The USGS reported that levels of seismicity at Anatahan were elevated during 5-10 March, but decreased significantly on 11 March. On 5 March, diffuse ash plumes drifted as far as 120 km NW and 95 km SE. During 5-7 March, continuous ash emissions produced plumes that rose to altitudes less than 3 km (10,000 ft) a.s.l. and drifted NE, SW, and W. On 8 and 9 March, a sulfur dioxide plume was detected by the satellite-based Ozone Monitoring Instrument (OMI) and drifted SW and WSW. According to reports from the Washington VAAC, a diffuse ash plume rose to an altitude less than 3 km (10,000 ft) a.s.l. and drifted W on 10 March.

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