著者

宮地 直道 富樫 茂子 千葉 達朗

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.49, no.5, pp.237-248, 2004

参考文献数

34

被引用文献数

4

---

A large-scale collapse occurred at the eastern slope of Fuji volcano about 2900 years ago, based on calibrated ¹⁴C age of a wood sample collected in the resulting debris avalanche deposit. The collapsed slide deposit, called "Gotemba debris avalanche deposit" (Goda), is distributed on the eastern foot of the volcano covering an area of more than 53 km² The source amphitheater is not preserved because it became covered by younger tephra erupted from the summit crater. This avalanche deposit is overlain by the "Gotemba, mudflow deposits" (Gomf) emplaced repeatedly after the avalanche. Some now units of the Goda and Gomf entered pre-existing rivers and were finally emplaced as fluvial deposits. The Goda is composed of debris-avalanche blocks, showing jigsaw cracks, along with smaller blocks ranging from several tens of centimeters up to l m in diameter. The debris-avalanche matrix is a mixture of smaller nieces of blocks and ash-sized materials due to mainly shearing and fragmentation of large blocks. Igneous rocks include fresh and altered gray basaltic lava, weathered tephra including red scoria and white clay. Petrographical and geochemical data indicate that most blocks were derived from the Older Fuji volcano. The volumes of the Goda and Gomf are about l.05km^3 and 0.71km^3 respectively, based on presently available geological and borehole data. Since the blocks of Goda are composed mostly of the products of the Older Fuji volcano and the older stage lavas of Younger Fuji volcano do not extend to the eastern foot of Fuji volcano, a bulge of Older Fuji volcano must have existed in the eastern flank of Fuji volcano preventing the older stage lavas to now to the east. This bulge collapsed in the form of three blocks from the foot of the mountain. The abundance of hydrothermally altered deposits in the Goda and the absence of fresh volcanic products within the Goda suggest its origin as a rupture inside the altered deposits possibly triggered by a large earthquake or phreatic eruption.

著者

安井 真也 富樫 茂子 下村 泰裕 坂本 晋介 宮地 直道 遠藤 邦彦

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.43, no.2, pp.43-59, 1998-04-30

被引用文献数

4

---

A large amount of pyroclastic materials (ca. 1.7 km^3) was erupted during the 1707 eruption of Fuji Volcano. Variety of lithic fragments has been recognized in the pyroclastic fall deposits, namely, accessory and accidental lava fragments, gabbros, and granitoids. A great variety of petrologic features is observed in gabbroic fragments consisting of olivine gabbro norite, gabbro norite, troctolite and anorthosite. The gabbros are divided into O, P and F groups on the basis of modal ratios of olivine, plagioclase and Fe-Ti oxide. O group mainly consists of plagioclase and olivine with minor amounts of pyroxenes and Fe-Ti oxide. O group is considered to have been adcumulated in the lower part of magma chamber because of their high depletion in incompatible elements, their well-sorted grain size and sedimentary structure. P group is composed of plagioclase, pyroxenes and minor amounts of olivine and Fe-Ti oxide. F group is similar to P group, but is enriched in Fe-Ti oxide. P and F groups are orthocumulates and may be solidified in the upper part and margin of magma chamber or dike because of their porphyritic texture. Such a variety of gabbros may correspond to the difference in location of the single gabbroic body beneath Fuji Volcano. The estimated source magma of the gabbros is similar to the basalt of Fuji Volcano in chemical and mineralogical compositions indicating that they are cognate origin. Chemical compositions of olivine and pyroxenes become magnesian and those of plagioclase become calcic with the decreasing of bulk-rock FeO^*/MgO ratio. It suggests that they are the products of continuous fractional crystallization. The magma of the 1707 eruption could have come up from under the gabbroic body, which was the solidified basaltic magma chamber, and have caught and brought the rocks from the gabbroic body up to the surface as cognate xenoliths during the eruption.

著者

宮地 直道 富樫 茂子 千葉 達朗

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.49, no.5, pp.237-248, 2004-10-29 (Released:2017-03-20)

参考文献数

34

被引用文献数

5

---

A large-scale collapse occurred at the eastern slope of Fuji volcano about 2900 years ago, based on calibrated 14C age of a wood sample collected in the resulting debris avalanche deposit. The collapsed slide deposit, called “Gotemba debris avalanche deposit” (Goda), is distributed on the eastern foot of the volcano covering an area of more than 53 km2 The source amphitheater is not preserved because it became covered by younger tephra erupted from the summit crater. This avalanche deposit is overlain by the “Gotemba, mudflow deposits” (Gomf) emplaced repeatedly after the avalanche. Some now units of the Goda and Gomf entered pre-existing rivers and were finally emplaced as fluvial deposits. The Goda is composed of debris-avalanche blocks, showing jigsaw cracks, along with smaller blocks ranging from several tens of centimeters up to l m in diameter. The debris-avalanche matrix is a mixture of smaller nieces of blocks and ash-sized materials due to mainly shearing and fragmentation of large blocks. Igneous rocks include fresh and altered gray basaltic lava, weathered tephra including red scoria and white clay. Petrographical and geochemical data indicate that most blocks were derived from the Older Fuji volcano. The volumes of the Goda and Gomf are about l.05km^3 and 0.71km^3 respectively, based on presently available geological and borehole data. Since the blocks of Goda are composed mostly of the products of the Older Fuji volcano and the older stage lavas of Younger Fuji volcano do not extend to the eastern foot of Fuji volcano, a bulge of Older Fuji volcano must have existed in the eastern flank of Fuji volcano preventing the older stage lavas to now to the east. This bulge collapsed in the form of three blocks from the foot of the mountain. The abundance of hydrothermally altered deposits in the Goda and the absence of fresh volcanic products within the Goda suggest its origin as a rupture inside the altered deposits possibly triggered by a large earthquake or phreatic eruption.

著者

安井 真也 富樫 茂子 下村 泰裕 坂本 晋介 宮地 直道 遠藤 邦彦

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.43, no.2, pp.43-59, 1998-04-30 (Released:2017-03-20)

参考文献数

22

---

A large amount of pyroclastic materials (ca. 1.7 km3) was erupted during the 1707 eruption of Fuji Volcano. Variety of lithic fragments has been recognized in the pyroclastic fall deposits, namely, accessory and accidental lava fragments, gabbros, and granitoids. A great variety of petrologic features is observed in gabbroic fragments consisting of olivine gabbro norite, gabbro norite, troctolite and anorthosite. The gabbros are divided into O, P and F groups on the basis of modal ratios of olivine, plagioclase and Fe-Ti oxide. O group mainly consists of plagioclase and olivine with minor amounts of pyroxenes and Fe-Ti oxide. O group is considered to have been adcumulated in the lower part of magma chamber because of their high depletion in incompatible elements, their well-sorted grain size and sedimentary structure. P group is composed of plagioclase, pyroxenes and minor amounts of olivine and Fe-Ti oxide. F group is similar to P group, but is enriched in Fe-Ti oxide. P and F groups are orthocumulates and may be solidified in the upper part and margin of magma chamber or dike because of their porphyritic texture. Such a variety of gabbros may correspond to the difference in location of the single gabbroic body beneath Fuji Volcano. The estimated source magma of the gabbros is similar to the basalt of Fuji Volcano in chemical and mineralogical compositions indicating that they are cognate origin. Chemical compositions of olivine and pyroxenes become magnesian and those of plagioclase become calcic with the decreasing of bulk-rock FeO*/MgO ratio. It suggests that they are the products of continuous fractional crystallization. The magma of the 1707 eruption could have come up from under the gabbroic body, which was the solidified basaltic magma chamber, and have caught and brought the rocks from the gabbroic body up to the surface as cognate xenoliths during the eruption.