Geochemical characterisation of major silicic explosive eruptions from the Azores in the last ~100 kyrs

Journal article

Volcanoes in the Azores have experienced large, silicic explosive eruptions, and these events have been concentrated mostly on the islands of São Miguel and Terceira. It is evident these eruptions are capable of dispersing tephra thousands of kilometers, owing to distal sedimentary sequences in Europe and Africa recording tephra sourced from the Azores, but the lack of detailed glass geochemical datasets precluded robust distal correlations to specific eruptions. Establishing the glass compositions of these large pyroclastic deposits will allow these Azores tephra to serve as chronostratigraphic markers. Here we present the major and trace element glass geochemical compositions of the largest known eruptions from Sete Cidades, Fogo, and Furnas volcanoes on São Miguel, and Pico Alto/Guilherme Moniz on Terceira. The glasses are predominately trachytic with 60.5–67.5 wt% SiO₂, 12–19 wt% Al₂O₃, 1–7 wt% FeOt, Eu <6.5 ppm, Zr <2400 ppm, and Yb <14 ppm. A notable feature is that the glass compositions of individual deposits are diverse, and often show distinct populations that are associated with different eruption phases. The melts erupted from central volcanoes on both islands are distinctive, with glasses erupted from São Miguel contain distinctively lower FeOt (<4.0 wt%), and higher Al₂O₃ (>16.8 wt%) and La/Y ratios (>1.6 ppm) relative to those erupted from Terceira. The dataset was used to establish that a cryptotephra layer in the Taforalt archaeological site in Morocco is linked to the Santa Bárbara eruption from Sete Cidades volcano >2000 km away. Interestingly, despite several tephra layers from the Azores being identified in Holocene sequences across Europe, there are no voluminous deposits on the Azores, except for Fogo A, within this timeframe. This confirms that the tephra is widely dispersed even in some of the smaller eruptions and this geochemistry dataset provides a useful framework for identifying Azores tephra in late Pleistocene distal records. However, the Azores volcanoes typically erupt compositionally diverse evolved melts that do not vary much between successive eruptions. Thus, it is often difficult to distinguish the eruption deposits based on glass compositions alone, and often chronostratigraphic information is also required to reliably correlate distal tephra to specific eruptions.

Authors: Horn, EL; McLean, D; Pimentel, A; Pacheco, JM; Aguiar, S

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Elsevier
• Journal: Journal of Volcanology and Geothermal Research
• Volume: 468
• Article number: 108446
• Publication date: 2025-09-15
• Acceptance date: 2024-09-14
• DOI: 10.1016/j.jvolgeores.2025.108446
• EISSN: 1872-6097
• ISSN: 0377-0273
• Language: English
• Keywords: tephrochronology; explosive volcanism; glass geochemistry; tephra; São Miguel; Terceira