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Instituto de Investigação
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Artigos em livros de actas ► Internacionais


Referência Bibliográfica

FRANÇA, Z., CRUZ, J.V., NUNES, J., FORJAZ, V.H. (2003) - Azores historical eruptions: geochemical features and related eruptive styles. Abstract Book - Week A, XXIII General Assembly of the International Union of Geodesy and Geophysics - IUGG 2003, Sapporo, Japan, A.558.


The archipelago of Azores, located in the North Atlantic Ocean (37-40ºN; 25-31ºW), is made of nine volcanic islands, with ages ranging from about 9 My to less than 300 000 y. Azores islands are in the triple junction between American, African and Euroasian plates, according to a complex geodynamic setting. Since the settlement in the 15th century 26 volcanic eruptions took place, 12 of which were subaerial events, in the islands of São Miguel, Terceira, São Jorge, Pico and Faial. In the other islands (Graciosa, Santa Maria, Flores and Corvo) there are not records of historical eruptions. However, at Graciosa, a lava flow with an age of about 2 00 yr. B.P. was identified. Historiacl eruptions have emitted basalts, basanites, hawaiites, mugearites, benmoreites and trachytes. The majority of the samples are undersaturated and belong to the alkaline and sodic series, with normative nepheline and olivine. A few samples lye in the subalkaline and potassic series field. The Differentiation Index show sharp differences comparing samples from different islands and between eruptions in the same volcanic complex. Harker diagrams, spider diagrams and the Rare Earth patterns also point out distinctive characteristics among different islands and among eruptions in the same volcanic center. The more evolved rocks result essentially from fractional crystallization on magmatic chambers related with composite volcanoes which summit is truncated by a subsidence caldera. At Pico island the more evolved rocks (mugearite/benmoreite) are related with a single lava flow extruded from Pico Mountain, a huge composite volcano less than 250 000 y old, without a summit caldera and underlaied by an embrionary magma chamber in development. Isotopic geochemistry of Pico, Faial, São Jorge and Terceira suggests that This Central Group island result from a single mantelic source, to which HIMU, EM and DMM reservoirs have contributed in different proportions. The characterization of historical volcanic eruptions relies essentially on the recorded testimonies, which have been more recently complemented by geochemical data and volcanological mapping, in order to characterize the eruptive styles. In that islands of central group historical eruptions were mainly from the strombolian type, with more or less long hawaiian phases. At São Miguel island, eruptive styles were more heterogeneous, including subplinian, plinian and phreatomagmatic eruptions asd well as strombolian and hawaiian events. The well-known Capelinhos submarine eruption (1957:58; Faial island) was in an early stage from the surtsyan type which evolved to a strombolian stage after the conduit isolation from seawater. Related to Capelinhos eruption are also hawaiian and phreatic phases, the latter located inside the caldera of Faial central composite volcano. The last eruption at Azores took place between 1998 and 2000, corresponding to a submarine eruption, about 5 miles W of Terceira island, in the proximity of the 1867 AD submarine eruptive center. The peculiar characteristics of this eruption allowed to definition of an eruptive style known as “serretian type”.