[Abstract]

The morphology of free crystals contained in fifteen pumice fall deposits from eight volcanoes in Japan was observed. Although most of the crystal faces are filmily covered with vesicular glass, broken surfaces are observed in many free crystals. The ratio of length of broken surface (b) to the total circumference of a crystal (a) "b/a value" in the 2-D picture was measured for plagioclase free crystals contained in the deposits. The b/a values vary from sample to sample and each sample has a wide distribution of the value on the frequency diagram. Previously, broken free crystals in ignimbrites have been reported in many cases, whereas enough has not been known for pumice fall deposits. From the results of this study, broken free crystals are common also in pumice fall deposits indicating that abundant broken free crystals are produced through the eruptive process of explosive eruptions like plinian and subplinian style.

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yasui@chs.nihon-u.ac.jp