La Palma (Canary Islands) eruption 2021: Forecast and hindcast of eruption duration

After 50 years of quiescence, the most voluminous eruption since historical records started at La Palma took place (September 19 – December 13, 2021, 85 days and 9 hours of duration). The observed deformation field during the eruption was consistent with a deflating reservoir located at Moho depth beneath Cumbre Vieja volcano. This reservoir has been previously studied and determined that magmas can stagnant and reside there over a wide range of time-scales varying from days-weeks to a few centuries. During the eruption, applying some basic principles we made a forecast of the eruption duration. Under the assumption of mass conservation in a close-system, the pressure drop, responsible for the observed deflation signal, controls eruption duration. On day 38th of the eruption and despite its simplicity, one of our models with a pressure drop equal to 1% of the initial reservoir overpressure provided an estimated duration of 86 ± 7 days. Therefore, less than halfway along the actual duration of the La Palma eruption, the proposed model was able to estimate a possible tight duration. This duration was consistent with historical records of eruption durations in La Palma ranging from 24 to 84 days. The lack of a priori bounds on the pressure drop necessary to cease the eruption made the forecast too uncertain to have operational impact. In hindsight, we empirically show that the pressure drop in the reservoir was actually limited by mass conservation. We conclude that the magma dynamics of the Moho reservoir was the dominant control on the eruptive volume evolution. This study demonstrates that near real-time forecasts of eruption durations will be possible in future similar eruptions at La Palma. Our study opens the possibility to explore the applicability of this method to other volcanoes.