The eruption of Tsumaya pyroclastic flow is one of the three successive activities which took place within Aira caldera, southern Kyushu. The activities started with an eruption of Osumi pumice fall, then followed the eruption of Tsumaya pyroclastic flow, and then ended with the eruption of large scale Ito pyroclastic flow. According to the occurrence of the deposit, it is interpreted that the eruptions occurred within such a short period that the dating method cannot distinguish each eruption. Many ¹⁴C ages related with the activity of Aira caldera are shown in Table 2. These ages show widely scattered values, from the youngest age (2, 460 Y.B.P.) to the oldest age (38, 900 Y.B.P.). Younger ages among them are considered to be due to contamination of modern carbon to the dated samples. In order to check the possible contamination of modern carbon to the charcoal samples alkali treatment by KOH solution were carried out on a sample in Tsumaya pyroclastic flow deposit. Then, the two dissolved fractions and the final residue were dated by ¹⁴C dating method (Table 1). These results agreed with the age of about 22, 000 Y.B.P. within the range of errors. This shows that no contamination of modern carbon occurred to the dated sample in this case. From discussions on the nature of ¹⁴C dating samples and from comparison with the ages by ionium dating and fission track dating, it is suggested that the possible age of the activity of Aira caldera is about 22, 000 years before present.

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Gravtiy survey over the Aso volcanic region was made by author et al from 1965 to 1966. Bouguer gravity anomalies over this area are characterizd by a strong negative anomaly on the Aso caldera. The gravity low of the Aso caldera shows characteristic feature of the “low anomaly type caldera” or "crater lake type caldera" as is pointed out by Yokoyama. Aso caldera is outlined by a circular somma and a range of more than a dozen central cones divides the caldera into two wide plains (atrio), the northern Asodani atrio and southern Nangodani atrio. Contour lines of Bouguer anomaly are ring-shaped on the middle of the caldera, which they are peas-shaped on the Asodani atrio area (Fig.1). The Asodani atrio is a vast horizontal plain rimmed by a caldera wall on the north and the frank of the central cones on the south. Geological evidences show that thick sedimentation of volcanic ashes and gravels occurred in this area when it once constituted the bottom of a huge caldera-lake. It is apparent that this thick volcanic sediment disturbes the structural interpretations of gravity data on this area. Fortunately the thickness of volcanic deposit can roughly be estimated by the observed period of ever-existing microtremor. Thus, Bouguer anomalies over Aso caldera without the effect of the low density deposit are obtained (Fig.8) This gravity map shows that there are ring-shaped contour lines of the low anomaly at the middle of the caldera, however, no abnormal feature is recognized elsewhere. This result shows that the mass deficiency due to the caldera formation which has been proposed by Yokoyama exists only on the middle part of the caldera. On the other hand, gravity anomaly at the area where no peculiar feature is recognized is due to the topography of the caldera having been formed by the collapse and enlargement of the caldera wall.

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Under the condition of the maximum fractionation, chemical variations were numerically calculated for zoned plagioclase grains crystallized from liquids with various initial compositions in the Ab-An simple system. The thermodynamic data for calculating compositions of coexisting liquid and crystal at a given temperature were obtained from the phase diagrams of Bowen (1913), Lindsley (1966), and Yoder et. al. (1957). The zoning patterns thus obtained are shown graphically in Figs. 5 and 6, in which a radius of crystal grain is represented by the cube root of the value of percentage solidified. Grains crystallized from initially An-rich liquids have a narrow rim with considerable compositional variation arround a large and nearly homogeneous core, while those from Ab-rich liquids gradually change the composition from center to rim.

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