著者

長谷川 健 菊池 文太 柴田 翔平 井村 匠 伴 雅雄 常松 佳恵 山本 裕二 大場 司 鈴木 和馬 戸丸 淳晴 楠 稚枝 岡田 誠

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.189-196, 2023-09-30 (Released:2023-11-02)

参考文献数

22

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Volcanic bomb is one of the most common eruption products around their source craters. Although paleomagnetic studies on volcanic bombs have a potential to provide high-resolution chronology of volcanic activity, particularly when compared with the known geomagnetic secular variation records, there are only a few such studies. In this contribution, we made an attempt to determine paleomagnetic directions from large (>1 m in diameter) volcanic bombs around “Tsubakuroswa craters”, located in Azuma volcano, for evaluating the potential use of volcanic bombs for paleomagnetic dating. Six oriented mini-cores were drilled from the central part of each large volcanic bomb, five in total, located on a gentle slope a few hundred meters south from the craters. All of the mini-cores were subjected to thermal demagnetization analysis, giving a well-determined characteristic remanent magnetization (ChRM) direction for each bomb as follows: site mean declination (Dm) of 350.6‒358.0º and inclination (Im) of 48.9‒50.8º with a 95 percent confidence limit (α95) smaller than 2.4º. The ChRM directions were consistent among the bombs, supporting the availability of volcanic bombs for further paleomagnetic dating research. Referring the geomagnetic secular variation record in this area, an all-site mean ChRM direction from the five bombs (Dec=355.5º, Inc=50.1º, α95=1.9º) most likely accounts for the derivation of the volcanic bombs by the Meiji Era (1893 CE) eruption. Historic pictures and descriptions are consistent with and support this interpretation. Previous reports suggested that the Meiji Era eruption did not eject magmatic materials and that the last magmatic eruption of this volcano was probably in 1331 CE. However, our results suggest that magmatic eruptions might have occurred here only ca. 130 years ago and may be largely affecting the current activity of this crater area. Our study suggests that volcanic bombs are potentially useful materials for paleomagnetic studies such as dating and establishing geomagnetic secular variation records.

著者

大場 司

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.117, no.6, pp.344-356, 2011-06-15 (Released:2011-11-03)

参考文献数

71

被引用文献数

2 13 1

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熱水変質鉱物火山噴出物は,水蒸気噴火によって火山地下よりもたらされる.近年の噴火事例研究と同様,高分解能火山層序学でも熱水変質鉱物火山噴出物の記載が増えている.近年の噴火事例研究では,噴火と火山下熱水系の関係が指摘されており,その熱水系の構造を理解するには,熱水性鉱床や地熱研究の知見が有益である.マグマ噴火頻度の低い火山には,火山体内部に発達した珪化変質帯や高度粘土化変質体由来の変質鉱物を多く含む噴出物が産する.一方,マグマ噴火頻度の高い火山の熱水変質鉱物火山噴出物には硫黄系鉱物が卓越する.熱水変質鉱物火山噴出物は主に降下堆積物として産し,フロー堆積物も認められる.噴火には斜面移動が関係する場合としない場合があり,それぞれマグマ上昇が関係する場合としない場合がある.マグマの組成や貫入頻度と,熱水変質鉱物を放出する水蒸気噴火の頻度や構成鉱物の間には密接な関係がある.

著者

宮本 毅 中川 光弘 長瀬 敏郎 菅野 均志 大場 司 北村 繁 谷口 宏充

出版者

東北大学東北アジア研究センター

雑誌

東北アジア研究 = Northeast Asian studies (ISSN:13439332)

巻号頁・発行日

no.7, pp.93-110, 2003-03-31

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Baitoushan volcano (Changbaishan volcano in China), situated on the border between China and North Korea, is one of the most active volcanoes in mainland Asia. The volcanism is divided into three stages, firstly the shield volcano-forming stage effusing large amount of basaltic lava flow, secondly volcanic cone-building stage consisting of trachytic magma, and finally summit caldera (called Tianchi caldera)-forming stage erupting the vigorous alkali-rhyolitic pyroclastics. Tianchi caldera has been formed by some huge eruptions (10th century, 4000yBP eruption and more) since Pleistocene. In caldera-forming stage, the time sequence of such huge eruption progress from Plinian pyroclastic fall to following large pyroclastic flow (ignimbrite). Based on our geological and petrological studies and Radiocarbon dating about wood and charcoal samples in pyroclastics, we found that 9th century eruption, unknown until now, has occurred before 10th century eruption. From the relation of stratigraphy and radiocarbon age the dormancy between this episode and 10th century one is estimated about a hundred year. The time sequence and eruptive style of this new episode is similar to 10th century eruption, but the magma composition is different.

著者

南 裕介 伊藤 順一 草野 有紀 及川 輝樹 大場 司

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.2, pp.39-57, 2023-06-30 (Released:2023-07-27)

参考文献数

39

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Akita-Yakeyama Volcano is an active stratovolcano located on Northeast Honshu island, Japan. Recent eruptive activity has occurred on the flank of the volcano in May 1997 and in the summit crater (Karanuma vent) in August 1997. These events indicate that Akita-Yakeyama Volcano has a high potential for future eruptions. In order to better understand the hazards posed by Akita-Yakeyama Volcano, this study focused on the modern explosive activity of Akita-Yakeyama during the last 6000 years. The authors conducted field observations and excavation surveys at outcrops, whole-rock chemical analysis, volcanic glass chemical analysis, and radiocarbon dating for intercalated paleosol layers. As a result, at least nine layers of pyroclastic fall deposits derived from Akita-Yakeyama during the last 6000 years were recognized, ranging from Volcanic Explosivity Index (VEI) levels of 1 to 2. In chronological order, the major pyroclastic fall deposits consist of AKY8 (45th to 47th century BC), AKY7 (10th to 29th century BC), AKY6 (2nd to 8th century BC), AKY5 (1st century BC to 2nd century AD), AKY4 (5th to 9th century AD), AKY3 (1678 AD), AKY2 (1892 AD), AKY1 (1951 AD) and 1997 eruption ejecta. The decreasing proportion of juvenile materials in eruptive deposits over the last 6000 years is consistent with a reduced magma contribution. It indicates that the development of the hydrothermal system is likely to play an important role in future eruption scenarios for Akita-Yakeyama Volcano.

著者

大場 司

出版者

Japan Association of Mineralogical Sciences

雑誌

岩鉱 (ISSN:09149783)

巻号頁・発行日

vol.86, no.7, pp.305-322, 1991-07-05 (Released:2008-03-18)

参考文献数

16

被引用文献数

3 3

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Geological survey has been carried out on Akita-yakeyama volcano, eastern Akita Prefecture, Japan to establish the evolutional history of the volcano. Furthermore, the volcanic products have been described in detail to contribute to the petrological investigation of the volcano, that is currently in progress. The volcanic activities can be divided into three stages in terms of mode of volcanic activity or characteristics of the erupted products, or both. Approximately 300, 000 years ago, the volcanic activity started within a caldera formed by the eruptions of the voluminous felsic pyroclastics at 1 million years ago. This is the beginning of the early stage; the new volcano erupted number of andesitic lavas of the pigeonitic rock series while pyroclastic flows and falls were less abundant. The shape and size of the volcano built during this stage is not clear since it was covered with the following volcanic products, and severe erosion altered it much. In the middle stage the lava flow activities have been associated by explosive eruptions: pyroclastic falls and flows were repeated from central vent. The alternation of lavas and pyroclastics could have completed a large stratovolcano. All the volcanic rocks during this stage belong to hypersthenic rock series. After middle stage, the activity ceased for a while, and hydrothermal alteration expanded to nearly all over the volcano. Consequently, collapses of the altered piles of lavas and pyroclastics have led to five horseshoe-shaped calderas around the summit. After the repose period, the late stage activity started with explosive central and flank eruptions of pyroclastic fall; lava flows as well as pumice falls followed. During this stage, two andesite lava domes and two dacite domes were constructed. In the central area of the summit crater a dacite dome was built and dacitic ash was erupted; this is the latest activity. All the rocks of the last stage are of the hypersthenic rock series as well.

著者

井村 匠 大場 司 中川 光弘

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.125, no.3, pp.203-218, 2019-03-15 (Released:2019-06-07)

参考文献数

42

被引用文献数

4

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十勝岳火山噴出物の変質火山灰粒子について岩石学的観察を行った.4.7ka噴火,3.3ka噴火,1926年噴火噴出物を対象とした.これらの火山灰粒子は様々な程度に変質し,鉱物組み合わせからシリカタイプ(シリカ鉱物のみ),ミョウバン石タイプ(シリカ鉱物+ミョウバン石±カオリン鉱物),カオリンタイプ(シリカ鉱物+カオリン鉱物)に分類される.いずれの鉱物組み合わせも酸性熱水変質を示している.全試料において,酸性変質部と未変質部からなる弱変質した火山灰粒子が卓越する.このような岩石組織は,熱水流通系における酸性熱水-岩石反応により形成したものである.これには多量の熱水が岩石と反応しながら排出されるような反応過程が考えられ,反応時間はごく短期間であった.これらの特徴は,マグマ貫入時に一時的に形成した高温酸性熱水系による非定常プロセスに由来している.以上の点はマグマ貫入頻度が高い十勝岳の火山熱水系の特徴であると考えられる.

著者

鹿野 和彦 大場 司 石山 大三

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 2017 (ISSN:24335320)

巻号頁・発行日

pp.95, 2017 (Released:2018-02-01)

著者

大木 郁也 大場 司

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2014年大会

巻号頁・発行日

2014-04-07

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秋田県南部に位置する三途川カルデラは,約1Ma以前に大規模火砕流を伴うカルデラ陥没により形成した.本地域には,カルデラ形成時に堆積したとされる虎毛山層が分布する.虎毛山層は,下位より虎毛山凝灰岩部層,皆瀬川凝灰岩部層からなる.虎毛山凝灰岩部層は,溶結凝灰岩,火山礫凝灰岩,凝灰質砂岩・黒色頁岩・礫岩の互層からなり,層厚は900mに達する.皆瀬川凝灰岩部層は,火山礫凝灰岩,凝灰岩,礫岩からなり,層厚は450mに達する.虎毛山層は,8層の火砕流堆積物(PDC-1~8),土石流堆積物(DF-1),湖成堆積物(LD-1)から構成される.これらの層序は,下位よりPDC-1,DF-1,LD-1,PDC-2~PDC-8からなる.各層の厚さはPDC-1が20m,DF-1が80m,LD-1が140m,PDC-2が50m,PDC-3が250m,PDC-4が200m,PDC-5が340m,PDC-6が160m,PDC-7が90m,PDC-8が30mである.火砕流堆積物は,塊状無層理の火山礫凝灰岩からなり,軽石と異質岩片を含む.しばしば,炭化木片を含み,脱ガスや柱状節理が発達する.PDC-4, 6は火砕流堆積物の基底部はグラウンドサージ堆積物からなる.このグラウンドサージ堆積物には,低角斜交層理が発達し,デューン構造が認められる.このうちPDC-6は,グラウンドサージ堆積物の下位にグラウンドブレッチャーが認められる.このグラウンドブレッチャーは,最大礫径2.5mの異質岩片を含む基質支持礫岩からなる.また,PDC-1, 3, 4, 8は特徴的に溶結相を伴う.溶結相には,ユータキシティック組織やスフェルライトが認められる.土石流堆積物(DF-1)は,層理が発達し,円礫を主体とする礫支持礫岩からなる.礫は平行に配向し,弱く逆級化する.湖成堆積物(LD-1)は,黒色頁岩及び凝灰質砂岩,礫岩の互層からなる.黒色頁岩にはラミナが発達し,凝灰質砂岩には葉理・層理が発達し,礫岩は塊状無層理である.湖成堆積物(LD-1)の上位のPDC-2は,水中環境での堆積を示唆する.8層の火砕流堆積物の存在は,本地域では火砕流が少なくとも8回発生していたことを示唆する.火砕流堆積物(PDC-4)の流向方向は,グラウンドサージ堆積物のデューン構造から,北東から南西方向であると推定でき,給源位置は滝ノ原火口であると推定した.休止期間を示す湖成堆積物(LD-1)が虎毛山層中部に狭在し,カルデラ陥没が少なくとも2回発生したと推定される.地層の走向は石神山周辺を中心とする半同心円構造をなし,その傾斜は半同心円の外側を向く.この構造は,石神山周辺を中心とするドーム状の隆起構造を示唆する.この隆起構造は,再生ドームであると考えられ,カルデラ中心域にあたる小安岳周辺の基盤岩の高まりの原因の一つである.再生ドームの形成と厚い火砕流堆積物の分布と環状割れ目の存在は,三途川カルデラがValles型カルデラである可能性を示唆している.

著者

大場 司

出版者

日本岩石鉱物鉱床学会

雑誌

岩鉱 (ISSN:09149783)

巻号頁・発行日

vol.88, no.1, pp.1-19, 1993-01-05 (Released:2008-03-18)

参考文献数

15

被引用文献数

5 9

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Magmatic evolution of Akita-yakeyama volcano, northeastern Japan, has been investigated based on bulk chemistry (major and trace), Sr isotopic features and mineral chemistry in combination with stratigraphical study. The main body of the volcano was built by eruptions of a large amount of hypersthene-augite andesite magma and a minor amount of olivine bearing hypersthene-augite andesite magma as lava flows and pyroclastic falls, and an edifice of stratovolcano was built. No evidence of magma mixing or crustal assimilation can be recognized in the olivine free andesite, but olivine-pyroxene disequilibrium in the olivine bearing andesite indicates magma mixing. After a repose period, a series of eruptions began from the central vent and erupted hypersthene-augite andesite magma as fine ash fall and hypersthene-augite-olivine andesite magma as lithic-rich pyroclastic falls. As stated above the olivine-free andesite has no evidence of magma mixing, but the olivine bearing andesite has the mineralogical and chemical evidence of magma mixing, such as disequiliblium phenocryst assemblage, reverse zoning of pyroxene and dusty plagioclase. Flank eruptions followed and erupted quartz bearing olivine-hypersthene-augite andesite magma, quartz bearing augite-hypersthene andesite magma and augite-hypersthene dacite magma as lithic-rich pyroclastic falls, lava flows and pumice fall. Among them quartz bearing andesites have high 87Sr/86Sr ratios, suggesting the involvement of crustal assimilation. At the end of these eruptions, augitehypersthene dacitic lava dome and olivine bearing augite-hypersthene dacitic lava dome intruded into central crater. Dacite magma of the lava domes has no or little evidence of magma mixing or crustal assimilation. Chemical variation of trace elements (Zr, Rb, Y) from the hypersthene-augite andesite of the main body to the augite-hypersthene dacite of the domes cannot be accounted for by fractional crystallization, and difference of degree of partial melting of the source material is required.

著者

林信 太郎 谷口 宏充 千葉 達朗 伊藤 英之 大口 健志 長谷中 利昭 大場 司 藤田 浩司 根本 靖彦

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.1997, no.2, 1997-09-21

被引用文献数

1