著者

辻 智大 岸本 博志 藤田 浩司 中村 千怜 長田 朋大 木村 一成 古澤 明 大西 耕造 西坂 直樹 池田 倫治 太田 岳洋 福岡 仁至

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.68, no.3, pp.129-160, 2023-09-30 (Released:2023-11-02)

参考文献数

49

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Kuju volcano, located within Beppu-Shimabara graben central Kyushu, Southwest Japan, has been active in the recent 200,000 years. The 54 ka Handa eruption, as large as VEI 5 or 6 and the largest one of the volcano, released large-scale pyroclastic flow deposits (Handa pfd; Kj-Hd) and a wide-spread tephra (Kj-D ash and Kj-P1 pumice fall deposits) that has been reported at more than 500 km from the source. The stratigraphic relationships among the deposits from the Handa eruption are important for volcanology and disaster prevention, and have been studied in various studies, but there is no consensus on the stratigraphy. In this study, we examined the stratigraphic relationships and the eruption history based on the stratigraphic and petrographic studies around Kuju volcano, as well as on Shikoku and Honshu Islands. As the results, the stratigraphic relationships were revealed as follows. 1) The pumice fall deposit, that has been named Kj-Yu, was previously included in Kj-D ash layers, but is revealed to be a much older ejecta than Kj-D ash, along with the tephras newly named Kj-Tb1 and 2. 2) The clay-rich layer just below Kj-D was previously considered to be soil, but it contains a large number of volcanic ash particles so that it is defined as Kj-Y ash layer. 3) Three light brown fine ash layers, newly named Kj-D-U2, 4 and 6, sandwich between the blue grey sandy ash layers i.e. Kj-D-U1, 3, 5 and 7, are revealed to be the co-ignimbrite ash derived from Kj-Hd 1, 2 and 3 pfd, respectively. It suggests that the Kj-Hd1, 2 and 3 pfd are interbedded with Kj-D-U ash layers. 4) Kj-P1 overlies on Kj-D-U7 ash layer that mantled the reworked deposit of Kj-Hd3. 5) Kj-P1 is divided into lower and upper units based on the grain-size analysis, petrography, the chemical composition of glass shards and the isopach maps. Kj-S pfd was formed in the same time as the upper unit. Based on the results, the eruption history is assumed as follows. Pre-Handa eruption: the activity was low and the small-scale explosive eruptions that had released the pumice and volcanic fragments in loam (Kj-Y), followed by a relatively large explosive eruption that had formed Kj-AL. Early phase: the eruption started with phreatic eruption, sub-plinian eruption that deposited the lower unit of Kj-D ash. Subsequently, the eruption changed to vulcanian eruptions that ejected Kj-D-U. This eruption continued for a long period time. During the time, three large-scale pyroclastic flow eruptions happened and has formed Kj-Hd1, 2 and 3. Their co-ignimbrite ashes generated from the Kj-Hd pfds were deposited as Kj-D-U2, 4 and 6. Lahar were generated after Kj-Hd2 and 3 deposition. This phase was terminated by the deposition of Kj-D-U7 ash. Late phase: the plinian plumes occurred twice and deposited lower and upper lalyers of Kj-P1. The second one is the largest plinian eruption in the whole volcano history, with a large umbrella plume producing a wide-spread tephra at more than 500 km from the source and an intraplinian pyroclastic flow (Kj-S).

著者

長谷川 健 柴田 翔平 小林 哲夫 望月 伸竜 中川 光弘 岸本 博志

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.187-210, 2021-09-30 (Released:2021-10-29)

参考文献数

57

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Based on detailed fieldwork, petrological and paleomagnetic investigations, we present a revised stratigraphy of deposits from the 7.6 ka eruption at Mashu volcano and the formation process of its summit caldera, eastern Hokkaido, Japan. As previously described, the eruption products consist of an initial phreatomagmatic unit (Ma-j) and the overlying three pumice-fall layers (Ma-i, -h, and -g), which are in turn overlain by pyroclastic-flow deposits (Ma-f). In the present study, we divide Ma-f into 4 subunits: Ma-f1/2, Ma-fAc, Ma-f3a and Ma-f3b in descending order. Ma-f3b is a valley-ponding, pumice-flow deposit with limited distribution. Ma-f3a comprises clast-supported facies (fines-depleted ignimbrite: FDI) and matrix-supported (normal ignimbrite) facies, the two changing across topography. The FDI is characterized by a gray, fines-depleted, lithic-breccia-rich layer with materials incorporated from the substrate. Impact sag structures from large (>50 cm) dacite ballistic blocks were recognized at the base of the Ma-f3a within 10 km from the source. Ma-fAc is a minor eruption unit consisting of accretionary lapilli. Ma-f1/2 is a most voluminous (8.8 km3), widely distributed and weakly stratified ignimbrite. Both Ma-f3a and Ma-f1/2 can be classified as “low aspect ratio ignimbrite (LARI)”. Dacite lithic fragments are ubiquitously observed throughout the sequence and are not considered to be juvenile; they have distinctly different chemical compositions from the pumice fragments in the early pumice-fall (Ma-g~Ma-i) and pyroclastic-flow (Ma-f3b) deposits, but those of pumice clasts in the late pyroclastic-flow units (Ma-f3a and Ma-f2) lie between the two on a FeO*/MgO vs. SiO2 diagram. The 7.6 ka caldera-forming eruption of the Mashu volcano was initiated by Plinian fall (Ma-j~-g), and then, a small-volume high aspect ratio ignimbrite (Ma-f3b) was deposited by a valley-confined pyroclastic flow that was generated by partial column collapse. After that, a violent pyroclastic flow was generated probably during a strong explosion of a dacite lava edifice on the summit of Mashu volcano. This flow emplaced Ma-f3a. The caldera collapse that followed the explosion generated a climactic pyroclastic flow that emplaced Ma-f1/2. Ma-f3a flow was extremely fast. Ma-f1/2 flow was related to sustained flow due to low settling velocity and high discharge volume. These are supported by field observations and numerical simulation that shows the ability of the flow to surmount high topographic obstacles and spread widely. The 7.6 ka caldera-forming process of Mashu volcano was driven not only by subsidence of roof block but also by violent explosions.

著者

及川 輝樹 中野 俊 荒井 健一 中村 圭裕 藤田 浩司 成毛 志乃 岸本 博志 千葉 達朗 南里 翔平

雑誌

日本地球惑星科学連合2018年大会

巻号頁・発行日

2018-03-14

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飛騨山脈南部の岐阜県・長野県県境に位置する乗鞍火山の最近約1万年間の噴火史を,テフラ層序とAMS 14C年代測定を基に明らかにした.なお本報告は,乗鞍岳火山防災協議会が行った調査を基にして,その後検討を加えたものである.かつて乗鞍火山における最近1万年間の活動中心は,火口縁に最高峰の剣ヶ峰(3026m)がある権現池火口の他,恵比須岳火口にもあるとされていた.しかし,恵比須岳火口起源のテフラとされていた噴出物は,年代や記載岩石学的特徴から,その火口起源のものではないことが明らかとなった.そのため,最近1万年間の活動中心は権現池火口周辺に限られる.最近1万年間における乗鞍火山の噴火活動は,テフラ層序に基づくと,少なくともマグマ噴火を2回,水蒸気噴火を10回行っている.マグマ噴火は,いずれも水蒸気噴火に始まるが,その後火山灰を放出する噴火とスコリアを放出する噴火がそれぞれ発生した.スコリアを放出する噴火は,その初期に小規模な火砕流も発生した.総テフラ噴出量は数100~1000万/m3オーダである.なお,権現池火口周辺から流れ出た溶岩のうち,保存のよい微地形が残存する溶岩も3ユニットあることから,溶岩を流す噴火も完新世に3回発生した可能性がある.また,個々の水蒸気噴火の総噴出量は,数10~数100万m3オーダとなる.最新の噴火は,約500年前に発生した水蒸気噴火である.およそ7300年前に降下した鬼界アカホヤ火山灰より上位のテフラユニットの数から算出した噴火頻度は,800年に一回となる.近隣の焼岳火山(100~300年/回)と比べると噴火頻度は少ないが,桁違いに少ないわけではない.