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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica

PACHECO, J.M., GASPAR, J.L. (1998) - The Graciosa caldera-forming eruptive sequence (Graciosa island, Azores). EC Advanced Study Course on "Volcanic Hazard, Monitoring and Risk Mitigation". S. Miguel, Furnas, Junho (Comunicação Oral).


The geological mapping of Graciosa Island allowed the identification of over a hundred volcanic deposits related to the construction of the Central Volcano. The eruptive history of this volcano was marked by a succession of sub-aerial episodes with periods of hawaiian, strombolian, subplinian and plinian activity, and hydromagmatic episodes including surtseyan and phreatomagmatic eruptions.
The caldera-forming event, dated of 12200 years BP, was related to an important hydromagmatic explosive eruption, responsible for the extrusion of pumice fall, pyroclastic flows, lahars and surges. This work describes the internal structure of the eruptive sequence and correlates the documented exposures around the volcano.
The eruptive sequence starts with a set of alternated lithic rich lapilli and hydromagmatic ash layers. Several massive flow units with an important lateral facies variation overlie these layers. Some units are massive layers of fine ash with scattered lithics, locally bearing fumarolic pipes, while others are lenses dominated by very coarse lithic material. Despite the presence of some distinctive syenites xenoliths, lithics are mainly basaltic and traquitic lava fragments from earlier eruptive events.
On top of these layers one can observe a thick sequence of sandwave facies surge deposits with several bomb sags formed by the impact of ballistic blocks. A change in the eruptive style is characterised by a fall layer, extremely rich in lithics. Some very large blocks (80 cm) are found near the coast, 2 km from the vent.
The top of the sequence is composed by several massive flow units with an important lateral facies variation, similar to those of the base, frequently cut by important erosion gullies.