Report on Anatahan (United States) — 30 June-6 July 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 30 June-6 July 2004
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2004. Report on Anatahan (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 30 June-6 July 2004. Smithsonian Institution and US Geological Survey.
16.35°N, 145.67°E; summit elev. 790 m
All times are local (unless otherwise noted)
On 2 July, the seismicity level at Anatahan rose nearly as high as it was in late April 2004. After the 2nd, seismicity declined slightly. The nature of the seismic signals suggested that Strombolian explosions occurred at intervals of tens of seconds to minutes. The Washington VAAC reported that during clear weather on 3 July, a ~30-km-long ash plume was visible below ~3 km a.s.l.
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.