Aso-2火砕流堆積物の二次流動

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Near the west rim of the Aso caldera, Aso-2 pyroclastic flow deposit, one of the four major eruption-cycle deposits erupted from the Aso area before the formation of present Aso caldera, is devided into Aso-2R, -2A and -2B subunits in ascending order. In this area, Aso-2A is densely welded to the very base of the deposit and adheres to the surface of the underlying Aso-2R unit without basal non- or partially- welded zone (Figs. 2, 3, 4). Some parts of Aso-2A deposit have evidence of secondary flowage by its extremely flattened essential lenses which are elongated in a direction on the foliation plane showing flow lineation (Figs. 5, 6, 7, 9). Aso-2R is homogeneous, lava-like deposit of pyroclastic origin formed by coalescence and homogenization of vitric shards accompanied by mass movement of viscous laminar flow. Various kinds of relict pyroclastic or vitroclastic structures and textures are found in Aso-2R in the field and under the microscope (Figs. 10-16). At Loc. 10 in Fig. 1 the basal face of Aso-2R shows "ropy surface" like pahoehoe lava surface indicating a certain amount of laminar flowage (Fig. 17), while at Loc. 1 Aso-2R has differentiated into scoriaceous top, central compact and massive or platy part and breceiated base, like those of a typical aa lava flow (Figs. 2 and 3) suggesting much longer traval distance of laminar flow which may exceeds 100 meters and may well be called rheoignimbrite defined by Rittmann (1962).
特定非営利活動法人日本火山学会の論文
1974-11-01

Aso-2火砕流堆積物の二次流動

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