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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica


GERTISSER, R., SELF, S., GASPAR, J.L., KELLEY, S.P., PIMENTEL, A., EIKENBERG, J., BARRY, T.L., PACHECO, J.M., QUEIROZ, G., VESPA, M. (2010) – Ignimbrite-forming eruptions on Terceira, Azores. Cities on Volcanoes 6th, Tenerife (Canarias), Espanha, 31 Maio - 4 Junho (Comunicação Oral).

Resumo


Ignimbrites of comenditic trachyte composition constitute a significant portion of the younger volcanic stratigraphy of the island of Terceira, Azores. The base of the most recent group of deposits, called the Upper Terceira Group, is marked by the Lajes-Angra Ignimbrite, an extensive ignimbrite sheet produced during the latest ignimbrite-forming episode some 20-23 k.y. ago. The Lower Terceira Group consists of ignimbrites and interstratified pyroclastic fall deposits and lava flows predating the Lajes-Angra Ignimbrite. Including the Lajes-Angra Ignimbrite, the exposed volcanic succession on Terceira includes at least seven formations containing ignimbrites plus two isolated exposures of other types of pyroclastic density current deposits. Most of these are from Pico Alto or, possibly, Guilherme Moniz, the middle pair of central volcanoes of the four that form Terceira. Radiocarbon ages complemented by 40Ar/39Ar age determinations on anorthoclase crystals from several ignimbrites suggest a narrow period of ignimbrite-forming volcanism dating from c. 86 ka to 20-23 ka, which followed a period of predominantly effusive basaltic to trachytic/rhyolitic activity. However, a recurrence of ignimbrite volcanism on the island cannot be completely discounted. The results from this study also indicate that the 40Ar/39Ar method can be successfully applied to date anorthoclase crystals in volcanic rocks younger than 100 ka from Terceira, although it does not provide the same precision as radiocarbon dating at the younger age range of these volcanic rocks.

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