Emissions of ash and gas at Anatahan, which began on 10 May, continued during 28 May to 3 June. During most of the report week, eruption clouds rose to heights around 3 km a.s.l. Based on a report from an aircraft, the Washington VAAC stated that ash from a fairly vigorous eruption on 31 May around 1025 rose to ~7.6 km a.s.l. By 2202 there were reports of ash to ~ 6 km a.s.l. The TOMS Volcanic Emissions Group reported detecting significant SO2 emissions from Anatahan during 10-30 May, although data gaps prevented measurements on some days.

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: Washington Volcanic Ash Advisory Center (VAAC), OMI Sulfur Dioxide Group