During 22-27 June the Air Force Weather Agency (AFWA) observed in satellite imagery a moderately dense cloud of ash and steam that rose to a maximum elevation of ~3 km (9,800 ft) a.s.l., and drifted W. Additional thin ash and volcanic fog (VOG) were visible to the W and N-NW of the island. On 26 June the AFWA also identified in satellite imagery a dense cloud of ash and steam rising to ~3.7 km (12,000 ft) a.s.l. moving towards the W, and VOG to the W, N and NE of the island. No particular seismic signal was associated with the eruptions. By 28 June the seismicity level dropped by about 80% from the continuously high levels of the last week.

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)