Discovery of woody pumices in greentuff and its geological significance

概要

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Analysis of giant (>1 m in diameter) pumices is useful for reconstructing submarine volcanism, as the types of giant pumice that can form during submarine eruptions depend on the style of the volcanic eruption, as well as the eruption environment. We here present the first discovery of giant pumices in the Green Tuff of the Ou Backbone Range in northeast Japan. Such pumices are characterized by elongated vesicles, which are a typical feature of woody pumice, and which indicate eruption on the sea floor at depths of >1,000 m. Thus, the woody pumices in the Green Tuff provide information about the environment of submarine volcanism at the time of eruption of the tuff. The woody pumices and Kuroko deposits in the study area are very similar to those found in modern back-arc rift zones, such as in the back-arc rift zones of the Okinawa and Izu-Ogasawara arcs. Therefore, we infer that the tectonics and volcanism of the Green Tuff in the study area resemble those of modern back-arc rift zones.