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São Miguel is the largest and most populated (>137,000 inhabitants) island of the Azores archipelago and comprises three trachytic central volcanoes that have been active during the Holocene: Sete Cidades, Fogo and Furnas. This is true for older deposits with incomplete preservation (e.g., Heiken and McCoy, 1984 Costantini et al., 2009 Longchamp et al., 2011) but also holds for recent and well-monitored eruptions (e.g., Spanu et al., 2016). Several studies have discussed the sensitivity of determining eruptive source parameters from deposits.
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The reconstruction of past eruptions on volcanic ocean islands (including the determination of eruptive source parameters) and consequently the assessment of their impact is greatly conditioned by various factors, including, but not limited to: (1) number of outcrops (small subaerial area of deposition), (2) outcrop accessibility (e.g., vegetation cover) and (3) preservation of primary features of deposits (degree of erosion and weathering). This threat is even greater for volcanic islands as the ocean is a strong impediment for the evacuation of large numbers of people. In case of a future explosive eruption, under westerly blowing wind conditions, the impact on São Miguel could be catastrophic, with long-term economic consequences.Įxplosive volcanic eruptions are recognized as one of the most violent natural events that pose a plethora of hazards to local communities, economic sectors, and infrastructure, but may also cause widespread disruption of air traffic (e.g., Mazzocchi et al., 2010 Wilson et al., 2012 Biass et al., 2014 Scaini et al., 2014). Most critically, the main harbor, only airport and hospital are located in the capital city of Ponta Delgada, ∼12 km SE from Sete Cidades caldera. Given the predominant winds blowing from westerly directions, Sete Cidades is considered the most hazardous volcano for the entire island of São Miguel with its current population of >137,000 inhabitants. This is the first evidence of a decimeter-thick deposit of Sete Cidades volcano in the central part of São Miguel Island, which allows to constrain eruptive source parameters and wind conditions. We here present evidence of a mid-distal deposit (>25 km from the vent) that can be correlated with the proximal (on Sete Cidades volcanic edifice) pumice fall deposit of the Santa Bárbara Formation based on deposit characteristics, textural features and geochemistry. The last paroxysmal explosive eruption took place at ∼16 ka, enlarging the caldera to its present dimensions, and is recorded by the Santa Bárbara Formation. Although currently dormant, it has been the most active volcano on the island in the last 5 ky, with at least 17 trachytic explosive eruptions, some of which sub-Plinian, that took place inside the summit caldera.
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Sete Cidades is the westernmost central volcano of São Miguel Island, Azores. However, the products of explosive eruptions on volcanic ocean islands are almost inevitably incompletely preserved as a significant portion of the erupted material is deposited into the ocean, thereby impeding our ability to accurately reconstruct past events, determine eruptive source parameters and ultimately assess the associated hazards. It is well known that explosive volcanic eruptions pose serious hazards to local communities and may have worldwide impacts. 5Department of Physics and Geology, University of Perugia, Perugia, Italy.4Institut für Geochemie und Petrologie, ETH Zürich, Zurich, Switzerland.3Instituto de Investigação em Vulcanologia e Avaliação de Riscos (IVAR), Universidade dos Açores, Ponta Delgada, Portugal.2Centro de Informação e Vigilância Sismovulcânica dos Açores, Ponta Delgada, Portugal.1Department für Geo- und Umweltwissenschaften, Ludwig-Maximilians-Universität, Munich, Germany.Ulrich Kueppers 1*, Adriano Pimentel 2,3, Ben Ellis 4, Francesca Forni 4, Julia Neukampf 4, José Pacheco 3, Diego Perugini 5 and Gabriela Queiroz 3