EFFECTS OF THE TSUNAMI OF 26 DECEMBER 2004 ON RASDHOO AND NORTHERN ARJ ATOLLS, MALDIVES BY EBERHARD GISCHLER 1 and REINHARD KIKINGER 2 INTRODUCTION We report our observations on Radhoo Atoll, located in the Maldives, during the tsunami of 26 December 2004. Observations were made on a marginal reef island and from a small boat in the lagoon of the atoll. Post-tsunami changes on some islands and reefs of Rasdhoo and nearby Ari Atoll are described. SETTING The Maldive archipelago is about 1,000 km long and up to 150 km wide encompassing an area of 107,500 km2 (Fig. 1). Some 0.3% of this area is formed by 1,200 islands, only 10 of which are larger than 2 km2. The maximum land elevation is 5 m above present sea-level. Geomorphologically, the Maldives form a N-S-trending double row of 22 atolls, separated by the Inner Sea up to 450 m deep. The Maldives are bounded bathymetrically by the 2,000 m contour, i.e., the archipelago rises steeply from the surrounding Indian Ocean seabed. The geological development of the Maldives since the early Tertiary was recently summarized by Purdy and Bertram (1993) and Belopolsky and Droxler (2003). Whereas the knowledge of the Tertiary development is well documented based on ODP drill sites and exploration wells and seismics, the knowledge on the Quaternary evolution of the Maldives is quite limited (e.g., Woodroffe, 1992; Kench et al., 2005). The climate is monsoon-dominated. During the wet monsoon from April to November winds blow to the NE, during the dry monsoon from December to March winds blow to the SW. Annually, most strongest and frequent winds blow towards the E (Fig. 1). Due to their proximity to the equator, the Maldives are largely storm-free. Water temperatures fluctuated annually between 28-30 °C during the past several years (COADS, grid 3-5°N, 72-74°E). Annual precipitation rates ranged from 1,000-2,000 mm during the 20th century (GHCN, Minicoy, Laccadives). The tidal range in the Maldives is 0.5-1 m. Rasdhoo Atoll is located in the western row of Maldivian atolls. It is a comparably small atoll with a maximum diameter of 9.25 km and a size of 62 knr (Fig. 2). The marginal reef is near-continuous and surface breaking. There are 5 sand and 'Johann Wolfgang Goethe-Universitat, Dept. Geoscienccs Geography, Frankfurt am Mam. dermam ; e- mail: gischler@em.uni-frankfurt.de 2 Kuramathi Bio Station, Rasdhoo Atoll, Maldives; and University of Vienna, Dept. Marine Biology, Aus- tria; e-mail: kikinger@utanet.at