Report on Anatahan (United States) — 23 July-29 July 2008
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 23 July-29 July 2008
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2008. Report on Anatahan (United States). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 23 July-29 July 2008. Smithsonian Institution and US Geological Survey.
16.35°N, 145.67°E; summit elev. 790 m
All times are local (unless otherwise noted)
The USGS reported that seismic tremor from Anatahan slowly increased during 16-26 July. A diffuse sulfur-dioxide plume drifted W on 17 July. According to a Washington VAAC report a low-level plume possibly containing ash drifted 50 km NW on 26 July. The Volcanic Alert Level was raised to Advisory and the Aviation Color Code was raised to Yellow. According to another Washington VAAC report on 28 July, an ash plume rose to an altitude of 1.5 km (5,000 ft) a.s.l. and drifted N. Seismic tremor continued to increase. A sulfur dioxide plume drifted NW. The Volcanic Alert Level was raised to Watch and the Aviation Color Code was raised to Orange. During 29-30 July, seismic tremor levels remained elevated, but possibly slightly decreased.
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.