火山
Online ISSN : 2189-7182
Print ISSN : 0453-4360
ISSN-L : 0453-4360
49 巻, 3 号
選択された号の論文の4件中1~4を表示しています
  • 増田 直朗, 渡辺 一徳, 宮縁 育夫
    原稿種別: 論説
    2004 年 49 巻 3 号 p. 119-128
    発行日: 2004/06/30
    公開日: 2017/03/20
    ジャーナル フリー
    Three lava flows: Nakahono, Nagano, Otogase lava flows and Otogase debris avalanche deposit were newly discovered on the western slope of Aso central cones, southwestern Japan. The Nakahono lava is biotite rhyolite (SiO2=73wt.%) and is distributed on only a floor of the Otogasegawa River. Occurrence age of the Nakahono lava is older than that of ACP3 or ACP5. The Nagano lava is pyroxene dacite (SiO2 = 68 wt.%) and comprises a pyroclastic cone (ca. 600m diameter). The Otogase lava is pyroxene rhyolite (SiO2=73wt.%) and forms a lava plateau (ca. 1km long and 200-300m wide). The lava is interbedded between the Aira-Tn ash (AT, ca. 29 calka) and the Kikai Akahoya ash (K-Ah, ca. 7.3calka). The Otogase debris avalanche deposit is possibly generated by a collapse of lava flow or dome on the vent of the Otogase lava. Based on the stratigraphic relationship with tephra deposits, the eruption age was estimated at ca. 20calka. In the period of younger stage than approximately 30calka, intermittent eruptions producing three rhyolite to dacite lava flows provide important information about eruptive history and magma evolution in the western part of Aso central cones.
  • 高橋 良, 中川 光弘, 中西 清, 吉本 充宏
    原稿種別: 論説
    2004 年 49 巻 3 号 p. 129-142
    発行日: 2004/06/30
    公開日: 2017/03/20
    ジャーナル フリー
    During the 20th century, two major eruptions of Hokkaido-Komagatake volcano occurred in 1929 and 1942. The 1929 eruption was one of the largest magmatic eruptions in Japan in the last 100 years, and has been investigated in detail. On the other hand, no systematic volcanological research has ever been done on the 1942 eruption. Since it has not been clarified whether the 1942 eruption was magmatic or phreatic, we decided to investigate the sequence and eruption mode of this event. At the summit, the 1942 eruptives overlie the 1929 pyroclastic fall deposits and are covered by ash-fall deposits of recent phreatic eruptions. Based on the photographs, documentary descriptions and sequence of the eruptive deposits, we were able to divide the eruption into three phases. Phase-1: Start of the eruption on November 16 at 8: 00 (JST) that formed NW-SE trending fissure 1.6 km in length. This was followed by eruptions at the main crater (1929 crater) that produced the pyroclastic surges (~8: 10). Phase-2: Eruption column had grown up from 8: 18-8: 20 and ash-fall started to spread towards the southeast portion of the 1929 crater. Phase-3: Decrease of the eruption activity at 8: 40 and continuous ash emission until the evening. We newly found fresh pumice and its fragments in the deposits associated with the Phase-1 event. This fresh pumice exhibits no alteration features and has similar petrological features to other historic pumice since 1640. This fresh pumice, however, could be distinguished from other historic pumice by the most silicic nature of its matrix glasses. Therefore, we concluded that the pumice was derived from the juvenile magma that caused the 1942 eruption. Considering the temporal evolution of the component of the eruptive materials, the 1942 eruption began with a phreatomagmatic eruption (Phase-1) followed by phreatic eruptions (Phase-2 and -3). Before the two major eruptions in 1929 and 1942, small eruptions occurred intermittently from 1919-1924 and 1934-1938, respectively. Based on these observations, the preceding eruptions could be possible precursor activities of subsequent magmatic eruptions. Since 1996, small phreatic eruptions have intermittently occurred in Hokkaido-Komagatake until 2000. The present state of the volcano, as we would like to emphasize, has been quite similar to the conditions prior to the two previous magmatic eruptions.
  • 筒井 智樹, 松島 健, 清水 洋
    原稿種別: 寄書
    2004 年 49 巻 3 号 p. 143-151
    発行日: 2004/06/30
    公開日: 2017/03/20
    ジャーナル フリー
    Pseudo Reflection Profiling (PRP) is introduced as an effective method for shallow seismic study of volcanoes. PRP is applied to seismic exploration records in Unzen Volcano in 1995 and two reflection profiles are obtained. A reflection profile of E-W line shows more complicated appearance of reflection phases in Heisei-Shinzan side than that in another side of the survey line. Another reflection profile of N-S line includes convex phases in the west of Heisei-Shinzan. Two of these reflections are common in both profiles and can be located at the boundary of the shallow aseismic region which was pointed out in the other geophysical study. Since these results are consistent with the previous reflection study and PRP requires free source location, PRP will be one of the useful method for surveying shallow structure of volcanoes.
  • 佐野 貴司
    原稿種別: 解説・紹介
    2004 年 49 巻 3 号 p. 153-156
    発行日: 2004/06/30
    公開日: 2017/03/20
    ジャーナル フリー
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