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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica

KUEPPERS, U., QUEIROZ, G., PACHECO, J. (2007) Eruptive and transportation processes during caldera-forming eruptions of Sete Cidades volcano, São Miguel, Azores. AGU Joint Assembly, Acapulco, México, 20 - 25 de Maio (Poster).​


Sete Cidades volcano forms the Western part of the island of São Miguel, Azores, which is hosting three active trachytic central volcanoes (Sete Cidades, Fogo, and Furnas). Volcanic activity in the archipelago exhibits a strong tectonic control and on São Miguel, the NW-SE
trending basaltic Terceira Rift is intersecting the central volcanoes. All three have erupted since the settlement of the island in the 15th century. The very Eastern part of the island is considered extinct.


The oldest dated subaerial rocks of Sete Cidades volcano exhibit an age of 210 ka. Morphology of the present summit caldera (5 km diameter, up to 350 m deep), stratigraphy, and distribution of the deposits suggest a multiple-stage evolution and at least three caldera-forming eruptions (CFE) are assumed to have occurred. 14C-dating revealed ages of 36, 29, and 16 ka, respectively, for the most recent ones. Today, the average slope angle is 12° and the maximum distance of the coastline from the caldera rim approx. 5 km. Assuming a comparable situation at the time of the CFE, a large portion of the eruptive products has probably not been deposited on land. After a pause of several thousand years, eruptive activity resumed approx. 5
ka ago and started filling the caldera. As deposits of minor thickness and distribution can be found between the deposits of the CFE, it is unclear whether the caldera formation is completely finished.


Climatic factors (e.g. precipitation, air humidity) have affected the deposits by erosion, weathering, and possibly significant reworking and caused dense vegetation on all flanks of the volcano. Still, it was possible to establish distribution and thickness of the deposits of the CFE and constrain differences in eruptive behaviour and transport/emplacement mechanisms. They are composed of air-fall deposits and pyroclastic density currents but show significant differences amongst them: (1) Degree of pre- and syn-eruptive magma-magma interaction and syn-eruptive magma-water interaction. (2) Ratio of juvenile/lithic content and basaltic/trachytic magma. (3) Degree of vesiculation and crystal content of the juvenile material. (4) Percentage of air-fall deposits within the deposits of a single CFE and the timing of their deposition. (5) Distribution of air-fall deposits. (6) Degree of welding. The results highlight the bandwidth of possible eruptive scenarios at this trachytic central volcano cut by an active rift. Based on the study of these eruptions, volcanic hazard maps can be produced that are essential for adequate risk assessment.