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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Artigos em livros de actas ► Internacionais


Referência Bibliográfica

SELF, S., GERTISSER, R., PIMENTEL, A., KELLEY, S.P. (2005) - Calderas and ignimbrites on Terceira, Azores. Proceedings of the Workshop on “Caldera volcanism: analysis, modelling and response”, p. 14.


The island of Terceira (Açores, Portugal) is 400 km2 in area and consists of four composite volcanoes with calderas (from youngest to oldest: Santa Barbara, Pico Alto, Guilherme Moniz, Cinco Picos) grouped along a basaltic fissure zone that transects the island from NW to SE. Three of the volcanoes are noteworthy for production of peralkaline felsic magmas, which have sometimes erupted explosively to produce ignimbrites and fall deposits. While little is known of the history of the older of these three (Guilherme Moniz), the two younger, active calderas appear to have formed incrementally: Pico Alto has had several ignimbrite-producing eruptions, with sub-plinian/lava dome-forming events occurring before, during, and after ignimbrite volcanism, and Santa Barbara has undergone numerous subplinian/lava dome-forming events. Ignimbrite eruptive volumes estimated from on-land deposits are small (~0.1-0.3 km3) but considerable amounts of material must have flowed into the sea.


The Lajes-Angra Ignimbrite, of comenditic trachyte composition and variously dated by 14C at 19-23 ka, is the youngest from Pico Alto caldera. This low-aspect-ratio ignimbrite is densely welded in places despite being only a few metres thick. A widespread ignimbrite veneer facies of this deposit attests that pyroclastic flows covered 2/3 of the island. At least five other ignimbrites have been identified so far in the sequences of deposits that underlie the Lajes-Angra deposit in sea cliffs around Terceira. It is not yet known whether these ignimbrites were all derived from Pico Alto, or whether any came from Guilherme Moniz caldera. New 40Ar/39Ar age determinations on anorthoclase crystals separated from ignimbrite pumice clasts, together with 14C ages for the younger units, support a rather narrow period of ignimbrite volcanism from 85 to 20 ka ago. The average time-interval between eruptions during this period is thus ~16 ka. The 20 ka elapsed since the most recent event suggests that ignimbrite volcanism from Pico Alto could still occur in the future, and may be a future threat to Terceira’s island-bound 60,000 population. Santa Barbara’s caldera is smaller (~2 km diameter) than Pico Alto’s (~4 km), and, based on stratigraphic and age relationships, this volcano could potentially enter an ignimbrite-forming phase.