On the Indonesian islands just north of Krakatau, some local strata contain suspiciously well-sorted, well-rounded pumice but are lacking the dense components commonly found in this type of material.
[At right: Anak Krakatau erupting in 1999. ]
Steven Carey and researchers from the University of Rhode Islandís School
of Oceanography and from the Volcano-
logical Survey of Indonesia have suggested that tsunamis, generated by the eruption of Krakatau in 1883, deposited rafts of pumice on the shores of these islands, which lie in the Sunda Strait between Sumatra and Java. Their article in the April issue of Geology outlines the important role tsunamis can play in depositing volcanogenic clasts, as well as in assessing volcanic hazards to coastal areas.
The inundation of coastal areas by tsunamis during the eruption of Krakatau deposited several relatively thin exotic sediment layers above the high-tide mark, says Carey. Understanding where these layers came from and how they were deposited on the islands is critical to helping predict how future eruptions may affect coastal areas.
Shallow submarine earthquakes, volcanic flank failure or the discharge of pyroclastic flow into the sea typically generate tsunamis. These giant waves wash volcanic fallout or pyroclastic flow material, which was floating on the sea surface, inland where it can later be traced back to the original eruption. On the islands north of Krakatau, a typical 1883 volcanic deposit is massive, poorly sorted sand with some pumice component. Carey and his co-workers found two anomalous layers, one on islands about 20 kilometers north of the volcano and another 35 kilometers north on the Sumatra coast. These layers suggest a different depositional method than the surrounding strata.
This first layer of pumice fragments looks very similar to deposits from a Plinian eruption ó an explosive eruption that releases a steady, turbulent stream of large volumes of fragmented magma and gas. But the pumice on these islands is better rounded than Plinian-fall pumice and is also missing the heavier lithic and crystal components typical of Plinian fragments. In order to identify how the pumice in this layer differs from other pumices of known origin, Carey used fractal analysis to compare the particle shapes with those in Plinian-fall deposits.
His team found that the particles were quantitatively more rounded than the clasts taken from fall or surge deposits, as if some post-eruption process had abraded them. Carey suspected that the pumice was not dropped on the island directly, but were dropped on the ocean and then rafted to the island. One option was that the pumice fragments abraded each other as the tsunami carried them up the coast of an island and deposited them there as it receded. The missing lithic and crystal components would have sunk into the water before deposition.
Carey and his co-workers also attribute the second unusual layer to tsunami deposition. This layer also contains rounded pumice, but is poorly sorted. The lithics and crystals missing from pumice in the other layers are present and mixed with broken coral pieces and carbonate sand. This deposit would also have been washed up in a tsunami, but here the water entrained local beach material, including fresh pyroclastic material from the eruption. In this case, the pumice did not drop its crystal and lithic cargo at sea.
Characterizing these deposits can help researchers locate and assess the origin of tsunami deposits. And by locating and linking these peculiar strata to tsunamis, researchers can begin to determine how far tsunamis of volcanic origin can travel and thus inundate far away shores.
Geotimes contributing writer