Reconstruction of Azorean eruptive scenarios through the correlation of proximal and distal tephras

Explosive volcanic eruptions are amongst the most hazardous natural phenomena due to their potential to affect large areas of land, ocean, and airspace. Thus, understanding how volcanic ash clouds disperse is of crucial importance for the mitigation of volcanic hazard. The Azores archipelago, in the middle of the North Atlantic, is an active volcanic region with an extensive geological record of explosive eruptions from several trachytic central volcanoes. Previous studies have reported distal occurrences of Azorean tephra as far as North Africa or the British Isles, but to date there are no reconstructions of tephra dispersal patterns. In the present work, we correlate cryptotephras with their source volcanoes and reconstruct plausible eruptive scenarios using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model.

Proximal trachytic tephra layers from Sete Cidades and Furnas volcanoes on São Miguel Island (Azores) have been successfully correlated with cryptotephras found in Morocco and Ireland, respectively, based on volcanic glass compositions and age constraints. The pumice fall deposit of Santa Bárbara eruption (18.7 – 19.5 cal ka BP) from Sete Cidades volcano has been geochemically correlated with cryptotephras in layer TAF_S1_R2 (< 26.5 – 24.4 cal ka BP) of Taforalt archaeological site, Morocco. Likewise, the deposits of three hydromagmatic eruptions of Furnas volcano showed good geochemical correlations with cryptotephras found in lacustrine sediments in Ireland, confirming previous studies: Furnas C (154 cal BC – 422 cal AD) compositionally matched cryptotephra layers MOR-T7, -T8, and -T9 (c. 280 AD, c. 150 AD, and c. 35 AD, respectively); Furnas I (1439-43 AD) has been correlated with MOR-T2 (c. 1400 AD); and Furnas 1630 (1630 AD) with PMG-5 cryptotephra (c. 1600 AD).

To reconstruct possible volcanic ash clouds trajectories from Sete Cidades and Furnas volcanoes to Morocco and Ireland, we used the HYSPLIT model to perform simulations of hundreds of eruptive scenarios based on eruption source parameters of Santa Bárbara, Furnas C, Furnas I, and Furnas 1630 eruptions, and daily atmospheric conditions between 2014 and 2021. Our results show that in 52% of the simulations tephra disperses towards North Africa and in 8% towards the British Isles. Also, in 9% of the cases tephra heads to both North Africa and the British Isles in the same simulation and in the other 31% of the cases tephra disperses in different directions.

Although the frequency of explosive eruptions in the Azores is relatively low, a future explosive event may have tremendous economic consequences not only to the archipelago, but also to the entire North Atlantic airspace, as the predominant westerly atmospheric circulation pattern will most probably disperse volcanic ash clouds across some of the world’s busiest air routes. Therefore, eruptive scenario modelling based on past eruptions is a fundamental tool to improve the assessment of volcanic hazard.