Middle–to–late Holocene explosive eruptions of Lonquimay volcano (Southern Andes of Chile): Stratigraphy, tephra dispersal, and pre-eruptive conditions

Abstract

The Lonquimay-Tolguaca Volcanic Complex (LTVC), located in Chile's Andean Southern Volcanic Zone (40°S), experienced its last eruption in 1988–90, profoundly affecting neighbouring rural communities. Yet, more than twenty explosive eruptions have been recognised and described at Lonquimay volcano from its Holocene tephra records. Knowing its eruptive past is one of the main concerns of the residents of Malalcahuello, 11 km E-SE from the LTVC. In this contribution, we aim to decipher the behaviour of Lonquimay, focusing on its most recent (i.e., Middle-to-Late Holocene, <5.5 ka) explosive history to address community-based questions with renewed scientific knowledge. We studied proximal-to-medial (i.e., 2–15 km) tephra sequences east-southeast from the LTVC. Seventeen tephra units were described. These are mainly formed by rhythmic alternations of both grey to white pumice beds, and black to brown scoria beds. Pyroclast textures consisting of scoria, dense juvenile fragments and pumices display compositions ranging from basalts to dacites. The study of tephra deposits suggests Strombolian to Sub-Plinian eruptive styles, whereas isopach-based volume estimates on the order of 10⁷ m³ from four individual deposits support a Volcanic Explosivity Index (VEI) of 3. This type of events has a 27–61 % probability of occurrence within the next 100 years. The studied tephra deposits were fed by mafic (basaltic and basaltic andesitic, 1050–1100 °C) and silicic (andesites and dacites, 890–940 °C) magma batches stored between ∼2 to ∼7.5 kbar. Magma evolution was mostly controlled by fractional crystallisation and occasional mixing events. Assessing tephra fall deposits simultaneously at multiple scales offers clues on the eruptive scenarios, magma processes and sources, as well as potential hazards of their parent volcanic systems. Along with contributing to scientific knowledge, these elements can enhance risk reduction strategies by integrating community questions about active volcanism into hazard research.