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Centennial-scale eruptive diversity at Volcán Calbuco (41.3°S; Northwest Patagonia) deduced from historic tephra cover-bed and dendrochronologic archives

Abstract:

Since the late-18th Century, eye-witness accounts have documented a wide-spectrum of eruptive activity sourced from Volcán Calbuco located in northwest Patagonia. Despite these observations there is very little known about the eruptive products themselves that can account for this eruptive diversity. In this study, we examine the tephrostratigraphic record post-dating the interval 1578–1702 cal. yr BP, with emphasis on historical eruptions (i.e. <130 years, including the 2015 eruption) at proximal to medial distances (<16 km from source) to characterize the composition, distribution, volume and style of these units. At least 11 discrete tephra units are recognized which are in accord with documented eruptive activity between ~1760 CE and 2015. Juvenile pyroclasts from these units span a narrow compositional range from basaltic-andesite to andesite (55–60 wt% SiO2), and contain plagioclase (71–73%), pyroxene (~21%), cristobalite (3–5%) and scarce olivine and Ti-magnetite (1–2%). The largest documented historic eruption occurred in 1893–95, and produced a thick mantle of coarse-grained tephra fallout (0.32–0.50 km3 non-DRE) accompanied by intense ballistic bomb barrage closer to source. The 1893–95 eruption is comparable to the 1961 and 2015 eruptions both in terms of magnitude and explosivity, despite pyroclastic density currents (PDCs) not being documented in 1893–95. Both the 1929 and 1961 eruptions generated lava-flows, tephra fallout and PDCs, from which the 1961 volume totalled ~0.17 km3 non-DRE, affecting valleys northeast, up to a distance of 6 km from the crater. In contrast, the 2015 eruption only produced PDCs, tephra fallout and ballistics (0.26–0.36 km3 non-DRE). Results obtained from dendrochronological analysis of Nothofagus dombeyi trees within the study area reveals growth suppression indicated by structural damage during the 1893, 1929 and 1961 eruptions, probably related to thicker overall accumulations of tephra. Collectively, the componentry, architecture and volume of historic (AD 1893, 1961 and 2015) Calbuco tephra indicate sub-Plinian parental events derived from more mafic products with disequilibrium textures, than smaller eruptions characterized by c. 60% wt. SiO2 probably triggered by crystal fractionation, providing a centennial time scale eruptive heterogeneity. This data is meaningful in terms of better understanding eruptive diversity at basaltic-andesite centers elsewhere that have high eruption frequencies (e.g. centennial time-scales).