James K. Muller, Marc-Antoine Longpré 

Basaltic fissure eruptions are chiefly characterized by sizable emissions of lava flows and volcanic gases, posing significant hazards. However, such eruptions may be punctuated by explosive episodes, which are comparatively poorly understood but may have important volcanic hazard and environmental implications. The 1730–1736 CE Timanfaya eruption on Lanzarote, Canary Islands, is a large basaltic fissure eruption characterized by a marked temporal–compositional trend from early basanite to late tholeiite lavas, but little is known on its associated pyroclastic deposits. Here we report field and geochemical data from the tephra deposits of the Timanfaya eruption to reconstruct eruptive style over time. Stratigraphic sections demonstrate the wide dispersal of the tephra blanket, with a bulk volume of ~0.44 km3. We find that nearly all distal tephras are characterized by low SiO2 content and high incompatible trace element concentrations that only match the compositions of tephras sourced from vents active in the eruption’s initial phase. This implies that violent explosive activity was restricted to the first few months of activity, followed by prolonged, dominantly effusive eruptive style. Isopleth data from a basal tephra layer suggest that eruption columns lofted to ≥8 km altitude. Explosive activity at Timanfaya was thus similar to that of the 1783–1784 Laki and 2021 Cumbre Vieja eruptions, highlighting the explosive potential of mafic volcanism in the Canary Islands. Our data suggest that early basanitic magmas were particularly volatile-rich and reveal a correlation between magma composition and eruptive style, although a causative link remains untested. We infer that the Timanfaya eruption released 11–26 Tg of sulfur (and 81–431 Tg CO2) to the atmosphere, but ice core evidence for stratospheric transport of sulfur is unclear. Yet, a number of climate proxy records suggest that the eruption had significant hemispheric climate and environmental consequences. For large-volume basaltic eruptions characterized by effusive and explosive eruptive styles, such as Laki and Timanfaya, Volcanic Explosivity Index values appear most representative of associated hazards and impacts when based on eruptive volume estimates.