Abstract
Prior to May 2008, it was thought that the last eruption of Chaiten Volcano occurred more than 5,000 years ago, a rather long quiescent period for a volcano in such an active arc segment. However, increasingly more Holocene eruptions are being identified. This article presents both geological and historical evidence for late Holocene eruptive activity in the 17th century (AD 1625-1658), which included an explosive rhyolitic eruption that produced pumice ash fallout east of the volcano and caused channel aggradation in the Chaiten River. The extents of tephra fall and channel aggradation were similar to those of May 2008. Fine ash, pumice and obsidian fragments in the pre-2008 deposits are unequivocally derived from Chaiten Volcano. This finding has important implications for hazards assessment in the area and suggests the eruptive frequency and magnitude should be more thoroughly studied.
Highlights
Chaitén Volcano is a small rhyolite volcano in southern Chile (Fig. 1)
The 2σ intervals of both groups yield an overall range of AD 1516-1658 if they are merged, but their most probable ranges overlap between AD 1625 and 1658 (Fig. 4), which is in good agreement with the age of the pyroclastic density current (PDC) deposit at the caldera rim and the historical accounts that we describe below
The chemical compositions of the pumice and obsidian in the pre-2008 deposits strongly suggest that Chaitén Volcano was the source
Summary
Prior to the onset of its May 2008 eruption, it was thought that Chaitén Volcano had been dormant for thousands of years (Lara, 2009; Carn et al, 2009). This apparent long quiescent period is suspicious in the Southern Andes where a high eruptive flux has been reported (e.g., up to 0.9 km3/ky in Puyehue-Cordón Caulle Volcanic Complex at 40oS; Singer et al, 2008) with at least 3-4 significant eruptions (VEI>3) each century (Siebert et al, 2010; Dzierma and Wehrmann, 2012). Historical eyewitness reports of unrest or eruption of Chaitén Volcano were absent, which is not surprising because human settlement in the Chaitén Bay area did not begin until 1921. Chaitén Volcano was ranked 40/95 in the national threat score (considering 95 geologically active volcanoes; Lara et al, 2006a; modified by Lara et al, 2011; methodology based on Ewert, 2007) and, not considered as a high-priority volcano for monitoring before 2008
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