On 6 April around 0300 an explosive eruption began at Anatahan and produced an ash plume to an initial height of ~15.2 km (~50,000 ft) a.s.l. On 5 April at about 2200 seismic signals began to increase slowly and the Washington VAAC began to see increased ash on satellite imagery. The seismicity peaked on 6 April around 0300. The U.S. Air Force Weather Agency reported an upper level ash plume at ~15.2 km (~50,000 ft) a.s.l. blowing E to SE, and a lower level ash plume at ~4.6 km (~15,000 ft) a.s.l. blowing SW; the plume extended more than 465 km. Earth Probe TOMS data on 6 April at 1046 showed a compact sulfur-dioxide cloud drifting E of Anatahan following the eruption.

On 6 April during 0400 to 0900 the seismicity at Anatahan decreased to near background. The seismicity surged for about 1 hour, with amplitudes about one half those reached during the earlier eruption, and subsequently dropped again to near background.

Prior to the 6 April eruption, during 31 March to 4 April the amplitudes of harmonic tremor varied, reaching a 2-month high on the 3rd. Small explosions occurred every one to several minutes, probably associated with cinder-cone formation. Steam-and-ash plumes drifted ~200 km, and vog (fog composed of volcanic gases) drifted ~400 km at altitudes below ~2.4- 4.6 km (~8,000-15,000 ft).

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, Joint Center for Earth Systems Technology (JCET), University of Maryland, Washington Volcanic Ash Advisory Center (VAAC)