Volcanic Geology of São Miguel Island (Azores Archipelago)

The Azores archipelago in the Atlantic Ocean is composed of nine volcanic islands and São Miguel is the largest and most volcanically active. During the past 5000 years several eruptions have taken place on the three active central volcanoes – Sete Cidades, Fogo and Furnas – and in the basaltic fissure systems of Picos and Congro. There is evidence that Furnas was in eruption when the first settlers arrived some time between 1439 and 1443. In the sixteenth and seventeenth centuries there were two explosive sub-Plinian eruptions, Fogo in 1563 and Furnas in 1630. The last eruption on land occurred in the Picos Fissural Volcanic System in 1652, involving the extrusion of lava domes. In 22 chapters, this volume considers the volcanic geology of the island under the headings of geological setting, volcanic history, geological hazards and risk assessment, volcano monitoring and natural resources.
Hydrogeochemical characterization of mineral waters in São Miguel Island, Azores Available to Purchase
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Published:January 01, 2015
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CiteCitation
J. V. Cruz, P. Freire, A. Costa, J. Fontiela, L. Cabral, R. M. Coutinho, 2015. "Hydrogeochemical characterization of mineral waters in São Miguel Island, Azores", Volcanic Geology of São Miguel Island (Azores Archipelago), J. L. Gaspar, J. E. Guest, A. M. Duncan, F. J. A. S. Barriga, D. K. Chester
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Abstract
A dataset on the chemical composition of mineral water discharges from São Miguel, 76 samples, depicts a large variability of chemical types and dissolved solids content. The distribution of the discharges shows an association with the three active Quaternary central volcanoes that dominate the geology of São Miguel, namely Furnas, Fogo and Sete Cidades, 74% of which are springs, 13% fumaroles, 12% wells and 1% hand-dug wells. Total dissolved solids (TDS) values are in the range of 159–20 957 mg l−1, discharge temperature varies from 15 to 99.5°C and waters are mainly strongly acid to slightly alkaline (pH ranging between 2.2 and 7.71). Springs discharge mainly from perched-water bodies, corresponding to Na-HCO3 and Na-HCO3-Cl-type waters, with a composition influenced by absorption of CO2 and mixing between meteoric water and boiling waters with a hydrothermal component. Instead the majority of the wells are distributed along the coast and depict chloride enrichment owing to mixture with seawater. The lower pH values are associated with boiling pools at Fogo Volcano with a SO4-dominated chemistry, resulting from steam-heating of shallow perched-water bodies.