Early morphodynamics of the sudden formation of beaches during the 2021 volcanic eruption of La Palma

Abstract

On 19 September 2021, a new Strombolian monogenetic volcano erupted on the island of La Palma (Canary Islands, Spain). During the 12-week eruption, a succession of lava flows progressed down the west-central slopes, descended down the coastal cliffs and advanced over a narrow island shelf, filling it in the form of lava deltas. After the stabilization of the lava deltas, the formation of beach-like sedimentary bodies could be observed. This works aims to document the rhythms and formation dynamics of these beaches in the first months after their emergence using orthophotographic materials obtained using unmanned aerial vehicles (UAVs). These technologies have allowed analysis of the subaerial coastal evolution of the lava deltas between October 2021 and May 2022 through high-resolution digital images of approximately weekly frequency. The study includes analyses of the dichotomous evolution (emergences, disappearances, disintegrations and coalescences), short-term surface and shoreline variability (by rates of variation, Shoreline Change Envelope and Net Shoreline Movement indicators) and medium-term surface and shoreline trends (by linear regressions and End Point Rate indicators). The observations show that the formation of shoreline sedimentary bodies was sudden, within 24–48 h after stabilization of the lava fronts, and that most were subsequently long-lasting. Despite their durability, the dynamics after their appearance show high morphological mutability. The analysis of the maritime regime has allowed us to interpret these early, abnormally changing dynamics as initial states of morphodynamic disequilibrium and adjustments towards morphological stability and equilibrium with the physical environmental conditions.