Report on Tanaga (United States) — 19 October-25 October 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 19 October-25 October 2005
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2005. Report on Tanaga (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 19 October-25 October 2005. Smithsonian Institution and US Geological Survey.
51.885°N, 178.146°W; summit elev. 1806 m
All times are local (unless otherwise noted)
Elevated seismic activity below young volcanic vents on Tanaga Island continued during 14-21 October, although the rate of small earthquakes reduced in comparison to peak values during early October. An unusual, several minute-long seismic signal on 17 October may have been a landslide or small phreatic explosion, but satellite images detected no airborne ash. The activity that began at Tanaga on 1 October was at the highest level recorded since the seismic network was installed in 2003, so the Concern Color Code remained at Yellow.
Geologic Background. Tanaga volcano, the second largest volcanic center of the central Aleutians, is the central and highest of three youthful stratovolcanoes oriented along a roughly E-W line at the NW tip of Tanaga Island. Ridges to the east and south represent the rim of an arcuate caldera formed by collapse of an ancestral edifice during the Pleistocene. Most Holocene eruptions originated from Tanaga volcano itself, which consists of two large cones, the western of which is the highest, constructed within a caldera whose 400-m-high rim is prominent to the SE. At the westernmost end of the complex is conical Sajaka, a double cone that may be the youngest of the three volcanoes. Sajaka One volcano collapsed during the late Holocene, producing a debris avalanche that swept into the sea, after which the Sajaka Two cone was constructed within the collapse scarp.