Activity surged to a moderately high level on 5 May, when an extensive ash-and-steam plume to 4.5 km a.s.l. (15,000 feet) was visible in all directions. Ash extended 768 km N, 130 km S to northern Saipan, and 111 km W. VOG extended in a broad swath from 3,000 km W, over the Philippine Islands, to 1,000 km N of Anatahan. By 9 May harmonic tremor amplitude had decreased to near background levels, with a corresponding drop in eruptive activity. As of 10 May the Air Force Weather Agency was reporting ash to about 3 km a.s.l. (10,000 feet) extending 400 km W and an area of VOG less than half that noted on 5 May.

Geological Summary. 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.

Source: Emergency Management Office of the Commonwealth of the Mariana Islands and United States Geological Survey Volcano Hazards Program