Prosesses of welding within pyroclastic flow deposits are simulated numerically. Basic equations to simulate the welding are the equation of heat conduction, the relationship between porosity and viscosity, and the relationship between viscosity and temperature. As the relationship between porosity and viscosity, a theoretical equation derived by Murray and other investigators is employed : ln φ=ln φ_o-(3/4)・(P/η)・t. These equations are solved iteratively by a high speed electronic computer. Standard model of the pyroclastic flow deposits is chosen as follows ; initial temperature is 650℃, thickness of deposits is 75 m, viscosity at 650℃ is 10¹⁵ poise, and physical properties of the deposits are nearly the same as those of pumices. Calculations are performed for the standard and various modified models. Results indicate that : (1) degree of welding is mainly controlled by viscosity, intial temperature, and thickness of the pyroclastic flow deposits ; (2) welding is ceased within the first several years after deposition ; (3) on the other hand, duration of cooling of the deposits is continued to tens to hundreds years, nearly ten times greater than that of welding ; (4) position of the most densely welded part is situated at about two third depht of the deposits regardless wide variation of the parameters of the models ; (5) if the value of the viscosity becomes less than that of the experimental data by the effect of high vapour pressure, welding is possible even if the initial temperature is lower than 600℃ ; (6) within the range of the parameters employed in this study, welding both in the upper 10 m of the deposits and in the layer thinner than 10 m seems improbable.

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The Tarumae volcano (1024 m.a.s.l.) which is situated at the eastern margin of South-west Hokkaido is the youngest of the three volcanoes formed after caldera depression of the Shikotsu volcano (Figure 1). The volcanic history of the Tarumae volcano can be divided into five stages as shown in Table 1. The volcano is composed only of pyroclastic materials except a lava dome formed in 1909. The plain immediately to the east of the volcano is covered by thick air-fall deposits consisting of Ta-a, Ta-b, Ta-c and Ta-d members in descending order. Each member consists of multiple fall units representing one cycle of eruption without dormant time. In the Ta-a member 8 fall units are discriminated and in the Ta-b, 10. The isopach of Ta-a and Ta-b members are shown in Figures 3 and 4, respectively. Petrographycally, the air-fall deposits are augite-hypersthene andesitic. Pumice flows correlative to Ta-a and Ta-b discharged from the volcano deposited in all directions except west and formed relatively gentle slope within the range of 7 km from the vent. Detailed stratigraphic study revealed that pumice flows took place twice during fall activities in each Ta-a and Ta-b cycle. The fact that the pumice flow deposit overlies rather thick fall unit suggests that the flow results from relatively large eruption. Pumice flow deposit in each time is not a single flow unit but multiple flow units. Individual flow unit is relatively thin and usually not welded. The volume of pumice fall deposite (Ta-a 1.9 km³, Ta-b 3.4 km³) are larger than that of pumice flow deposits (1.8 km³ in total). These figures suggest that the eruptive activities of the Tarumae volcano in Ta-a and Ta-b stages were very explosive and the vertical eruption column reached to a considerable elevation and that large proportion of ash and pumice in eruption column deposited as pumice falls and minor proportion as pumice flows. From the evidences as above mentioned the Tarumae pumice flow deposits are considered to be formed by same mechanism of St. Vincent glowing avalanche proposed by HAY (1959) or by the base surge proposed by MOORE (1967). The Tarumae volcano was formed only by repeated activities of pumice fall and pumice flow without any lave except latest extrusion of central lava dome. This type of volcano may be found in small andesitic volcanoes so far thought to be usual “stratovlocano”.

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Calderas are classified into salic and mafic types according to their association with either salic pyroclastic flows or mafic lava flows. The salic type corresponds to the low gravity anomaly type, and the mafic type to the high gravity anomaly type of YOKOYAMA. When the longer diameters of the two types of calderas in Japan and the world are plotted against the SiO₂ contents of the pyroclastic flows, or lava flows, they are distributed in diverging U-shaped areas, i.e., the size of salic type calderas becomes larger with increasing SiO₂ content, and that of mafic type calderas increases with decreasing SiO₂ content. The colossal amounts of salic magmas and their high explosivity explain the larger size of the more salic type calderas, while the lower viscosity of the more mafic magmas explains the larger size of the more mafic type calderas. The relation between the diameters and the depths of the salic type calderas of Japan is examined. For smaller calderas, less than 7 km in diameter, there is some linear relation between the two parameters, but for the larger ones the depths are nearly constant at 500～700 m, irrespective of their diameters. Comparison is made with artificial explosion and lunar craters, and their genesis is discussed.

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