Furnas volcano, one of three active trachytic stratovolcanoes on Sao Miguel, is considered to be one of Europe’s most hazardous volcanoes. Approximately ten explosive eruptions have occurred over the past 3200 years, including the most recent in 1630 AD. Approximately 1,500 people who live within the caldera in the village of Furnas, as well as several thousand living in close proximity, are at significant risk given the potential for a future eruption. Our work builds on previous detailed stratigraphic studies of the Furnas 1630 AD deposit (Cole et al., 1995) in order to further understand the nature of the magmatic processes leading to the highly evolved and explosive eruptions of Furnas volcano.

We have analyzed a suite of 31 stratigraphically controlled samples that represent a time sequence through the Furnas 1630 AD eruption. Samples from throughout the deposit are dominantly crystal-poor trachyte pumices with very limited major element variation, with the exception of minor late erupted, crystal-rich trachyte and latite. In contrast to the limited major element variations, trace element variations are large, with ~2-fold variations in incompatible elements such as Zr. Major and trace element systematics, combined with Sr, Nd and Pb isotope data, suggest that the Furnas 1630 AD magmatic system evolved primarily by fractional crystallization of a sanidine dominated assemblage, with minor assimilation of hydrothermally altered syenite wallrock. Although late erupted pumice and dome material is less evolved than earlier erupted pumice, the deposit does not exhibit a monotonic compositional gradient as observed in trachyte deposits of the neighboring Fogo volcano (Snyder et al., 2004). In contrast, the Furnas 1630 AD deposit exhibits two “normally zoned”sequences separated by a “reversely  zoned ”sequence, in which there is a change in the sign of the variation in Zr concentration with time.

These systematics, coupled with the late occurrence of crystal-rich trachyte and a latite whose composition appears unrelated to the trachytes, suggest that the eruption may have been triggered by the intrusion of a latite magma into a crystal mush zone at the base of a compositionally zoned trachytic magma body that had complex geometry and/or multiple chambers.