Report on Anatahan (United States) — 7 May-13 May 2003
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 7 May-13 May 2003
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2003. Report on Anatahan (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 7 May-13 May 2003. Smithsonian Institution and US Geological Survey.
16.35°N, 145.67°E; summit elev. 790 m
All times are local (unless otherwise noted)
On 10 May around 1700 an eruption began at Anatahan, a volcanic island which has had no historically documented eruptions. Scientists on a small ship about 10 km away saw an ash plume that eventually rose to ~12 km a.s.l. The eruption occurred primarily from Anatahan's eastern crater and observers did not see lava flows. No precursory activity was recorded (the island of Anatahan is uninhabited and lacks working seismometers) and no signs of the impending eruption were seen by scientists who visited the island on 6 May.
According to the Washington VAAC, an ash cloud was visible on satellite imagery beginning around 1730 on 10 May. The next day around 0655 ash was seen moving in three different directions; WNW at a height around 5.5 km a.s.l., SW around 8.5 km a.s.l., and two separate and smaller ash plumes were drifting SE at heights around 13.4 km a.s.l.
Local authorities issued a special advisory on 11 May stating that ". . . the general public especially fisherman, tour operators and commercial pilots are advised to stay away from the island of Anatahan until further notice from the Office of Emergency Management." As of 13 May ash emissions continued and a hot spot was visible on satellite imagery. Residents of the small island of Anatahan (part of the United States Commonwealth of the Northern Mariana Islands) were evacuated in 1990 after a shallow earthquake swarm. The most recent reported seismicity occurred in 1993.
Geologic Background. 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: The Margins Mariana Subduction Factory Imaging Project Research Team (from Washington University, St. Louis; Scripps Inst. of Oceanography; and CNMI Emergency Management Office), US Geological Survey Hawaiian Volcano Observatory (HVO), Washington Volcanic Ash Advisory Center (VAAC), Marianas Variety, Voice of America News