著者

小林 淳 萬年 一剛 奥野 充 中村 俊夫 袴田 和夫

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.245-256, 2006

参考文献数

45

被引用文献数

1 4

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We discovered a set of phreatic explosion deposits, herein referred to as the Owakidani tephra group, on the northern slope of Mt. Kamiyama and in the Owakidani fumarolic area of the Hakone Volcano. The tephra group is the product of the volcanic activities since the latest magmatic eruption of Hakone Volcano at around 2.9ka. It comprises five units named Hk-Ow1 to Hk-Ow5 in the ascending order. Both Hk-Ow1 and Hk-Ow2 comprise tephra fall deposits and secondary debris flow deposits. In addition to these deposits, Hk-Ow2 is also associated with surge deposits. Hk-Ow3, Hk-Ow4 and Hk-Ow5 consist of tephra fall deposits. The ash of these tephra fall deposits and the matrix of the secondary debris flows are principally composed of clay, altered lithics and secondary minerals supposed to be of fumarolic area origin. It is possible that Hk-Ow1 and Hk-Ow2 erupted from a fissure on the northeastern ridge of Mt. Kamiyama, while Hk-Ow3, Hk-Ow4 and Hk-Ow5 erupted at Owakidani. No juvenile material was found within the deposits of these eruptions except for Hk-Ow2, while the surge deposit of Hk-Ow2 contained trace amounts of volcanic glass fragment. Although it is considered that the principal nature of the eruptions of the Owakidani tephra group is phreatic, the deformation of the edifice around the source area implies the possibility of magma intrusion to shallow levels. Based on the calendar ages of the Owakidani tephra group and the stratigraphic position of the Kozushima-Tenjosan tephra, we estimated that Hk-Ow3, Hk-Ow4 and Hk-Ow5 erupted in relatively short intervals between the latter half of the 12th and 13th centuries. On the other hand, Hk-Ow1 and Hk-Ow2 erupted at around 3 kyr BP and 2kyr BP, respectively. The eruption ages of the Owakidani tephra group generally correspond to the seismic events that occurred in the Kozu-Matsuda Faults and the Tanna-Hirayama tectonic line. It is suggested that the activity of the Hakone Volcano may be closely related to the tectonic events in this region.

著者

町田 洋 新井 房夫 村田 明美 袴田 和夫

出版者

公益社団法人 東京地学協会

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.83, no.5, pp.302-338, 1974

被引用文献数

25 17

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This paper presents the basic data for tephrochronology of the Middle Pleistocene events of South Kanto, Central Japan. The tephra preceding to the last interglacial age (the Tama tephra) is found in highly dissected terraces (Fig. 1) much thicker than the younger tephra formations (Fig. 2). It is subdivided into five formations from T-A to T-E in order of increasing age (Table 1 and Fig. 2). This classification is based not on the eruptive history of the volcanoes but on the sequence of terraces. The relationship between these two is shown in Fig. 2.<BR>Of the four columnar sections (Fig. 6), section 1, synthesized from many sections in the Ooiso Hill (Fig. 3), 25 km east of the Hakone volcano, provides a standard section for South Kanto. There are so many tephra layers of the Tama stage (about 150 m in thickness) preserved in a relatively fresh state, that they are expected to cover a long time in the Middle Pleistocene. A detailed description is therefore needed for specifying any particular tephra bed. 75 marker beds have been selected, and described in terms of lithological and petrographical characteristics as in Table 2 and Fig. 4. The most valuable for characterizing marker tephras are the refractive indices of orthopyroxene and hornblende.<BR>The distribution and the petrographic character of tephras indicate that the tephras mainly comprising two pyroxene phenocrysts might come chiefly from the Old Somma of the Hakone volcano (Fig. 8-1 and 8-2) and partly from the Ashitaka volcano. Whereas such salic tephras with abundant hornblende, biotite and other phenocrysts as GoPi and TE-5 might originate from distant volcanoes (Fig. 8-3).<BR>As a result of precise examination, tens of the marker beds have been traced eastward to the environs of Tokyo and Yokohama (Fig. 6), where the standard Quaternary sequence had been established. Furthermore, several marker beds have been recognized within the Middle Pleistocene marine sediments in the Boso Peninsula. By use of these the Quaternary stratigraphy already established in South Kanto is significantly revised (Table 3 and 6).