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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Comunicações orais ► em encontros internacionais


Referência Bibliográfica

PORRECA, M., KUEPPERS, U., PIMENTEL, A., HESS, K.-U., PACHECO, J.M. (2011) – Stratigraphy of the 16 ka caldera-forming eruption of Sete Cidades volcano (Azores) and emplacement temperatures of pyroclastic density currents by paleomagnetic analyses. IUGG XXV General Assembly 2011, Melbourne, Austrália, 28 Junho - 7 Julho (Comunicação Oral)


Sete Cidades caldera (Azores) was formed by volcanic eruptions occurred in the last 36 ka. In this work, we interpret the stratigraphy of the last major collapse event (16 ka BP) and estimate the emplacement temperature of the related pyroclastic density currents (PDC) deposits.

The investigated deposits are dominated by massive lapilli tuffs emplaced by PDCs and locally thick but volumetrically subordinate pumice fall deposits. The deposits are entirely unwelded and often show a basal, well stratified surge deposit. At final stages, the activity was mostly magmatic with emplacement of coarse pumice- and lithic-rich fallout deposits.
Thermal remanent magnetization (TRM) analysis was performed on lithic clasts collected from the PDC deposits (134 clasts from 16 sites) in order to estimate their emplacement temperatures. The estimated emplacement temperatures are higher than the Curie temperature of the magnetic minerals present in the clasts, i.e. more than 500-580ºC. Only few clasts provide a minor emplacement temperature, around 400-460°C.
The estimated temperatures have been compared with those of simultaneous thermal analysis (STA) performed on obsidian clasts collected from the same deposits. The results of STA analysis show that the obsidian chips are fresh and unaltered glass and do not become foams before of 800°C.
The overall results indicate that the PDCs were emplaced at elevated temperatures between 500° and 750°C. Our innovative and multidisciplinary approach has shown consistent results allowing to better understand the emplacement processes of unwelded deposits.