On 11 May the Air Force Weather Agency (AFWA) reported thick ash rising to 4.2 km altitude (14,000 feet) and moving WNW. The thick ash extended in a triangular shape from the summit 444 km (240 nm) to the WSW through 510 km (275 nm) to the NW. A layer of thin ash at 3 km altitude (10,000 feet) extended beyond the thick ash another 1,000 km (550 nm). A broad swath of VOG extended over 2,200 km (1,225 nm) W nearly to the Philippines and over 1400 km (775 nm) NNW of Anatahan. Although the ash plume diminished over the next few days and was not as thick, it remained significant, rising to 2.4 km (8,000 feet) and extending 370 km (200 nm) WNW on the 13th. Scientific personnel from EMO and the USGS repairing and installing equipment the next day reported hearing a continuous roaring sound from 2-3 km W of the active vent. They also saw ash and steam rising by pure convection, not explosively, to 3 km altitude (10,000 feet).

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