Volcanic and seismic activity increased at Anatahan during 14-21 March in comparison to previous weeks. During 14-17 March, seismicity increased and steam rose a few hundred meters above the volcano. The inner East crater had been nearly filled with lava flows and lapilli since early January. According to seismic data, a small eruption began on 18 March at 1544. On 19 March the Washington VAAC issued an advisory that an ash plume was visible on satellite imagery below 4 km (13,100 ft) a.s.l. Small explosions that began late on 20 March lasted for 14 hours. No emissions were visible on satellite imagery.

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)