit comprises a surface clastic zone, an upper dense obsidian zone, an upper banded obsidian zone, a central thick rhyolite zone, a lower banded obsidian zone, a lower dense obsidian zone, and a lower clastic zone. The dense obsidian is > 98% glass with microlites of mainly magnetite and plagioclase, and rare plagioclase phenocrysts. Obsidian and rhyolite within single lava flows have similar bulk-rock compositions and number density of microlites, although the rhyolite contains glass with perlitic cracks and a large amount of crystalline material (spherulites and lithophysae), while the dense obsidian contains 0.4-0.8 wt.% H2O. These geological and petrological features indicate that the formation of obsidian and rhyolite layers in the lava units was controlled mainly by the timing of the vesiculation and degassing of magmas, in addition to the cooling effect.

著者

和田 恵治 佐藤 鋭一 佐野 恭平 向井 正幸

出版者

特定非営利活動法人 日本火山学会

雑誌

日本火山学会講演予稿集 2011 (ISSN:24335320)

巻号頁・発行日

pp.2, 2011-10-02 (Released:2017-02-10)

著者

安田 裕紀 佐藤 鋭一 和田 恵治 鈴木 桂子

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.60, no.4, pp.447-459, 2015

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Eruption interval between Hb- and Px-types pyroclastic-flows from the Ohachidaira caldera of Taisetsu volcano, central Hokkaido, Japan, was estimated from the paleomagnetic directions. Px-type pyroclastic-flow deposit rests on Hb-type one, and gravel beds are intercalated between them. Oriented 138 samples were collected from 13 sites for paleomagnetic analysis. The paleomagnetic direction of Hb-type pyroclastic-flow deposit shows a normal polarity with a westerly declination(overall mean is N=7, D=-27.1°, I=66.3°, α_<95>=2.7°, k=511.2), while that of Px-type pyroclastic-flow deposit shows a normal polarity with an easterly declination (overall mean is N=6, D=19.8°, I=67.5°, α_<95>=4.6°, k=213.8). The two paleomagnetic directions are significantly different, and the time interval between the two pyroclastic eruptions is estimated to be more than about 100±40 years based on the geomagnetic secular variation in China, Russia, Europe, North America, and Japan.

著者

佐藤 鋭一 和田 恵治

出版者

特定非営利活動法人日本火山学会

雑誌

日本火山学会講演予稿集

巻号頁・発行日

vol.2004, 2004-10-19

著者

和田 恵治 中村 瑞恵 奥野 充 佐藤 鋭一

出版者

特定非営利活動法人日本火山学会

雑誌

日本火山学会講演予稿集

巻号頁・発行日

vol.2003, 2003-10-11

被引用文献数

1

著者

和田 恵治 石崎 直人 佐藤 鋭一

出版者

一般社団法人 日本地質学会

雑誌

日本地質学会学術大会講演要旨 第114年学術大会(2007札幌) (ISSN:13483935)

巻号頁・発行日

pp.560, 2007 (Released:2009-01-30)

参考文献数

4