One deep drilling was carried out by the Taisei Co., on the northeastern caldera floor, 500 m above sea level, of Aso volcano for the purpose of getting information on both underground structure and hot spring (Fig.1). According to our observation of core samples, we show the drill hole section as Fig.2. Noticeable facts are as follows : 1) From the surface to 320 m in depth, the sediments consisting of volcanic sand bed, tuff bed, gravel bed and clay bed exist. They have deposited in the caldera lake. 2) Underneath the sediments, the Kuzyu pumice flow forming the welded tuff in lower part are laid more than 150 m in thickness. This pumice flow is divided into two parts at -453 m. The upper part is whitish grey and the lower part is grey in colour. Both the flows show the different character in chemical composition as Table 1. 3) The granite of Tamana type exists underneath the Kuzyu pumice flows. The surface of the granite is thought to be an erosion surface.

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Examples are presented in which arrangement of lateral and post-caldera cones indicates the probable regional stress field in the late Quaternary. Izu-Oshima, Hakone and Fuji are among the larger stratovolcanoes to the southwest of Tokyo, the former two having collapse calderas on their summit. Parasitic and post-caldera cones and craters of the three volcanoes are following a trend of similar direction, some of them being produced by fissure eruptions on their flank. The zones trend in the direction of about N 35° W at Fuji, N 45° W at Hakone and N 30° W at Oshima. Dikes are also found in these zones running mostly parallel to them. The trend of fissures of fissure eruptions on the flanks of Fuji and Oshima is also similar to the zones of recent activity. Because sites of flank eruption are regarded as points where radially formed dikes around the central magma column have penetrated the flank, the above described distribution of craters in these volcanoes would indicate a concentration of radial dikes in a specific direction at the three adjacent volcanoes. Considering that dikes are fossil tension cracks formed perpendicularly to the axis of the minimum principal compressional stress, the concentration of parasitic craters can be explained by the stress field caused by the pressure increase in the magma column superimposed on the preexisting regional field with the maximum principal stress axis in a NW direction. The nearly identical, preexisting stress field in the three adjacent volcanoes suggests that the field is part of a more regional one including the area of the volcanoes. This suggestion is strongly supported by the presence of active, conjugate strike-slip faults in the same general area, i.e. on the Izu-peninsula. The maximum compressonal axis indicated by the faults is again in a direction of about N 30° W and oriented horizontally. The last movement of the fault system was observed in 1930, at the time of the Kita-Izu earthquake, whose magnitude was 7.0.

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The three pyroclastic flow deposits are exposed in the Goino area, southern part of Kagoshima City, South Kyushu. They are, in order of superposition, the Goino pyroclastic flow, the Ata pyroclastic flow, and the “Shirasu”. The stratigraphic relation between the former two pyroclastic flows is not clarified by the field survey. It is, however, concluded that the Goino pyroclastic flow is older than the Ata pyroclastic flow because the zircon from the Ata pyroclastic flow is colorless in mass color indicating the middle Pleistocene to the Recent, while that from the Goino pyroclastic flow is dull rose-pink indicating the early to middle Pliocene. The peterographic characters of these three pyroclastic flows are as shown in Table I. The Ata pyroclastic flow in this area is variable in lithological features, but the optical constants for the constituent minerals coincide exactly with those of the flow exposed in the Osumi Peninsula and Makurazaki areas. Throughout the flow deposit, are enclosed many granitic xenoliths derived from the South-Osumi granodiorite. It is consistent with the conclusion that the Ata pyroclastic flow magma is intimately related to the South-Osumi granodiorite constructing the basement of the Ata caldera in chemical composition (S. Taneda et al., 1966). The “Shirasu” exposed in this area, is red to greyish pink in color, about 20 meters in thickness, not consolidated and not welded. It is a primary pyroclastic flow deposit from the Aira caldera volcano in the latest Pleistocene.

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