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


Referência Bibliográfica

SELF, S., GERTISSER, R., KELLEY, S.P., GASPAR, J.L., PACHECO, J.M., QUEIROZ, G., PIMENTEL, A. (2005) - The style and tempo of ignimbrite volcanism on Terceira, Azores, and related hazards. VMSG Annual Meeting 2005 - Joint with Geochemistry Group, Open University, Milton Keynes, Reino Unido, 5 - 7 Janeiro (Comunicação Oral).​


The island of Terceira (Azores, Portugal) consists of four large volcanoes (Pico Alto, Santa Barbara, Guilherme Moniz, Cinco Picos) grouped along a basaltic fissure zone that transects the island from NW to SE. The island is noteworthy for the voluminous production of peralkaline felsic magmas. Peralkaline trachyte and rhyolite magmas formed extensive lava flows and domes, but also erupted explosively to produce pumice fall and pyroclastic flow deposits (ignimbrites).


The Lajes-Angra Ignimbrite, of comenditic trachyte composition and dated by 14C at ~19,000 y BP, is the latest in a history of ignimbrite eruptions from the active Pico Alto volcano. This event produced a low-aspect-ratio ignimbrite that is welded in places despite being only a few meters thick. A widespread ignimbrite venner facies of this deposit attests that pyroclastic flows covered most of the island. At least five similar ignimbrite-forming eruptions pre-dating the Lajes-Angra Ignimbrite have been identified so far in the sequences of deposits exposed above sea level around Terceira. It is not yet known whether the earlier ignimbrites were all derived from Pico Alto, or whether came from the two apparently extinct volcanoes Guilherme Moniz and Cinco Picos. 14C ages indicate that these eruptions occurred earlier than 30,000 y BP, which presented an age-dating challenge.


We present new 40Ar/39Ar age determinations on anorthoclase crystals separated from pumice clasts from the ignimbrite. These dates both overlap the 14C ages and extend to older ages. The preliminary data support a rather narrow period of ignimbrite volcanism from ~86 to ~19 ka BP. This suggests an average time interval of ~12 kyr between ignimbrite eruptions during this period. The occurrence of longer time gaps between ignimbrite eruptions, and the 19 ka elapsed since the most recent event, suggests that ignimbrite volcanism could still occur in the future, and may be a future threat to Terceira’s 60,000 population.


Regarding the pyroclastic flow hazard on Terceira, Pico Alto is considered to be more likely to have a significant explosive eruption than neighbouring Santa Barbara volcano, which apparently has not produced a pyroclastic-flow-forming eruption but has had less hazardous eruptions producing lava domes and small-volume sub-plinian fall deposits. Ignimbrite eruptions on Terceira appear to begin without any opening plinian phases. Pyroclastic flows that originated from Pico Alto (and possibly Guilherme Moniz or Cinco Picos), in the central part of the island, generally followed topographic depressions towards the N and S coasts. The pyroclastic flows also spread over flat interfluve surfaces where they left thin and inconspicuous deposits, the significance of which may be readily missed when assessing the past record of the island’s volcanic activity. It is now recognised that large areas, previously thought to be free of hazard from pyroclastic flows, were covered by thin ignimbrite veneer deposits. The well-populated coastal plain between Lajes and Praia da Vitoria, which includes the airport and a NATO base, may be especially susceptible to pyroclastic flows from Pico Alto.