火山
Online ISSN : 2189-7182
Print ISSN : 0453-4360
ISSN-L : 0453-4360
49 巻, 2 号
選択された号の論文の4件中1~4を表示しています
  • 宮縁 育夫, 星住 英夫, 渡辺 一徳
    原稿種別: 論説
    2004 年 49 巻 2 号 p. 51-64
    発行日: 2004/05/20
    公開日: 2017/03/20
    ジャーナル フリー
    The stratigraphy and chronology of late-Pleistocene (after deposition of AT ash) tephra layers erupted from post-caldera central cones of Aso Volcano, SW Japan, are evaluated through study of the thick tephra sequence preserved mainly atop the pyroclastic-flow plateau east of the caldera. Twenty andesite to basaltic-andesite scoria-fall deposits (YmS20-YmS1 in ascending order) and two pumice-fall deposits (NbP2 and NbP1) were identified as maker beds for the interval ca. 29-13ka (calibrated 14C age). The tephra layers were produced mostly by scoria and ash eruptions of Nakadake, the only active post-caldera central cone, and their bulk volumes calculated from isopach maps range from 0.01 to 0.9km3. The radiocarbon ages of buried soils just below individual tephra layers reveal that YmS20 to YmS15 were erupted at ca. 22-21 ka and YmS20 to Ym-S1 at ca. 18-16ka. Total tephra volume during 29-13ka is estimated at about 1.3km3 (DRE). Production of tephra was mainly concentrated in two periods : 22-21 ka (0.8km3) and 18-16ka (0.5km3).
  • Minoru YOSHIDA, Kentaro TSUCHIYA
    原稿種別: 論説
    2004 年 49 巻 2 号 p. 65-72
    発行日: 2004/05/20
    公開日: 2017/03/20
    ジャーナル フリー
    Fluorine and chlorine have been determined for all the products of the 1783 activity of Asama volcano. The activity displayed various features that are characteristic of andesitic eruptions, i.e., pumice falls, two types of pyroclastic flow (the Agatsuma and Kambara pyroclastic flows), and the Oni-oshidashi lava flow. Their F-C1 variation patterns differ from each other according to the nature of the eruption. The pumice samples have very uniform and the highest Cl contents of all the eruptive products, despite their vesiculated texture. This is explained by the enrichment of volatiles in the uppermost part of the magma column at the beginning of the eruption. The Agatsuma pyroclastic deposit samples show the lowest Cl and F contents and the largest variations. An intense release of gas from the pyroclastic flow after the effusion is thought to have caused this. The F and Cl contents of essential blocks in the Kambara pyroclastic flow deposits, together with the field observations, suggest that the release of gas from the essential material contributed little to the fluidization of this pyroclastic flow. In conclusion, the transition from one mode of eruption to another observed in the 1783 activity of Asama must be intimately related to the variation of the volatile contents of the magma at the time of the effusion.
  • 山縣 武彦, 高島 勲, 渡辺 公一郎, 井沢 英二
    原稿種別: 論説
    2004 年 49 巻 2 号 p. 73-81
    発行日: 2004/05/20
    公開日: 2017/03/20
    ジャーナル フリー
    Thermoluminescence (TL) ages are obtained for 28 samples from domes and debris avalanche deposit of Younger Unzen Volcano. Weighted mean ages are 4.1 ± 0.8 ka for Tenguyama lava, 3.8 ± 0.6 ka for Shichimenzan lava, 3.1 ± 0.6 ka for Fugendake-sancho lava, 3.4 ± 0.6 ka for Kazaana lava, 15 ± 2 ka for Myokendake lava, 25 ± 4 ka for Kunimidake lava. Six TL ages for Tarukidaichi debris avalanche deposit range from 20 ± 2 ka to 30 ± 3ka. We success to distinguish individual dome age by TL dating. Source rock of Tarukidaichi debris avalanche deposit is Kunimidake lava and age range of formation is estimated as 25 ka to 7.3 cal ka by TL dating and field observation. We gave new and useful age data of Younger Unzen Volcanic lavas by TL dating with age range about 25 ka to 3 ka.
  • 高橋 康
    原稿種別: 論説
    2004 年 49 巻 2 号 p. 83-102
    発行日: 2004/05/20
    公開日: 2017/03/20
    ジャーナル フリー
    Mt. Eboshidake is located in the western part of the Eboshi volcano group (western Eboshi volcano group), central Japan. According to the result of geological survey and the petrographic features, the western Eboshi volcano group is composed of 9 volcanic units such as; Iwayakan’non, Kokuzo, Kanahara, Omuro, Ofuji, Gotomiki, Lower Eboshidake, Narahara, and Upper Eboshidake lavas. Based on the newly measured paleomagnetic polarities and reported K-Ar radiometric ages, the volcanic activity of the western Eboshi volcano group was reconstructed as follows. The volcanic activity started about 1 Ma and during the early Pleistocene age, the volcanism was characterized by eruptions of thin basaltic andesite - andesite lavas from scattered vents, which formed small volcanic edifices. Then thick andesite - dacite lavas erupted in the initial period of middle Pleistocene, and formed clusters of lava domes. After that, the volcanic activity of the Eboshi stratovolcano started about 0.8 Ma, and after the dormant period (around 0.4 Ma) when Iwashimizu gravel bed of about 30m thick with two marker tephra beds was piled up, the basaltic andesite - andesite lavas erupted again until about 0.4-0.3 Ma. These lavas erupted from central vent and formed main edifice of Eboshi stratovolcano. The whole rock chemistry of 69 samples from the western Eboshi volcano group and eight samples from the eastern Eboshi volcano group was analyzed using XRF. The result shows that the rocks of the western Eboshi volcano group are classified into the basaltic andesite - andesite group (SiO2 : 51-58%) and the andesite - dacite group (SiO2 : 59-64%). The basaltic andesite - andesite group is mainly tholeiitic and more depleted in MgO content than the calc-alkaline andesite - dacite group. Most of these rocks are plotted in the medium-K field on the K2O vs. SiO2 diagram. This feature is similar with the other volcanoes in the eastern Eboshi volcano group and the Asama volcano.
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