Since the papers by WILLIAMS (1941, 1942), the definition of caldera has changed little. Most important discovery has been the recognition of the Valles-type calderas and resurgent cauldrons closely related to them (SMITH and BAILEY, 1962, 1968). Most important varieties of the collapse calderas are ; 1) Kilauea-type, 2) Crater Lake-type or Krakatoa-type, and 3) Valles-type calderas. Very small varieties of the Crater Lake-type calderas, transitional to craters, are frequently met with throughout the Japanese volcanic field (tentatively named the Haruna-type). A shallow, inverted cone-shaped structure has been strongly proposed by YOKOYAMA based on his recent geophysical investigations of Japanese Crater Lake-type calderas. Similar funnel-shaped structure is compatible with the recent geological findings : 1) Occurrence of the basement rocks at shallow depths within caldera floor, 2) very high rate (one caldera per 3, 000 years) of caldera formation in the Japanese islands in late Quaternary, 3) findings of funnel-shaped volcanic structures related to large-scale pyroclastic eruptions.

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This paper is only concerned with the calderas of low gravity anomaly type (or Krakatoan type). As an example of the subsurface structure of the calderas of this type, that of Hakone Caldera is discussed on the bases of gravity anomalies and the results of the drillings there. It is proved that there is the Tertiary basement at rather shallow depth under the deposits of ejecta from the post-caldera volcanoes at some parts of the caldera. Therefore, the caldera would not have collapsed as a whole into “magma reservoir” if any. As general problems, mass deficiency on the Japanese calderas are compared with those on the Canadian meteorite craters. Caldera formation consists of the two actions, i.e. explosion and outflow of the ejecta : The former is the main factor of meteorite craters while the latter is a characteristic of volcanic calderas and becomes dominant with the increase of their diameters. A hypothesis saying that enlargement of caldera openings would occur between the stages of pumice fall and pumice flow, is proposed. This may explain the pressure drop of pumice flow eruptions and also a large amount of pumice flow compared with that of pumice fall.

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In the active volcano field in Japan, the investigations are not many on the underground structure of volcanoes by seismic prospecting. In March, 1968 and 1969, the explosions (explosives ; about 500kg) by the Geological Survey of Japan were executed at Chigasaki, the northern extremity of Izu-Oshima. We observed the seismic waves by these explosions at the permanent and temporary observation points, and obtained the underground structure of Oshima using travel time. In case of the structure as a parallel layer over a half space, the velocities of P and S wave were 2.2km/sec and 1.1km/sec in the upper layer, and 4.6km/sec and 1.9km/sec in the lower part. The thickness of the layer was estimated to be about 800m. Presuming as two parallel layers over a half space, the velocities of P waves were 2.0km/sec in the first layer, 3.1km/sec in the second layer and 4.6km/sec in the lowest part. In this case, the thicknesses of the first and the second layer were about 400m and 600m, respectively. We obtained the arrival direction of P wave at each point using the orbital analysis. These directions are systematically deviated from the direction of wave source (shot point) as Fig. 10, and such a result seemd to be caused by the heterogeneous effect of the volcanic structure.

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There exist more than ten Quaternary calderas of various scales in Hokkaido. Most of them were formed by collapse after eruption of abundant pyroclastic materials, especially in the fashion of pyroclastic flow. Accordingly they belong mostly to Krakatau type of Williams (1941), though only Tokachi depression is of a major volcano-tectonic type. These calderas are classified into the following four groups from their scale (diameter) and reviewed particularly. Group Diameter (km) Name of Caldera I ＞2 Naka-machineshiri of Me-akan Tarumai Kamui-nupuri of Mashu II 2～10 Ohachi-daira of Daisetsu Kuttara Nigorikawa Atosa-nupuri Mashu III ＞10 Toya Shikotsu Akan Kutcharo IV Major volcano-tectonic depression Tokachi Most of the calderas belonging to groups III and II were formed at the end of the Pleistocene, though the largest depression of the last listed type occurred in the Pliopleistocene age and the small calderas (group I) collapsed after the formation of the central or peripheral cones in and around the larger calderas. It is the most remarkable that nearly all the calderas of Krakatau type (groups III and II) not only in Hokkaido but also in the whole Japan, sank towards the end of the Pleistocene. The catastrophic activity at the caldera formation stage begins generally in the form of air fall of pyroclastics and ends at the culmination of pyroclastic flows. But the frequent repetitions of such activity resulted at last in the formation of large caldera such as Kutcharo and volcano-tectonic type one as seen around Tokachi. It is clearly explained that the chemical compositions of the explosion products change often towards a regular trend within one cycle or through several cycles of eruption. The most interesting fact that the larger calderas are usually related with the more abundant volume of ejecta of the more felsic composition as shown in the following table, is ascertained and discussed genetically. Group I II III IV Volume of ejecta (km³) 0.2～2 3～6 20～100 300 Composition of ejecta Andesite Andesite (partly dacite) Dacite Rhyolite Genetically andesitic and dacitic magmas related with the ejecta from the calderas of groups I～III are considered to have been derived from the basaltic magma, while a tremendous amount of rhyolitic magma whose ejection resulted in volcano-tectonic depression is suggestive of the other origin different from the above.

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