During 9-14 January, the eruption of Anatahan volcano stabilized, as explosion signals became larger and generally less frequent than previously observed, averaging a few explosions per minute. Early on 16 January, the eruption suddenly stopped for a couple of hours, then the level of instrumentally recorded activity surged to a new high 50 percent above the previous high. Later on 16 January, the eruption declined slowly for several hours before it stopped again, this time for about 8 hours before it returned to the level of 9-14 January. Early on 18 January, the activity level again surged to its second highest level so far. Then around 1000 activity declined to the level of 9-14 January.

With the current high level of eruptive activity, ash could be in the air out to a few tens of kilometers from Anatahan. The Emergency Management Office has placed Anatahan Island off limits until further notice and concludes that, although the volcano is not currently dangerous to most aircraft, conditions may change rapidly, and aircraft should pass upwind of Anatahan or farther than 100 km downwind from the island and otherwise exercise due caution within 50 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