Report on Anatahan (United States) — 21 May-27 May 2003
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 21 May-27 May 2003
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2003. Report on Anatahan (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 21 May-27 May 2003. Smithsonian Institution and US Geological Survey.
16.35°N, 145.67°E; summit elev. 790 m
All times are local (unless otherwise noted)
Volcanic activity that began at Anatahan on 10 May continued during 21-27 May. On 21 May an ash plume was visible on satellite imagery at a height of about 4.9 km a.s.l. drifting W. Around 23 May, typhoon Chan-hom caused a shift in the prevailing wind direction near Anatahan and ash was transported S over the islands of Saipan, Tinian, Rota, and Guam. Due to the ash, airlines cancelled several flights to and from Saipan and Guam international airports. In Saipan and Guam an ash advisory, and a volcanic haze advisory, respectively, were issued on the 23rd stating that residents should be cautious of lower air quality due to the plume of ash, SO2, and other gases over the islands that morning. By 24 May the plume had drifted away from the islands. According to the Washington VAAC, satellite imagery on 24 May at 1413 showed the ash plume at a height around 5.2 km a.s.l. caught up in the cyclonic circulation of the typhoon moving S, then SE. By 27 May the ash cloud was around 3 km a.s.l. and drifting E.
Geologic Background. The elongate, 9-km-long island of Anatahan in the central Mariana Islands consists of a large stratovolcano with a 2.3 x 5 km compound summit caldera. The larger western portion of the caldera is 2.3 x 3 km wide, and its western rim forms the island's high point. Ponded lava flows overlain by pyroclastic deposits fill the floor of the western caldera, whose SW side is cut by a fresh-looking smaller crater. The 2-km-wide eastern portion of the caldera contained a steep-walled inner crater whose floor prior to the 2003 eruption was only 68 m above sea level. A submarine cone, named NE Anatahan, rises to within 460 m of the sea surface on the NE flank, and numerous other submarine vents are found on the NE-to-SE flanks. Sparseness of vegetation on the most recent lava flows had indicated that they were of Holocene age, but the first historical eruption did not occur until May 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera.