第三紀玄武岩の火道例

The purpose of this report is to describe the volcanic vent of a new type in the Neogene. The vent is situated in Tatenoyama, Marumori-machi, about 40 km SSW of Sendai City, North Japan. Its inner structure has became visible gradually every year because it was the stonepit. The Ryozen formation consists largely of basaltic lava and its pyroclastic rocks, and the Kaneyama formation consists of sandstone, siltstone and mudstone. Both formations are distributed around the vent, and are correlated with the lower part of the Neogene. In these formations, faults of both directions of NW-SE and WNW-ESE are widely developed, and have been of great significance to the formation of volcanism and sedimentary basin in the early Miocene. Volcanism of the vent occurred in the following order. 1st stage: The volcanic activity started with the opening of fissures that take directions of NW-SE and WNW-ESE, which occurred in connection with the fracture movement, namely, the formation of a fanshaped pit crater. 2nd stage: The successive explosive activities were accompanied with the outburst of a large amount of coarse ash, spherical lapilli and ball bomb. 3rd stage : The intrusion of lava and vent breccia, last stage: The emission of a little amount of ash, lapilli, breccia and ball bomb. Macroscopically, the lava speciments comprise two varieties. One of the two varieties is black in colour and very hard, and the other is redish purple in colour and remarkably porous. The volcanic products of the vent are petrographically basalt with olivine phenocrysts, and rarely with microphenocrysts of augite, groundmass of intergranular texture which consists of plagioclase, olivine, augite, magnetite, ilemenite and rarely interstitiary silica minerals. Formerly, the writer has described numerous volcanic vents and necks in the Neogene. These are circular-shaped or elliptical-shaped in horizontal cross section, and are constructed of a marginal part which consists of pyroclastic rocks containing enormous granitic fragments derived from the basement, and of a central part which consists of intrusive lava including a lot of xenoliths and xenocrysts. These vents are connected to volcanic products accumulated on the surface of the basement. The above-mentioned facts may show that magma injected the basement, while violently destroying it. On the other hand, the volcanic vent described in detail in this report easily erupts basaltic magma along the fissure which occurred owing to the fracture movement. The volcanic activity of this vent ends in simple style, and never occurs again from the same crater.