Abstract

Drift pumice collected in July 1969 from a beach on the eastern side of Eua Island, Tonga, is virtually identical to pumice stranded on islands along the Great Barrier Reef and the western islands of Fiji in late 1964 and 1965. This pumice is characterized by a predominantly bimodal chemical composition, consisting of distinct lumps of light or dark pumice. The refractive index of the dark pumice glass averages about 1.530 and that of the light pumice, about 1.507. Refractive indices of the light and dark pumice varieties from the different sites of collection appear identical within the limits of accuracy of the measurements. Microprobe analyses of plagioclase, augite, hypersthene, and magnetite in pumice from Eua Island and One Tree Island on the Great Barrier Reef also support the essential identity of the pumice from these localities, and it is shown that the pumice is mineralogically and chemically very similar to dacites of Fonualei and Falcon Islands, Tonga. Field reconnaissance of volcanic islands in Tonga has so far failed to reveal a subaerial source for the pumice; eruption from a submarine volcano in the Tonga-Kermadec area seems likely. The volume of volcanic material erupted and transported across ocean basins as drift pumice may be considerable and may be an important source of the volcanic material found in pelagic sediments.

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