Chapter 9 The volcanic history of Furnas Volcano, São Miguel, Azores

Abstract

Furnas is the easternmost of the trachytic active central volcanoes of São Miguel. Unlike the other central volcanoes, Sete Cidades and Fogo, Furnas does not have a substantial edifice built up above sea-level. Although not as dominant as the other two volcanoes, Furnas does, however, have an edifice rising from the basal basaltic lavas exposed on the north coast to around 600 m asl on the northern rim of the main caldera. In common with Sete Cidades and Fogo, Furnas had major trachytic explosive eruptions in its volcanic history that emplaced welded ignimbrites. In the last 5 ka Furnas has had 10 moderately explosive trachytic eruptions of sub-Plinian character; two of these have taken place since the island was settled in the mid-fifteenth century. A future eruption of sub-Plinian magnitude is a major hazard posed by Furnas Volcano. Even when not in eruption, Furnas is a hazardous environment. Its fumarolic fields discharge high levels of CO 2 and concentrations in some area of Furnas village present a risk to health; the steep slopes and poorly consolidated volcanic materials are prone to landslides, in particular when triggered by earthquakes or following heavy rain, as was the case in 1997, when landslides caused severe damage and casualties in Ribeira Quente.

3-D interpretation of short-period magnetotelluric data at Furnas Volcano, Azores Islands

“…Subsequently, a new generation of hydrothermal systems develop, destroying or replacing the older ones. As pointed out by Guest et al (2015), the area of the Furnas Caldera has a documented history of frequent eruptions since the formation of the Povoção Caldera at least 30 ka ago.…”

“…Furthermore, the observed conductive zone C2 does not extend towards the south, and we do not find a connection to the ocean there. One may speculate that the lack of southward connection originates from the recent explosive volcanism within the caldera, which has been migrating from the NE to SW since at least 1.9 ka ago (Guest et al 2015) and may have destroyed earlier fluid pathways. It has to be noted, however, that ocean water is not the only potential source of salinity as the fluid associated with the C2 zone may derive from mixing with highly saline magma-derived fluids (Nicholson 1993;Yardley & Bodnar 2014).…”

“…The inner caldera is smaller and was formed around 10-12 ka BP, which is responsible for the steep topography of more than 500 m in our study area. It contains several younger eruptive centres, a shallow caldera lake and several zones of intensive fumarolic activity within the inhabited caldera (Guest et al 1999(Guest et al , 2015. Phlegrean fields volcano (Naples, Italy) may be comparable to Furnas volcano in several aspects, such as the existence of a caldera and the composition of the hydrothermal fumarolic emissions.…”

Biased Volcanic Hazard Assessment Due to Incomplete Eruption Records on Ocean Islands: An Example of Sete Cidades Volcano, Azores

“…The presence of MME in trachytic pumice clasts from Fogo and Furnas volcanoes has never been described (N. Wallenstein personal communication; cf. Guest et al, 1999Guest et al, , 2015Wallenstein, 1999;Wallenstein et al, 2015), making it a unique characteristic of the SBF deposits of 7C volcano.…”

Chapter 1 Volcanic geology of São Miguel Island (Azores Archipelago): introduction

“…Detailed geological accounts of the two central volcanoes, which topographically dominate the island, are presented by Queiroz et al (2015) for Sete Cidades and Wallenstein et al (2015b) for Fogo; the authors reconstruct the history of the volcanoes paying special attention to caldera-forming events and the recent intracaldera sub-Plinian and phreatoplinian eruptions. A brief account of the volcanic history of Furnas by Guest et al (2015) is an update of an early paper by Guest et al (1999) in which new information is presented. The geology of the basaltic fissure systems that link the central volcanoes is described by Ferreira et al (2015).…”