著者

新堀 賢志 長井 雅史 金子 隆之 藤井 敏嗣 中田 節也 吉本 充宏 安田 敦 青柳 正規

出版者

東京大学

雑誌

東京大學地震研究所彙報 (ISSN:00408972)

巻号頁・発行日

vol.82, no.2, pp.119-178, 2008-01-18

著者

安井 真也 高橋 正樹 金丸 龍夫 長井 雅史

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.4, pp.293-325, 2021-12-31 (Released:2022-02-22)

参考文献数

36

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The Asama-Maekake volcano has been active during the last 10,000 years. Large-scale eruptions that occurred in the 18th and 12th centuries have been well studied, whereas little information is available for older eruptions. In this paper, we aim to reconstruct the history of this volcano in detail through a combination of extensive geological survey and 14C dating. The observation and description of twenty-one trench excavations, two drilling core samples, and many outcrops enabled us to build a stratigraphy of the eruptive products in much greater detail than ever before. The trench excavation sites cover an area of nearly 180 degrees around the volcano. These sites were selected mainly in the medial area at distances between 5 and 10 km from the summit crater. Many older deposits buried by thick younger deposits were found. The pyroclastic fall deposits of this volcano vary from a thick pumice layer to pumice grains scattered in the black soil, indicating a variation in the scale of sub-plinian eruptions. More than 120 samples for 14C dating were taken from the black soil immediately beneath the pyroclastic fall deposits. Some charcoals contained in the pyroclastic flow deposits were also subjected to dating. The 14C dating results were used for the correlation of deposits of different localities and distributions of some pyroclastic fall deposits older than 2000 years were revealed. The qualitative evaluation of the number and scale of eruptions throughout history might be possible using these data. Four classes of eruptive scales are recognized in the pyroclastic fall deposits in this study. Class 1: Defined by the isopach line for 128 cm thickness being able to be drawn on the map and the area enclosed by the 64 cm isopach line being more than 25 km2. The deposits are recognized at distant locations more than 50 km from the summit crater. Class 2: Defined by that the isopach line for 64 cm thickness being able to be drawn on the map and the area enclosed by the 16 cm isopach line being more than 15 km2. Class 3: The deposit of this class is recognized as a distinct layer that continues horizontally at each locality. In most cases, the observed thickness is less than several tens of centimeters and generally no structure can be observed. Class 4: This class comprises scattered pumice grains in the soil, for which the measurement of thickness is impossible. The deposits of classes 3 and 4 are seldom found at distances farther than 15 km from the crater. Most of the pyroclastic fall deposits of the period between 9400 and 3100 cal BP are of Classes 3 and 4. On the other hand, a large-scale eruption (Class 1) occurred about 2000 years ago, generating pyroclastic fall deposits in distant areas of more than 50 km from the crater. The recurrence interval of large-scale eruptions during the last 2000 years is estimated to be about 700 years. This is less frequent than in the period prior to 2000 years ago. Therefore, a change in eruption mode occurred about 2000 years ago when eruptions became infrequent but large in scale.

著者

長井 雅史 小林 哲夫

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.124, no.1, pp.65-99, 2015-02-25 (Released:2015-03-11)

参考文献数

43

被引用文献数

10 11

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Ioto (Iwo-Jima; Sulphur Island) is a volcanic island located at the volcanic front of the Izu-Bonin arc about 1250 km south of Tokyo. The island consists of a central cone and southwest rim of a submarine caldera with a diameter of about 10 km. The rocks of the volcano are trachyandesite and trachyte, which are seldom found at a volcanic front. High rates of geothermal activity and crustal uplift have been observed, which are considered to be related to magma intruding at a shallow depth. Therefore, Ioto volcano is considered to be an active resurgent dome. However, eruptive history, including the process and timing of caldera formation, has not been clarified. Eruptive history based on our recent field survey, dating, and chemical analysis is as follows. A pre-caldera edifice was formed by volcanic activity of trachyandesite-trachyte magma in a subaerial and subaqueous environment. The magma composition and types of eruption were similar to those of the post-caldera edifice. It is still unclear when the caldera was formed. The caldera floor, which was a sedimentary basin with shallow marine sediments and a subaqueous lava flow, has been present at least since 2.7 cal kBP. Furthermore, a small volcanic island covered with trees used to exist in the Motoyama area. The complicated sequence of the Motoyama 2.7 cal kBP eruption is described as follows. First, on the volcanic island or in the surrounding shallow water, an explosive phreatomagmatic eruption occurred that formed subaqueous welded tuff (Hinodehama ignimbrite) and a subsequent thick subaqueous lava flow (Motoyama lava). While the Motoyama lava was still hot, the eastern part collapsed. The collapsed mass was quenched to form large blocks similar to pillow lava. A subsequent large phreatomagmatic eruption occurred, destroying the hot Motoyama lava, the older edifice, and the marine sediment. The resultant subaqueous pyroclastic flow generated the Motoyama pyroclastic deposit. Then, the eruption center shifted to the Suribachiyama area, which is just outside the southwest caldera rim. Deposits from three different eruption periods have been identified—lower, middle, and upper pyroclastic deposits—and a lava flow that erupted during the middle pyroclastic period. The lower unit was formed by a subaqueous eruption at a deeper level; the middle deposit was formed by a phreatomagmatic explosion at a shallow depth; and, the following lava emission generated a lava island. The upper pyroclastic deposit was generated by a combination of phreatomagmatic and Strombolian eruptions. Although the ages of these eruptions are not obvious, the first phase of the eruption occurred during the period between 2.7 cal kBP and 0.8-0.5 kBP, which is estimated from the age of the upper marine terrace X (Kaizuka et al., 1983). The eruption of the upper deposit occurred before AD 1779 (ca. 0.2 kBP). The eruptive products described so far are covered with younger sediment from marine terraces and spits. Recently, small-scale deposits from phreatic explosions accompanied by geothermal and uplift activities have been found distributed throughout the island, but juvenile material has not been confirmed to exist in the products.

著者

大塚 宜明 金成 太郎 飯田 茂雄 長井 雅史 矢原 史希 櫻井 宏樹

出版者

札幌学院大学総合研究所 = Research Institute of Sapporo Gakuin University

雑誌

札幌学院大学人文学会紀要 = Journal of the Society of Humanities

巻号頁・発行日

no.100, pp.83-99, 2016-10-01

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本論では,置戸黒耀石原産地における先史時代の人類活動解明のための基盤構築を目的として,置戸黒耀石原産地調査で採集した黒耀石原石・黒耀石製石器の観察結果と,黒耀石原産地推定分析の結果を報告し考察を行った。 検討の結果,置戸黒耀石原産地を構成する所山・置戸山の黒耀石は,原産地においてはそれぞれ独立して分布することが詳細に明らかになった。人類活動については,(1)置戸黒耀石原産地内で採取可能な黒耀石原石を原料とした石器製作が個々の原産地で行われていること,(2)置戸山2遺跡採集の所山産黒耀石製石器の存在から,置戸黒耀石原産地を構成する個々の原産地が全く無関係ではないこと,(3)遠隔地産黒耀石がみとめられる置戸安住遺跡が石器や原料の搬出入の拠点である可能性,を明らかにした。 以上の検討結果から,置戸黒耀石原産地には,置戸黒耀石原産地と遠隔地を結ぶ大規模な人類の動き,置戸黒耀石原産地と直近の生業地である常呂川中・下流域を結ぶ中規模な動き,そして置戸黒耀石原産地内の原産地間を結ぶ小規模な動きといった,黒耀石をめぐる先史時代人の様々な活動痕跡が刻まれていることが明らかになった。

著者

長井 雅史 小林 哲夫 三輪 学央 棚田 俊收 上田 英樹

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 2016 (ISSN:24335320)

巻号頁・発行日

pp.157, 2016 (Released:2017-02-07)

著者

長井 雅史 金成 太郎 弦巻 賢介 柴田 徹 杉原 重夫

出版者

The Geological Society of Japan

雑誌

日本地質学会学術大会講演要旨 (ISSN:13483935)

巻号頁・発行日

pp.111, 2015 (Released:2016-03-31)

著者

長井 雅史 金成 太郎 柴田 徹 弦巻 賢介 杉原 重夫 横田 彰宏

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125年学術大会(2018札幌-つくば) (ISSN:13483935)

巻号頁・発行日

pp.109, 2018 (Released:2019-08-16)

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

新堀 賢志 長井 雅史 金子 隆之 Fujii Toshitsugu Nakada Setsuya Yoshimoto Mitsuhiro Yasuda Atsushi Aoyagi Masanori

出版者

東京大学地震研究所

雑誌

東京大学地震研究所彙報 (ISSN:00408972)

巻号頁・発行日

vol.82, no.2, pp.119-178, 2007

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An archaeological excavation site at the northern foot of Mt. Vesuvius in Italy provided a three-dimensional outcrop with a height of 8m to study its volcanic succession. Through a stratigraphical study of sediments and chemical analyses of juvenile materials, the timing and the sequence of the burial processes of the villa, which is attributed to Emperor Augustus, have been revealed. The sediments filling the villa can be divided into five stratigraphical units (Group1, Group2, Group3A, Group3B, and Group3C) by the presence of soil. The lowermost unit (Group1) directly covering the partially collapsed Roman building includes air-fall deposits, surge deposit, and epiclastic flow deposits. One of charcoals found in this unit give an age of 1500yBP, and the juvenile scoria have the same compositional range as ejecta of the AD472 Sub-plinian eruption, and differ from ejecta of major eruptions. The next three units (Group2, Group3A, and Group3B) include thick epiclastic flow deposits interbedding air-fall deposits. The uppermost unit (Group3C) consists of alternating scoria and ash-fall layers and an overlying ash-fall layer. The petrographical features and the composition of juvenile materials coincide with those of the AD1631 Sub-plinian eruption. From these geological and geochemical features, the burial process of the Roman villa is described as follows. When the AD472 eruption started, the villa had partially collapsed. This damaged building was mantled by an air-fall deposit a few tens of centimeters thick. The remaining building was soon struck by several phases of lahars, and was buried up to a height of 5m. The villa experienced at least five eruptions, and their ejecta and subsequent lahars buried the building further. The last eruption, which completely buried the villa, was the AD1631 eruption. This reconstructed scenario suggests lahars generated just after the eruptions were major agents in the burial of the Roman villa.

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.