Seismicity at Anatahan increased abruptly on 24 April at 1052 to a level that had not been reached since the summer of 2003. Around 0600 on 26 April, seismicity leveled off. Later that day, during two 30-minute observation periods, scientists saw regular puffs of yellow/brown steam and ash emitted at 1- to 2-minute intervals. The rate of emissions virtually matched the rate of seismic events during that time. During 24-26 April, steam-and-ash plumes reached a maximum height of about 600 m above the volcano. According to the Washington VAAC, on 24 April a thin plume from Anatahan was visible on satellite imagery ~1 km above the volcano and extending NW from the volcano. Aircraft were warned to proceed with caution near the volcano.

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: Agence France-Presse (AFP), Washington Volcanic Ash Advisory Center (VAAC), Emergency Management Office of the Commonwealth of the Mariana Islands and United States Geological Survey Volcano Hazards Program