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em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica

KUEPPERS, U., PACHECO, J., QUEIROZ, G. (2008) - The last caldera-forming eruption of Sete Cidades volcano, São Miguel, Azores. IAVCEI 2008 General Assembly, Reykjavik, 17 - 25 de  Agosto (Comunicação Oral).​


​The Azores archipelago comprises 9 islands near the triple junction of the African, American, and Eurasian Plates. Sete Cidades volcano forms the Western part of São Miguel island, hosting three trachytic volcanoes (Sete Cidades, Fogo, Furnas). Volcanic activity in the archipelago exhibits a strong tectonic control. The position of Sete Cidades Volcano is constrained by several intersecting tectonic elements leading to trachytic volcanism from the central volcano as well as basaltic and trachytic flank eruptions.


Morphology of the present summit caldera, stratigraphy, and distribution of the deposits suggest a multiplestage evolution and three caldera-forming eruptions (CFE) have been suggested. 14C-dating revealed ages of approx. 36, 29, and 16 ka, respectively (Queiroz, 1997). After a pause, eruptive activity resumed approx. 5 ka ago and deposits of at least 17 eruptive episodes can be found (Queiroz et al. 2008).


The deposits of the last CFE are dominated by massive lapilli tuffs from quasi-steady but variably energetic pyroclastic density currents (PDC) and locally thick but volumetrically subordinate pumice fall deposits.


The PDC deposits are unwelded and often show a fine basal layer with a high fragmentation index and accretionary lapilli suggesting a phreatomagmatic start. At later stages, the activity was mostly magmatic and the deposits frequently contain charcoal wood fragments. Pumice clasts exhibit a pronounced rounding and are usually smaller than 5 cm. Lithic clasts of up to 50 cm are generally angular, only at high lithic fractions, abrasion may have acted effectively. The lithic breccia associated with the edifice collapse is only rarely cropping out. The deposition of the final fall deposits has been greatly affected by winds from W and SW and can only be found on the N and NE sector. Juvenile pumices generally contain a large number of magmatic mafic enclaves suggesting that the intrusion of basaltic melt into the trachytic reservoir caused the eruption. The variety of erupted lithics is large and includes obsidian and ash layers with drying cracks. This study suggests that the eruption took place inside the already existing caldera with shallow crater lakes and domes and substantially increased it.