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

相澤 正隆 安井 光大 井村 匠

出版者

地学団体研究会

雑誌

地球科学 (ISSN:03666611)

巻号頁・発行日

vol.75, no.2, pp.131-134, 2021

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.