Tephra dispersal and composition reveal the explosive onset of a large basaltic fissure eruption: Timanfaya, Lanzarote, 1730–1736 CE

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 and potential environmental repercussions. Here we report field and geochemical data from tephra deposits to reconstruct the temporal evolution of eruptive style and provide constraints on the impact of the Timanfaya eruption. Stratigraphic sections demonstrate the pulsatory nature of explosive activity during the eruption and the wide dispersal of the tephra blanket, for which a minimum bulk volume of ~0.44 km³ is derived. Isopleth data from a basal tephra layer tied to an early, particularly powerful eruption pulse suggest that eruption columns lofted to ≥8 km altitude. 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 the most violent explosive activity, responsible for the emplacement of the tephra blanket, was restricted to the first few months of the eruption, after which eruptive style was dominated by lava effusion and mild cone-building Strombolian activity. We argue that explosive activity at Timanfaya was similar to that of the 1783–1784 CE Laki and 2021 Cumbre Vieja eruptions and highlight the explosive potential of mafic volcanism in the Canary Islands. Trace element proxies for volatile elements suggest that early basanitic magmas were particularly rich in CO₂, sulfur, and chlorine. We infer that the Timanfaya eruption released 65–388 Tg CO₂, 9–23 Tg S, and 2–9 Tg Cl to the atmosphere; however, ice core evidence indicates that little to no sulfur reached Greenland, consistent with tropospheric transport of gas emissions. Some climate proxy records show anomalies that may be related to Timanfaya, but the environmental impact of the eruption beyond Lanzarote remains unclear.