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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica

ZANON, V., PACHECO, J., PIMENTEL, A. (2007) - Growth and evolution of São Roque offshore tuff cone, São Miguel Island (Azores): geomorphological evidences and facies analysis. IUGG XXIV General Assembly, Perugia, Itália, 02 - 13 Julho (Poster).​


The sin-eruptive and post-eruptive history of São Roque tuff cone, its geological setting and volcanological parameters were studied in detail, to understand the role of tectonic activity in the birth and morphological evolution of a relatively simple and small volcanic edifice.


This centre is located westward of the town of Ponta Delgada, inside the village of São Roque, in the island of São Miguel, and it is composed by two main bodies characterized by the different steepness of their slopes and numerous islets measuring only a few square meters. All these bodies are aligned along a curve that constitute overall a segment of an ellipse and represent the remnant of a tuff cone.


The tectonic setting of the area follows a regional trend, defined by extensional NW-SE trending faults which probably represent the emerged segment of the Terceira Rift. The morphology of this area is in fact dominated by the diffuse presence of numerous scoria cones with basaltic composition, sometimes aligned along fissural fractures.


The volcanic rocks which constitute these outcrops have a hydromagmatic origin and were erupted between 20890 (±240) and 8700 (±200) years ago. Judging from the different beddings of surge layers, it seems that magma emissions occurred along a fissure that probably opened progressively from SE to NW, forming small edifices through the rapid accumulation of wet sediments. Sin-eruptive partial collapses greatly modified the original morphology of these edifices, probably also allowing sea water to continuously flow into the eruptive fissure. The complex interaction of these two factors controlled the depth of fragmentation alongside the fracture, producing different kinds of deposits, in which the ash-lapilli ratio varied considerably. In these circumstances, also tide cycles probably played a major role, even causing periods of subaerial eruptive conditions, in which small-lived fire fountaining episodes generated agglutinated scoriae. The high water content of these deposits caused sin-eruptive and post-eruptive remobilization that resulted in collapses and some small-scale landslides. Furthermore, the post-eruptive accumulation of heavy tephra over unconsolidated tuff and littoral sands caused the selective sinking of some parts of the deposits, which resulted in the total destruction of all the southeasternmost deposits and in the formation of numerous small islets, separated by radially arranged channels in the northwesternmost sector. Fractures produced during these local partial collapses interested also a lava flow unit, emplaced several years after the end of São Roque centre, indicating that this process went on for a long time.


Finally, a transtensive fault and a set of direct faults with local importance, all WNW-ESE trending, operated the last dissection of the outcrops, generating high instability in the main bodies, where  falling and rolling of blocks are still frequently occurring.