A quantitative study of five thousand years of volcanism on Sao Miguel, Azores

Abstract

The activity of the three stratovolcanoes on the island of Sao Miguel is documented by tephrochronology, and during the past 5000 years a total of some 57 volcanic eruptions have taken place, mostly of magnitudes 4-6 on Tsuya’s scale. Approximately half were trachytic, and half basaltic. Each stratovolcano has a caldera within which each has had one historic eruption. The trachytic eruptions were predominantly explosive, and most took place from vents situated within the calderas. Isopach and isograde maps of most of the resulting pumice fall deposits are given. The basaltic eruptions produced both lava flows and pyroclastics, and isopach and isograde maps are given for some of the main fall deposits. The Agua de Pau volcano has had particularly large explosive eruptions, several of them (including Fogo A, the largest in the past 5000 years) being of plinian type. The output of the three volcanoes over the 5000 years is equivalent to 4.6 km 3 of dense rock, at which rate the exposed parts of the volcanoes could have accumulated in 150000 years. At least half of the erupted material is trachytic, a proportion typical of the entire accessible parts of the volcanoes. The 50 known eruptive vents of the past 5000 years are distributed in a zone 55 km long by 8 km wide which may lie above a major fracture zone. Some eruptive fissures trend obliquely across this zone, suggesting right-lateral movement along the fracture. Basaltic eruptions were confined to a much smaller area than in the preceding millennia perhaps due to the formation, at the time of the great Fogo A eruption 5000 years ago, of a broad trachytic magma chamber underlying the Agua de Pau and Furnas volcanoes which basaltic magma has since been unable to penetrate.

Chronic exposure to volcanic environments and chronic bronchitis incidence in the Azores, Portugal

“…Sa˜o Miguel, which is the largest island (757 km 2 ), has its geology dominated by the three active volcanoes of Sete Cidades, Fogo, and Furnas. Presenting fumarolic fields and hydrothermal vents (Booth et al, 1978;Cruz, 2003), and the always present ''rotten egg'' odor, the village of Furnas is built upon actively degassing ground inside Furnas caldera (Baxter et al, 1999), where the last eruption occurred at 1630 AD (Cole et al, 1995). Volcanic gases in this area are typically water vapor, carbon dioxide, H 2 S, SO 2 , with lesser amounts of other gases (Baxter et al, 1999;Ferreira and Oskarsson, 1999).…”

Essential and non-essential trace metals in scalp hair of men chronically exposed to volcanogenic metals in the Azores, Portugal

“…Presenting fumarolic fields and hydrothermal vents (Booth et al, 1978;Cruz, 2003), and the always present "rotten egg" odor, the rural village of Furnas, in S. Miguel Island, is built upon actively degassing ground inside a volcanic caldera (Baxter et al, 1999), where the last eruption occurred at 1630 AD (Cole et al, 1995). This village has about 1541 inhabitants (Serviço Regional de Estatística dos Açores, 2003), where the age groups 0-14, 15-24, 25-44, 45-64, and over 64 represent 22%, 16%, 27%, 19%, and 16% of the local population, respectively.…”

Baseline Levels of Metals in Volcanic Soils of the Azores (Portugal)

“…The Furnas volcano has an older caldera of about 7 × 5 km and with a 290-m-depth depression, enclosing a younger caldera, with a diameter of approximately 5 km, where two subsidence events took place in the last 5,000 years (Guest et al, 1999). This explains the origin of the depressions where the Furnas village (Booth et al, 1978) and the Furnas lake are found today (Zbyszewski, 1961). The growth of the Furnas central volcano started about 100,000 years ago, mainly as a result of eruptions of trachytic pumice.…”

“…Their activity, throughout the last 5,000 years, is revealed by 57 volcanic eruptions, resulting in the production of over 4.6 km 3 of dense rock, and the current hydrothermal manifestations that occur mainly in the Fogo and Furnas volcanoes (Booth et al, 1978;Cruz, 2003). Furnas is the easternmost of the three active volcanoes on the island of São Miguel.…”