北海道のカルデラについての 2・3 の問題

抄録

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.