An initial dating of eruptive activity at Agua Volcano, Guatemala

Abstract Volcán de Agua, also known locally as Hunahpú, is a largely unstudied volcano in Guatemala, situated less than 10 km south of the city of Antigua Guatemala, between the active volcanoes of Pacaya and Fuego. The population centres of Ciudad Vieja, Santa María de Jesús, Amatitlán and Palín also lie within 15 km of Agua’s summit, with a total population of approximately 550,000. There is little published work on either its eruptive behaviours or bulk chemistry, with no documented historical eruptions. Dating of its past activity is unknown, and the style of activity remains poorly constrained. Other volcanoes along the Guatemalan cordillera have shown a range of activities in the last two centuries, ranging from effusive to Strombolian at Pacaya in the south to the VEI 2–6 explosive eruptions at Santa Maria to the north west. As part of this study, we analysed samples from surface-exposed deposits that provide evidence for at least one phase of basaltic activity that generated pyroclastic density current (PDC) deposits. These deposits are poorly sorted and contain very angular to sub-rounded, weakly vesicular clasts. The deposit’s geometry suggests an overbank flow, indicating that it escaped an established drainage channel as the current changed direction. Charcoal fragments identified within the deposit were subjected to reflectance analysis, which indicates a depositional temperature of approximately ~300 °C. Radiocarbon dating of this charcoal yielded an age of 15,454 ± 41 BP. This represents the first published date of eruptive activity at Agua. Given the location of the dated deposit and lack of capping deposits, it may be possible that Agua has been inactive throughout the Holocene.