The third historical eruption of Anatahan began on 5 January, with explosions occasionally occurring through about 19 January. Near mid-day on 20 January seismicity at the volcano dropped abruptly to near background levels, indicating that explosions had ceased. Degassing may have continued. The apparent cessation of Strombolian activity lasted until late 22 January, when explosions suddenly resumed. The eruption peaked at about 0700 on 23 January. Pilots reported ash to heights of 3-4.5 km. After the peak in activity the explosions decreased somewhat, but were still frequent and strong as of 24 January.

The government of the Commonwealth of the Mariana Islands (CNMI), placed Anatahan Island off limits until further notice and concluded that, although the volcano is not currently dangerous to most aircraft within the CNMI airspace, conditions may change rapidly, and aircraft should pass upwind of Anatahan or beyond 30 km downwind from the island and exercise due caution within 30 km of Anatahan.

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