Gray Pumices Drifted from Fukutoku-oka-no-ba to the Ryukyu Islands

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  • KATO Yuzo
    Department of Marine Sciences, University of the Ryukyus

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  • 福徳岡の場から琉球列島に漂着した灰色軽石
  • フクトク オカ ノ バ カラ リュウキュウ レットウ ニ ヒョウチャクシタ ハ

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Since the late of May, 1986, many pumices have drifted to the Ryukyu Islands. These pumices are characterized by their size, color and components. The pumices vary in size: some are about 5 mm and the maximum size is about 20 cm. Most of these pumices are gray and a few of them are drak- or light-gray. The pumices contain several percent of essential black xenolithes which vary in size from 1 mm to 5 mm and rarely 2 cm. The rock name of pumices is olivine-augite-bearing trachyte and that of xenolithes is forsterite chromian diopside trachybasalt. These characteristics of the pumices correspond with those of pumices erupted from Fukutoku-oka-no-ba in the north of the Mariana Islands in January 18-21, 1986. Moreover, the chemical composition of host pumice (free from xenolith), bulk composition (including xenolith) and mineral chemistry of phenocryst also correspond with those of the pumices effused from Fukutoku-oka-no-ba. Based on these facts, it is evident that the pumices drifted to the Ryukyu Islands had their origins in Fukutoku-oka-no-ba. Judging from the data concerning the current and wind around the area through Fukutoku-oka-no-ba toward the Ryukyu Islands, it is confirmed that the pumices reached the Ryukyu Islands from Fukutoku-oka-no-ba after four months under the effect of current and wind which run both to the direction of west.

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