HR: 14:30h
AN: OS43A-03    [Abstracts]
TI: Onshore Boulder Deposits Near Krakatau Volcano, Indonesia
AU: * Setja Atmadja, C M
EM: Claudia.Maxcia@ait.ac.th
AF: Asian Institute of Technology, 58 Moo 9 Km.42 Paholyothin Highway, Klong Luang, Pathumthani, 12120, Thailand
AB: Onshore coral blocks were found at two sites in 60 km radial distance from Krakatau volcano which erupted and triggered tsunamis in Sunda Strait in the years 416 and 1883. The greatest of the 1883 eruptions generated a tsunami that ran up as high as 36m and reportedly moved coral boulders larger than 300 m³. At one of these sites, Gubug Garam on the east shore of Lampung Bay, Sumatra (S 5° 36' E 105° 24'), more than 40 boulders rest on a 70m wide horizontal coral platform in the inter tidal region. Their volumes range from 1 m3 to > 4m³. Their long axes increasingly align with the trend of the coastline the farther the blocks were transported. Inland, on ground underlain by beach sand and volcano-clastic deposits, more than 20 coral blocks up to 3 m³ as much as 500 m from the shore were found. Most of these boulders are embedded in the sediments. Digging beneath six of the boulders, it was found that they rest on top of pumice and tuff, which in turn overlies former topsoil. Above the pumice and tuff, the material surrounding the blocks is the present topsoil which consists of clayey silt or silty clay with some shell and branching coral fragment. The other site, near Anyer, Java (S 6° 4' E 105° 53'), has a similar topographic setting. The reef platform, 75 m wide, is littered with more than 20 blocks as large as 3 m³. Inland, as much as 150 m from the coastline, 5 very large coral blocks, the largest measuring 300 m³ were found. As at Gubug Garam, these blocks rest on pumice and tuff that are underlain by former topsoil. We thus infer that the blocks at both locations where displaced during the same event.
DE: 1862 Sediment transport (4558)
DE: 4564 Tsunamis and storm surges
SC: Ocean Sciences [OS]
MN: 2007 Joint Assembly