Welded tuffs and related pyroclastic deposits in northeastern Japan

Welded tuffs and related pyroclastic deposits are distributed at many localities in northeastern Japan, especially around the volcanoes of the Nasu volcanic zone running from north to south, but they are absent from the region along the Japan Sea. Their geological age varies from the Miocene to the Holocene, those of the Pleistocene being predominant in amount. Petrographically they cover rather a wide range from andesite to rhyolite, among which dacite is most common. The welded tuffs are always compact and hard, with well-developed columnar jointing, carrying parallel-layered obsidian lenticules; and various stages are observed from loose pyroclastic deposits to lava-like welded tuffs. Petrological, petrochemical, and physical properties of these deposits are studied in some detail. From these data some genetic consideration is given for the mechanism of welding, and also for the relation between the nature of parental magma and the formation of such pyroclastic deposits.