Rui Quartau

Rocky coastlines of newly formed lava deltas change rapidly after their emplacement if they are exposed to energetic waves and where they are internally weak. Such rapid changes can be studied in detail using time-lapse remote sensing data from recent eruptions. Two lava deltas were created during the 2021 La Palma eruption. We use 0.5-m-resolution Pléiades satellite images to characterize their evolution on daily and monthly intervals over 14 months, providing young lava-coastline change data of high temporal resolution for the first time. Coasts of these two deltas are classified into pocket beaches and rock promontories. The pocket beaches were formed from eroded volcanic materials. Fluctuations in their positions correlate strongly with tidal water level. Within the first 39 days, annualised coastal retreat rates of the promontories reached 712 m/yr, the fastest retreat rates found so far in young lava coasts, before decreasing with time. We interpret this change as due to the presence of friable materials on the lava fronts (e.g., clinker), which were easily eroded by waves initially, in contrast with later slow retreat due to more resistant lithologies within lava flow interiors. Slowed coastal retreat may also be due to attenuation of waves crossing a growing submarine platform in front of the deltas and protection from clastic materials formed by collapsed sea cliffs. In addition, the coastline of the northern delta became more crenelated in plan-view and retreated quickly. That retreat followed a similar systematic slowing found in other new rocky coasts formed by historical eruptions. In contrast, coastline of the southerly delta was more rounded and changed minimally. These deltas experienced two markedly different histories despite lying less than ~1 km apart, which can be explained by different patterns of lava flow emplacement and internal structure.