Abstract

Inundation of coastal areas by tsunamis during the 1883 eruption of Krakatau volcano led to the deposition of unusual pumice- enriched deposits. Fractal analysis of pumice shapes and lithologic characterization of the deposits suggest that the source of the abundant pumiceous material was widespread pumice rafts on the surface of the Sunda Straits that formed by fallout and pyroclastic flow activity. The rafts contained pumices rounded by particle-to- particle abrasion and were strongly depleted in dense components, such as lithics and crystals, by differential settling. Stranding of the floating pumice is inferred to have occurred during the receding phase of tsunamis after they had inundated low-lying coastal areas. Other pumice-bearing tsunami deposits contain significant amounts of coral fragments and nonvolcanic beach sediment. These units represent redeposition of beach and shallow-water sediments that were mixed with varying proportions of primary pyroclastic material. The Krakatau example illustrates the great diversity of lithofacies that may occur in deposits formed from volcanogenic tsunamis. Recognition of such deposits in coastal areas near centers of active explosive volcanism may provide an additional criterion with which to assess volcanic hazards.

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