著者

下司 信夫 小林 哲夫

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.51, no.1, pp.1-20, 2006-02-28

被引用文献数

2

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Volcanic history of Kuchinoerabujima Volcano in the last 30,000 years is reconstructed based on tephra stratigraphy. Kuchinoerabujima is a volcanic island which is a cluster of at least nine volcanic edifices; Gokyo, Jyogahana, Ban-yagamine, Takadomori, Noike, Kashimine, Hachikubo, Furutake and Shintake. Eruptions within the last 30,000 years occurred from Noike, Hachikubo, Furutake and Shintake volcanoes. Two major pumice and scoria eruptions occurred between 15 and 11 ka after an inactive period since ca. 30ka. NoikeYumugi tephra (15-14ka, DRE>0.06km^3), erupted from the summit of Noike Volcano, consists of Yumugi pumice fall deposit and Nemachi pyroclastic flow deposit. Furutake-Megasaki tephra (12-11 ka, DRE ca. 0.8km^3) erupted from Furutake Volcano and consists of Furutake agglutinate, Furutake scoria flow deposit and Megasaki scoria fall deposits. Volcanic edifice of Older Furutake was built during the 12-11 ka eruption. Eruption style changed around 10ka, after the collapse of Older Furutake Volcano. Activities of Yougner Furutake and Shintake Volcanoes are characterized with effusion of lava flow and no major pumice eruption is recognized. Lithic tephra erupted from Younger Furutake and Shitake Volcanoes within the last 10,000 indicates repetitive Vulcanian-type and phreatomagmatic eruptions. All historical eruptions since 1841 occurred at and around Shintake crater and were Vulcanian-type explosions with emission of magmatic materials and phreatic explosions.

著者

笠原 順三 阿部 なつ江 荒井 章司 下司 信夫 熊谷 英憲 丸山 茂徳 小原 一成 小原 泰彦

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.117, no.1, pp.1-4, 2008-02-25 (Released:2010-02-10)

参考文献数

17

著者

下司 信夫

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.118, no.6, pp.1254-1260, 2009-12-25 (Released:2010-03-23)

参考文献数

10

被引用文献数

11 14

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A large pyroclastic eruption occurred around 7.3 ka from the Kikai caldera about 30 km north of Yakushima Island. Its pyroclastic flow and fall deposits covered the entire area of Yakushima Island and may have influenced the evolution of unique floras and faunas of Yakushima Island. Detailed field survey revealed that the Koya pyroclastic flow deposit spread from NW to SE, covering almost the entire area of Yakushima. A part of the southern coastal area remained from the pyroclastic flow due to local alignment of topographic ridges and valleys, which acted as barriers to the pyroclastic flows. Possible tsunami deposits associated with the Kikai-Akahoya eruption were discovered in the area below ca. 50 m above sea level along the northern coasts of Yakushima and Kuchinoerabujima Islands.

著者

星住 英夫 宮縁 育夫 宮城 磯治 下司 信夫 宝田 晋治

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.1, pp.91-112, 2022-03-31 (Released:2022-04-26)

参考文献数

71

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Aso volcano produced four huge ignimbrite-forming eruptions named Aso-1, 2, 3 and 4 in ascending order, among which Aso-4 is considered the largest eruption in Japan in the last 1 million years. This paper describes the tephra sequence between the Aso-4 and Aso-3 eruptions (Aso-4/3 tephra group). The reconstruction of the eruptive history for Aso-4/3 tephra group presented here provides a valuable contribution to the understanding of caldera volcanism by outlining the preparatory processes of a catastrophic ignimbrite eruption. The eruption sequence of the Aso-4/3 tephra group, which is composed of at least 37 units of pumice-fall, scoria-fall, and ash-fall deposits, is divided into five stages. Stage 1 is characterized by the eruption of mafic scoria (VEI 3-4) during 133-114.1 ka, after the eruption of Aso-3. Stage 2 is characterized by the frequent eruptions of mafic scoria and ash (VEI 3-4) during 114.1-108.4 ka. The magma composition became more felsic during explosive eruptions (VEI 3-4) from 108.4-104.7 ka (Stage 3). During the most active stage from 104.7-97.7 ka (Stage 4), voluminous felsic pumice-falls erupted (VEI 4-5). The ABCD tephra (97.7 ka) is the largest plinian pumice-fall deposit of Aso volcano. Stage 5 (97.7-88 ka) is a relatively dormant period, during which only a biotite dacite pumice-fall was deposited (VEI 4). The low number of eruptions during stage 5 suggests that the magma supply rate decreased during the 10 thousand years that preceded the Aso-4 ignimbrite eruption. The estimated total tephra volume for the Aso-4/3 tephra group is 23 km3, which corresponds to 10 km3 in dense rock equivalent (DRE). The estimated the long-range tephra discharge rate (0.23 km3 DRE/ky) is similar to that in the post-caldera stage of Aso-4 (0.2 km3 DRE/ky).

著者

安間 了 山本 由弦 下司 信夫 七山 太 中川 正二郎

出版者

一般社団法人 日本地質学会

雑誌

地質学雑誌 (ISSN:00167630)

巻号頁・発行日

vol.120, no.Supplement, pp.S101-S125, 2014-08-31 (Released:2014-12-26)

参考文献数

65

被引用文献数

1 3

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世界自然遺産・屋久島の生物多様性を支えるのは海洋性の環境の中に現出する高山地形である.多くの海洋島が火山からなるのに対して,屋久島の基盤を構成するのは四万十帯の砕屑性堆積岩類と屋久島花崗岩である.本巡検では高山を形成する屋久島花崗岩の貫入機構を,正長石巨晶の定方向配列,岩脈の分布,貫入に伴う母岩の変形と接触変成作用の観察を通して議論する.母岩の四万十帯の地層や枕状溶岩の産状,付加体中での圧密,メランジュやデュープレックス構造の形成,地震による液状化構造がどのような順序で発達したかを観察し,付加体の変形史とメランジュの認定基準について議論する.また鬼界カルデラの噴火に伴う火砕流堆積物の産状,噴火による地震が引き起こした液状化などの構造を観察し,海中における爆発的噴火がもたらしうる災害のシナリオを検討する.

著者

山元 孝広 石塚 吉浩 下司 信夫

出版者

一般社団法人 日本地質学会

雑誌

地質学雑誌 (ISSN:00167630)

巻号頁・発行日

vol.126, no.3, pp.127-136, 2020-03-15 (Released:2020-07-31)

参考文献数

35

被引用文献数

1 2

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富士山東方の静岡県小山町の新東名高速道路の2018年度建設工事現場で,大規模な火山性の斜面崩壊堆積物を確認し,大御神岩屑なだれ堆積物と命名した.この堆積物は,大洞山東斜面に堆積していた富士火山起源のスコリア降下火砕物が表層崩壊を起こしたもので,その南東山麓に長さ4.5 km,最大幅1.5 km,体積9.3×106 m3の規模で広がっている.堆積物直下に神津島天上山テフラの降下層準があること,直上土壌の14C 暦年代から,発生時期は平安時代のAD 838から10世紀前半に特定された.この期間中には東海・南海連動の巨大地震であるAD 887の仁和地震が起きており,この地震動によって斜面崩壊が発生した可能性が強い.

著者

中島 隆 下司 信夫 及川 輝樹

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117年学術大会(2010富山) (ISSN:13483935)

巻号頁・発行日

pp.255, 2010 (Released:2011-03-31)

参考文献数

4

著者

下司 信夫 中野 俊

出版者

国立研究開発法人 産業技術総合研究所 地質調査総合センター

雑誌

地質調査研究報告 (ISSN:13464272)

巻号頁・発行日

vol.58, no.3-4, pp.105-116, 2007-08-31 (Released:2014-05-22)

参考文献数

17

被引用文献数

4 4

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口之島火山は琉球弧火山フロント上に成長した安山岩質の複成火山であり,0.3 Ma以降少なくとも 10 個の角閃石安山岩質溶岩ドームが輝石安山岩質の火山体の上に成長している.軽石流堆積物からなる大勝火砕流堆積物は約 4 万年前ごろにウエウラ火山から噴出した.大勝カルデラの形成後,横岳,南横岳,北横岳の少なくとも 3 つの溶岩ドームが形成された.複数の火砕流がこれらの溶岩ドームの形成に伴って発生し,そのうち南横岳から噴出した火砕流堆積物からは 1.9 万年前の年代が得られている.7,900 年前ごろの横岳・南横岳・北横岳の馬蹄形崩壊によって岩屋口岩屑なだれ堆積物が発生した.この崩壊地形の内部に前岳火山が成長した.落しの平,燃岳火山及びそのほかいくつかの小規模な溶岩ドームが前岳溶岩ドームの形成後に成長した.口之島火山の過去 4 万年間の噴出率はおよそ 8.5 × 104m3/yr と見積もられ,こられは琉球弧や東北日本弧火山フロントの代表的な火山に比べてかなり小さい.燃岳火山は口之島の中で最も新しい溶岩ドームである.燃岳 溶岩ドームの山頂部には幾つかの爆発火口が開口しており,これらは前岳溶岩ドーム上で水蒸気爆発が繰り返し発生したことを示している.最新の水蒸気噴火は 18 世紀以降の可能性がある.

著者

下司 信夫

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.61, no.1, pp.101-118, 2016-03-31 (Released:2017-03-20)

被引用文献数

3

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Large-scale pyroclastic eruption is one of the most awful natural disasters on the earth. Though their frequency is relatively low compare to the lifetime of human society, large-scale pyroclastic eruption can make serious impact on the global environment. Frequency of the volcanic eruptions shows a negative correlation against their scale: global frequency of the eruptions larger than VEI7 is approximately ten per 10,000 years, whereas more than 10 eruption of VEI4 occur every 10 years. The storage of voluminous magma within a shallow crust is a key process for the preparation for large-scale eruption. Inactive thermal convection in highly-crystallized magma bodies and visco-elastic behavior of the surrounding host rock can allow the stable storage of voluminous felsic magma at the neutral buoyancy level in the upper crust. Segregation of interstitial melt to form a melt pocket in highly-crystallized magma body can cause smaller scale of eruptions, whereas the remobilization of entire part of magma chamber will result a large-scale eruption with caldera collapse. Rupture and collapse of the roof rock of magma chamber induced by rapid decompression of magma chamber is the fundamental process of the eruption of voluminous magmas within short period. The decompression of magma chamber activates the slip of ring fault at the marginal portion of the roof and consequently the caldera starts subsidence. The collapse is controlled by the decompression inside the chamber and the strength of the roof rock. Ring fault turns to an open ring facture through which the voluminous magma can erupt to produce large ignimbrite. The volume of magma erupts during a caldera-forming eruption against the total magma chamber volume show negative correlation against the chamber size. This means that the large fraction of magma can remain even after caldera collapse particularly in large magma chamber. Evaluation of "precursory process" for catastrophic eruption is important to understand the driving mechanism of catastrophic eruption and also the hazard assessment. Accumulation of magma and building of a large-volume magma chamber within the earth’s crust is a long-term preparation process for catastrophic eruption. Short-term process for catastrophic eruption is the destabilization and rupturing process of the magma chamber.

著者

下司 信夫

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.127, no.2, pp.175-189, 2018-04-25 (Released:2018-06-11)

参考文献数

47

被引用文献数

2 2

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The fundamental structure of a collapse caldera is subsidence of a block(s) into a magma chamber with the evacuation of a massive volume of magma from the chamber. Decompression of a magma chamber caused by the extraction of magma from a magma chamber coinciding with pyroclastic eruption, effusive eruption, and lateral magma migration drives caldera collapse. Caldera-forming eruptions exhibit wide variations. Caldera-forming pyroclastic eruptions are characterized by a high eruption rate, whereas caldera-forming effusive eruptions have a much lower effusion rate. Caldera collapse caused by a pyroclastic eruption generally occurs within one day, whereas incremental collapses continue for up to one month in the case of some effusive eruptions. Collapse calderas also have wide structural variations. The aspect ratio of the roof of the magma chamber controls the development of caldera faults. The development of multiple caldera faults with a high aspect ratio causes piecemeal collapses, whereas a low aspect ratio results in the subsidence of coherent blocks detached by a simple ring fault. With the progress of caldera subsidence, the caldera structure develops from a flexural down sag to a double-ring fault system, and finally reaches an upward-flaring “funnel shaped” caldera following an intense collapse of the caldera wall. Calderas vary widely in size from 1 km to 100 km, and can also be divided into at least three classes by their internal structures. The largest group of more than 20 km across is characterized by flexural down-sag deformation. The intermediate-size group is characterized by well-developed caldera-border faults. The smallest calderas of less than 10 km across may have piecemeal structures. The existence of a pre-caldera volcanic edifice is also an indicator of differences in the magmatic system. Some calderas form at the summit of a pre-existing stratovolcano or shield volcano, whereas some large calderas form in a cluster of small volcanoes, involving non-volcanic basement rocks. A structural model of caldera development should involve these wide spectrums of collapse calderas. The development of collapse calderas is controlled by variations of magmatic activity, such as eruption style, eruption rate, and duration of eruption, as well as the architectures of their magma storage systems.

著者

白尾 元理 中田 節也 金子 隆之 長井 雅史 鴨野 岳人 下司 信夫 野上 健治 平林 順一 金沢 敏彦

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 (ISSN:24335320)

巻号頁・発行日

vol.2000, 2000

著者

下司 信夫 嶋野 岳人 長井 雅史 中田 節也

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.47, no.5, pp.419-434, 2002

参考文献数

36

被引用文献数

6

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Erupted magma of the 2000 eruption of the Miyakejima volcano changed from basaltic andesite to basalt during the caldera formation, from aphyric basaltic andesite with SiO₂=54 wt.% to plagioclase-phyric basalt with SiO₂=51.5 wt.%. Whole-rock compositions of the basaltic andesite of the June and July eruptions are plotted on the extension of the temporal variation of the previous eruptive materials, suggesting that the andesitic magma erupted in June and July eruptions were driven from the magma system worked for the last 500 years. Petrological character of the basalt in the eruptive materials of August, by contrast, is different from the previous lavas of the Miyakejima volcano. This shows that a new basaltic magma ascended to the shallow magma system after the caldera collapse. Identical ratio of the incompatible elements among the eruptive materials of the 2000 eruption and the recent eruptions suggests that they were driven from a common parental magma.

著者

長井 雅史 大野 希一 下司 信夫 中田 節也 千葉 達朗 中山 聡子 大学合同観測班地質グループ

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 (ISSN:24335320)

巻号頁・発行日

vol.2001, 2001

著者

中野 俊 下司 信夫

出版者

国立研究開発法人 産業技術総合研究所 地質調査総合センター

雑誌

地質調査研究報告 (ISSN:13464272)

巻号頁・発行日

vol.59, no.3-4, pp.197-201, 2008-11-20 (Released:2013-08-28)

参考文献数

9

被引用文献数

2 4

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鹿児島県,トカラ列島の北部に位置する小臥蛇島は直径0.5×1 km,角閃石安山岩- デイサイト質の小さな火山島で,底面の直径5×10 km,比高800 m以上の海山の山頂部に当たる.この島の東半分は熱水変質を被っており,2005年及び2006年の現地調査では数ヶ所で噴気・温泉活動を確認した.最近,後期更新世のK-Ar年代が報告されたが,小臥蛇島は第四紀火山であるだけでなく,噴気活動の存在から現在でも活動的な火山であることが明らかになった.

著者

下司 信夫

出版者

一般社団法人 日本地質学会

雑誌

地質学雑誌 (ISSN:00167630)

巻号頁・発行日

vol.109, no.10, pp.580-594, 2003-10-15 (Released:2008-04-11)

参考文献数

28

被引用文献数

7 11

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設楽火山岩体の一部を構成する大峠火山岩体の形成過程は,コールドロン形成期の火砕噴火のステージと,後コールドロン期の岩脈・シート群の貫入のステージに分けられる後コールドロン期における貫入岩体の形成は,大峠コーンシート群の形成から設楽中央岩脈群の形成に漸移し,ほぼ同時期に大峠ストック群が形成された大峠火山岩体は主にアルカリ苦鉄質岩からなり,少量のカルクアルカリ質デイサイトを伴う全岩組成の時間変化は,後コールドロン期の初期に発生した浅部のマグマ溜りへの未分化な苦鉄質マグマの再供給によって後コールドロン期の貫入岩体が形成されたことを示唆する

著者

中田 節也 長井 雅史 安田 敦 嶋野 岳人 下司 信夫 大野 希一 秋政 貴子 金子 隆之 藤井 敏嗣

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.110, no.2, pp.168-180, 2001-04-25 (Released:2009-11-12)

参考文献数

18

被引用文献数

41 43

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The 2000 eruption of Miyakejima volcano started with a submarine eruption of basaltic andesite on the morning of June 27, which occurred following earthquake swarms during the previous night. The main phase of the summit eruption began, being associated by a sudden subsidence of the summit area on July 8. Continuous collapsing of the summit area that had continued until midAugust, resulted in the formation of a caldera with the volume of about 0.6 km3. Phreatic (or phreatomagmatic) eruptions took places during the growth of the caldera, although the total volume of eruptives was about 11 million m3. which is smaller by one magnitude than the caldera volume. Eruptives are enriched with hydrothermally altered materials such as smectite and kaolinite.The manner of the first collapse suggests the existence of a large open space under the summit just before the subsidence. Judging from geophysical observation results, the open space may have ascended in the manner of stoping. Successive formation of open spaces at deeper levels is likely to have caused the continuous collapse of the summit area. These open spaces may have been generated by magma's migration from under Miyakejima to the west. The migration is considered to have continued by August 18.It is likely that an inflow of underground water to the open spaces generated a hydrothermal system, where the open spaces acted as a sort of pressure cooker that built up overpressure of eruptions. The hydrothermal system was broken by the largest eruption on August 18, and the eruption column rose about 15 km above the summit. A boiling-over type of eruption occurred on August 29, whereby sufficient overpressure of steam was not built up, resulting in the generation of low-temperature ash cloud surges moving very slowly.

著者

磯崎 行雄 丸山 茂徳 柳井 修一 下司 信夫 佐野 修 笠原 順三

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.2, pp.187-189, 2010-04-25 (Released:2010-07-06)

参考文献数

15

著者

丸山 茂徳 磯崎 行雄 柳井 修一 下司 信夫 佐野 修 笠原 順三

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.6, pp.959-962, 2010-12-25 (Released:2011-03-17)

参考文献数

28

著者

下司 信夫

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 (ISSN:24335320)

巻号頁・発行日

vol.1999, 1999