The third eruption in 2005 at Anatahan apparently began on 21 March when seismicity increased. Seismic amplitudes peaked on the 25th and faded out on the 26th. Near the peak on the 25th, the Air Force Weather Agency detected a hot spot on the island on satellite imagery, and reported an ash plume briefly reaching ~5.8 km (~19,000 ft) a.s.l. The plume height soon declined to below 3 km (10,000 ft) a.s.l., and by near midday on the 27th the plume had transitioned from ash and steam, to steam and vog (fog composed of volcanic gases). On the 27th the plume extended ~240 km (~130 nautical mi) SW.

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.

Sources: Emergency Management Office of the Commonwealth of the Mariana Islands and United States Geological Survey Volcano Hazards Program, Washington Volcanic Ash Advisory Center (VAAC)