The third historical eruption of Anatahan began on 5 January. Occasional, small long-period events were noted as early as 2 January, followed by harmonic tremor early on 4 January, which increased in size through midday 5 January. No large events or explosion signals were associated with the onset of the eruption, which probably occurred middle to late 5 January. Guam tower reported a low plume of thin ash and gas up to ~150 m above the volcano early on 6 January. The VAAC reported a plume at 1225 on 6 January that was 60 km long and 20 km wide blowing westward.

Beginning on 6 January, harmonic tremor gave way to frequent signals of Strombolian explosions. An overflight was accomplished on 7 January by Emergency Management Office personnel, who reported ash rising well above 1.5 km over the volcano. A dome was visible in the crater and bombs were observed rising less than 600 m above the crater.

After 6 January, the amplitude of the explosion signals increased slowly, roughly doubling by 1000 on 10 January, a time when explosions occurred every 3-10 seconds. Later on 10 January, the amplitude of the explosion signals plunged suddenly; by the morning of 11 January the amplitudes again underwent a similar two-fold rise and fall in amplitude.

This ongoing Strombolian eruption was very similar in nature and size to the previous eruption of April-July 2004. As a result, the Commonwealth's Emergency Management Office placed Anatahan Island off limits until further notice and concluded that, although the volcano was not currently dangerous to most aircraft passing nearby, conditions could change rapidly. They noted that aircraft should traverse either upwind of the island, or more than 100 km downwind of the island. Aircraft traveling within 50 km of Anatahan should exercise caution.

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: Washington Volcanic Ash Advisory Center (VAAC)