富士山の地質と新富士溶岩の古地磁気

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Geologic classification and distributions of the effusives of Fuji Volcano were com-piled mainly based on Tsuya (1968,1971) and the framework of geologic age by Miyaji (1988). The effusives can be classified into those of the Ko-Fuji and Shin-Fuji Volcanos, and that of the Shin-Fuji Volcano can be divided into 3,old, middle and young. The age of the boundary between the old and middle of Shin-Fuji Volcano is ca. 6300 years, and between middle and young is ca. 2200 years. The enormous amount of effusives of Fuji Volcano is related to the collision tectonics in central Japan, induced by motion of the Philip-pine Sea Plate. Paleomagnetic samples were collected from 11 sites of the old lavas and 3 sites of the middle aged lavas of the Shin-Fuji Volcano. Three oriented blocks were sampled from each site. The intensities of natural remanent magnetization (NRM) of most of the samples ranged from 5 to 25 A/m, and median destructive field (MDF) was 15-40mT. The direction of stepwise demagnetized NRM is stable and can be used for paleomagnetic studies. The positions of the virtual geomagnetic pole (VGP) are distributed around North Pole within latitudes higher than 60°N. VGP distributions are significantly different for the sites on the "same" lava. NRM intensities, MDF, anisotropy of magnetic susceptibility, and anhysteretic remanent magnetization (ARM), are useful to characterize the rock magnetism of lavas. The 3 blocks from 2 sites of the middle aged lavas of the Shin-Fuji Volcano have extremely strong intensities ranging from 40 to 80 A/m and different rock magnetic character from the other samples with 5-25 A/m of a NRM intensity. The sampling sites are located near the lower margin of the lava flows. The strong NRM can be estimated by the precipitation of reduced iron in the lava flow, contaminated by growing vegetation.
静岡大学の論文

富士山の地質と新富士溶岩の古地磁気

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