富士火山北西山麓本栖湖畔の864年青木ヶ原溶岩の形態について

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Aokigahara Lava Flow erupted in A.D. 864 covers the northwestern foot of Fuji volcano. It consists of two flows: Ishizuka Lava erupted from a parasitic vent at the foot of Ohmuroyama, and Nagaoyama Lava effused from a cinder cone of Nagaoyama. Ishizuka Lava flowed down to Motosuko Lake, spreading laterally like an alluvial fan in front of the lake. Approximately 36 flow lobes that protruded from the aa front, pinch and swell, occasionally bifurcating into several branches from swollen portions of the lobes. Most lobes have a few longitudinal clefts with similar features to crease structures of rhyolitic and andesitic lava domes and to be banded on cleft walls on pahoehoe tumuli. The flow lobes are classified into three types on the basis of surface structures. Type-I lobes are characterized by well-developed clefts, which exhibit two distinct features: Stripe-type has alternating stripes of red and grey on the cleft wall parallel to the elcngation of the lobes. The stripes exist only for a depth a cm from the surface of the walls. The groundmass of the grey stripes contains abundant dendritic crystallites, while that of the red stripes consists of palagonitized glass; Blister-type develops blisters with pipe vesicles on the cleft wall. It does not have stripes, however, they may be present at different levels on the same cleft wall. Type-II lobes have clinkers with stripes similar to those of Stripe-type clefts of Type-Ilobes, however, they do not develop clefts. Type-III lobes are thinner than Type-I obes and are covered with red platy clinkers, giving the whole lobe a reddish appearance. Clefts are poorly developed. Continuous supply of lava into a flow lobe, which ceased advancement, causes tensile stress on solidifying crusts. We suggest that stepwise opening of cracks resulted in the difference in crystal density and habit of stripes on the clefts. Occasional opening of the crack and quenching hot semi-molten lava exposed at the crack tip under water resulted in the fomation of red stripes, followed by solidification of the new crust beneath the crack tip. Subsequent fracturing of newly formed brittle crust resulted in the formation of the grey stripes. The Blister-type clefts were formed when the supply of lava temporaily stopped and reslarted again. Solidification of the crust and the formation of blisters proceeded when lava within the lobe was static. We conclude that Type-I was produced by intermittent supply of lava into a static flow lobe. Type-II lobes were moving during inflation of the lobe. As a result, cleft walls with stripes were disrupted to produce clinkers. Type-III was produced by breaking of crust immediately after its fomationd uring rapid emplacement of the lobe. Therefore, the shear rates during the fomation of flow lobes increase from Type-I, II, to III lobes.
1999-08-30

富士火山北西山麓本栖湖畔の864年青木ヶ原溶岩の形態について

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