著者

山元 孝広

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.4, pp.257-271, 2006-08-31

被引用文献数

3

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The Sashikiji 2 (S_2) Member in the products of Izu-Oshima volcano was formed by an explosive eruption accompanied with caldera depression. This member is characterized by breccia called as a "low-temperature pyroclastic flow deposit". In this paper, the S_2 breccia is re-examined based on stratigraphy, grain fabrics, grain-size distributions and modal compositions. The S_2 Member is divided into six units from S_2-a to S_2-f in ascending order. The S_2-a unit consists of scoria, bomb and aa lava flows from flank fissures. The S_2-b unit is made up of well-bedded ash and fine-lapilli from the summit. The S_2-c unit is composed of matrix-supported breccia, locally filling valley bottoms and containing abundant deformed soil fragments and woods. The S_2-d unit consists of reverse to normal grading, clast-supported breccia with ash matrix, covering topographic relief in the whole island. The S_2-e unit is composed of dune- to parallel-bedded lapilli and ash in the proximal facies. The S_2-f unit is clast-supported breccia with and without ash matrix. New ^<14>C ages of wood fragments in the S_2 Member have been determined as about Cal AD 340. Although the S_2-c and -d units are previously interpreted to the low-temperature pyroclastic flow deposit, these units are quite different in sedimentological features as follows. The grain fabric measurements have revealed that the S_2-d unit has a-type imbrication showing the longest axis of grains parallel to the flow direction. On the other hand, the S_2-c has random fabric of grains. The grain size distribution of the S_2-d unit shows a bimodal nature having subpopulations at phi -1.0 to 1.0 and coarser than phi -2.5. The bimodal nature and a-type imbrication suggest that the two transport processes overlap; the load of a turbulent suspension is not all in true suspension as the coarser population may travel in a cast-dispersion mass flow. The S_2-c unit shows a polymodal grain size distribution with multi subpopulations from coarse to fine. The poor sorting, massive appearance, valley-confined distribution, and random grain fabric of the S_2-c unit are characteristic of deposition from a cohesive flow without formation of traction-related bedforms or sorting of different grain sizes by turbulence. The modal composition measurements have indicated that the S_2-c and -d units lack essential scoriceous or glassy fragments. This evidence indicates that both units are derived from steam explosions due to outburst of highly-pressurized geothermal fluid within the edifice. The S_2-c unit was plausibly generated by remobilization of phreatic debris around the summit caused by ejection of condensed water from a plume or heavy rainfall. The S_2-d unit was a pyroclastic density current deposit resulted from collapse of a highly-discharged phreatic plume. Estimated velocities of the current are 150 to 30m/s based on suspended grain sizes.

著者

高橋 正樹

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.39, no.4, pp.191-206, 1994-09-20

参考文献数

66

被引用文献数

6

---

Late Quaternary polygenetic volcanoes in Japanese islands can be classified into two types based on the distance of vent migration : the stable and unstable vent types. The eruptive center of unstable vent type has migrated longer distance (exceeding 1.5 km within 10,000 years) than that of the stable one. The stable vent type constructs a symmetrical cone, contrarily the unstable vent type shows an elongated cluster of small stratovolcanoes. The unstable vent type is generally present in the regional or local extensional stress field, because independent open fractures are easily developed under the extensional stress regime ; the occurence of each type also depends on the crustal strain rate and/or strength of the basement. The eruption rate of stable vent type is larger than that of the unstable one ; it is probably because the high efficiency of magma transport to the surface can be realized by the stable vent.

著者

小山 真人 山岡 耕春 早川 由紀夫 白尾 元理

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.1991, no.2, 1991-10-14

著者

隅田 まり 勝井 義雄 河内 晋平 山崎 哲良 前田 仁一郎 後藤 芳彦 宮地 直道

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.1989, no.1, 1989-04-06

著者

井村 隆介 小林 哲夫

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

巻号頁・発行日

vol.2001, no.2, 2001-10-01

被引用文献数

1

著者

吉本 充宏 中村 賢太郎 濁川 暁 寺田 暁彦 石崎 泰男

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.2013, 2013-09-29

著者

宇井 忠英 柴橋 敬一

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.20, no.2, pp.51-64, 1975-08-01

被引用文献数

1 3

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Volcanic activity at the snow-capped summit of Mt. Chokai was first noticed by the captain of scheduled airline on March 1, 1974. The activity began with swarm of volcanic earthquakes, succeeded by fumarolic activity and finally explosion took place and a few craters were formed at eastern (late February-early March) and western (late April) foot of 1801 lava dome (Shinzan). The ejecta were exclusively fine-grained air-fall ash and accidental blocks. The blocks, formed the mud flow mixed with melted snow, rushed down the slope of volcanic edifice. Essential materials, such as bombs, air-fall scoria, or lava flows were not erupted. Rapid melting of snow was supposed to have been triggered by the formation of fumaroles caused by ascent of hot magma and associated juvenile gas. Thermal energy consumed for melting and evaporating snow is calculated as 3×10^<21> ergs. Total volume of mud-flow deposit is around 3×10^4 cubic meters, and that of air-fall ash is an order of 10^5 cubic meters. The entire area which showed thermal activity is approximately 700×200 meters, elongated in east-west direction. Distribution of earthquake foci was also trending east-west just passing the prehistoric summit and parasitic vents. Direction of vent alignments is the same for the most volcanoes in northeastern Japan, and is supposed to reflect regional stress field.

著者

田沢 堅太郎

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.36, no.4, pp.419-430, 1991-12-25

被引用文献数

1

---

Studies on coastal deposits and geomorphology of Oshima island have revealed some uplifts and subsidences associated with the volcanic activity of Oshima volcano. Uplifts and subsidences of the island are inferred by subtracting the sea levels at the time from the heights of the past sea surface recorded on coastal deposits. Two series of large-scale eruptions starting at the ages of O_<41> (7,780±160 yB.P.) and S_2 (1,360±40 yB.P.) have similar characteristics as follows. (1) At the ages of O_<42> (8,200-7,800 yE.P.) and O_1(1,500-1,400 yE.P.) just before the two series of large-scale eruptions, lagoons were formed along the west coast of Oshima island, suggesting the subsidence of the island. (2) At the beginning in each series, a large-scale steam explosion occurred at the summit crater to generate a debris flow. (3) In each series, the volume of the eruptive materials rapidly increased at the beginnings and gradually decreased after those times. (4) Volcanic activities were explosive at the beginnings in the series and gradually changed to effusive. (5) Oshima island was remarkably and rapidly uplifted at the beginnings of the two series of the large-scale eruptions and was gradually subsided. (6) In the stage of uplifling of the island, the scale of eruptions had been enlarging, and in that of subsiding, the scale of eruptions had been getting smaller.

著者

田沢 堅太郎

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.29, no.1, pp.1-15, 1984-04-01

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Izu-Oshima volcano consists mostly of subaerially deposited volcanic products. On the sea cliff, however, the water-laid deposits are intercalated in the O_<42> (about 7, 500 yB.P.) and O_1 (about 1, 500 yB.P.) members of the Older Oshima Group. Layers of coastal gravels are observed in the Y_5-Y_1 (A.D. 1338-the present) members of the Younger Oshima Group near the present coast. These layers show imbricate structure indicating landward flow. They are interpreted as wind-transported from nearby coastal beach. These nearshore deposits were presumably resulted from the variation of the sea level in height. In the O_1 age, lagoons were formed along the coast. In the S_2 age (about A.D. 550), remarkable emergence of the shoreline occurred. After that, the island has kept subsiding gradually.

著者

横山 泉

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.58, no.1, pp.91-102, 2013-03-29

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桜島火山には多くの寄生火口が地質及び地形の面から認められている.また,その噴火史において寄生火口の噴火がしばしば記録されている.寄生火口の分布パターンを調べるのに,色んな方法が提案されてきたが,ここでは,火山中心から半径方向の密度分布(km^2当たり)を調べた.一般論として,火山の下に点力源を仮定して,地表面で直応力の分布と水平差応力の分布を考え,岩石の強度を考慮すると,寄生火口の生ずる地点の見当がつく.それは,地表で力源を伏角51°で見る山腹の地点で,火山中心に対して対称な2点である.多くの火山では,対で生ずることは少ない.桜島火山の寄生火口の火道が主火道から分岐する深さを求めると,深さが3kmと10kmの2群となる.これらの深さと既に推定されているマグマ溜まりとの関連について触れた.桜島火山の歴史時代の(1471年以降の3)回の大噴火は総て,山頂に対称的に対をなして形成された.このことは力学的には正常であるが,事例としては例外である.ただ,分岐の深さが10kmの場合(1779〜80年噴火),山頂に対して対称位置に寄生火口が生じていない.この例外的な事例は,桜島地下で,浅部と深部で地殻構造が異なることに起因するのかも知れない.更に,寄生火口が再噴火しない機構について仮説を述べた.次の桜島火山の噴火地点は何処であろうか.山頂火口か,それでなければ,寄生噴火である.その場所は統計的に,山体の中心軸から約2.5km或は8.5kmの円環上で,かって噴火したことのない地点が考えられる.寄生火口の火道が主火道から分岐する機構が未解明である限り,これ以上のことは言えない.

著者

中野 健秀 ベサナ グレンダ 楠根 高志 大倉 敬宏 安藤 雅孝 プノンバイヤン レイモンド

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.1996, no.2, 1996-11-05

著者

高山 和喜 斎藤 務 木下 利博 藤井 直之 山岡 耕春 谷口 宏充

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.1992, no.2, 1992-11-09

著者

松本 〓夫 林 正雄 石橋 澄

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.21, no.3, pp.207-208, 1976-12-15

著者

横山 勝三

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.45, no.4, pp.209-216, 2000-08-28

被引用文献数

1

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The Ito ignimbrite, the product of a big eruption at Aira caldera about 24,500 y B. P., is distributed very extensively around the caldera in south Kyushu. The region within about 70 km from the center of the caldera was the previously known extent of distribution of the ignimbrite. Recent field research revealed, however, Iocal but extensively-scattered distribution of the ignimbrite in many places beyond the previously known extent of distribution northwest to northeast of the caldera. The farthest site of distribution of the ignimbrite is located about 90 km north of the caldera, indicating that the Ito pyroclastic flow originally spread at least 20 km farther than the previously known extent. The ignimbrite in the remote region is characteristically fine-grained compared with the one near the source. Both pumice and lithic fragments in the ignimbrite decrease, as a whole, in size with distance from source. However, the size of lithic fragments increases in the mountainous area beyond 70 km from source. This is because lithic fragments were incorporated into the pyroclastic flow from local land surface probably due to increased turbulence of pyroclastic flow during the passage on the irregular basal relief. The most remote ignimbrite, at a site 90 km from source, attains to about 35 m in thickness and contains abundant lithics of 5-15 cm in diameter, suggesting that the Ito pyroclastic flow spread farther beyond.

著者

小坂 丈予 平林 順一

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.25, no.2, pp.108-109, 1980-07-01

著者

小坂 丈予 湊 一郎 森 彰 大平 洋子

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.19, no.1, pp.39-40, 1974-07-01

著者

金子 克哉 伊藤 公一 安部 祐一 岩堀 功大

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.2010, 2010-10-09

著者

木 澤綏

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.1987, no.2, 1987-10-05

著者

津久井 雅志 中野 俊 齋藤 公一滝

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.53, no.2, pp.79-91, 2008-04-30

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Based on geological and archaeological data as well as historic documents, we review eruptions and earthquakes which have occurred during the 9th century in central and east Japan. The results reconfirm vigorous activities on Izu Arc, Izu-Oshima(〜838AD < N_3, N_2, N_1 < 886AD), Niijima(〜857AD and 886AD), Kozushima(838AD), Miyakejima(832AD and 850AD), and at Fuji volcano(800AD, 838AD < < 864AD, 864AD) during the 9th century. Beside these events, a big eruption of Niigata Yakeyama volcano had likely occurred in 887AD. Chokai volcano also erupted in 871AD, and 810-823AD. Collapse of Yatsugatake volcano took place in 887, probably invoked by a strong shock in 887. In addition, earthquakes with a magnitude from 7 to 8 had taken place along the Itoigawa-Shizuoka active fault system (in 841 or 762AD), Nagano fault system(887AD), Echigo plains(863AD), Shonai plains(850AD), Akita plains(830AD) and Nankai trough(887AD). As a result, we can point out a linkage of big eruptions and seismic activities in the 9th century over 800km long crossing Japan Arc. Geologically this seems to be a surface expression of East-West compression along eastern margin of Amurian Plate over 800km which was driven by the eastward motion of the plate. It is noteworthy to mention that the very similar volcanic and seismic activities have been occurring in the last 50 years to those happened in the ninth century, in their source areas and manners.

著者

吉田 大祐 渡邉 篤志 河野 裕希 雑賀 敦 平尾 暁彦

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

巻号頁・発行日

vol.2006, 2006-10-23