Rapid changes of the lava-delta coastlines formed by the 2021 volcanic eruption on La Palma, Canary Islands

Coastlines of new lava deltas change rapidly after their emplacement if they are exposed to energetic waves and where they are internally weak. Those rapid changes potentially make them easy to study with modern remote-sensing data. We use 0.5-m-resolution Pléiades satellite images to characterize how coastlines of two lava deltas created during the 2021 La Palma eruption evolved on daily and monthly intervals over 14 months, providing young lava-coastline data of high temporal resolution for the first time. The coasts are classified into pocket beaches and rock promontories. The pocket beaches were formed from eroded volcanic materials. Fluctuations in their shoreline positions correlate strongly with tidal water level. Within the initial few days, coastal promontories retreated by up to 2 m/day, then decreased with time. We attribute this change to the presence of friable materials on the lava fronts (e.g., clinker), which were easily eroded by waves, in contrast with later slow retreat due to more resistant lithologies within flow interiors. Attenuation of waves crossing a growing submarine platform in front of the deltas and protection from clastic materials formed by collapsed sea cliffs may have also slowed coastal retreat. In addition, the coastline of the northern delta became more crenelated in plan-view and retreated quickly, in contrast with a more rounded coastline of the southerly delta, which changed minimally. These two deltas experienced markedly different histories despite lying less than ~1 km apart, which can be explained by different patterns of lava flow emplacement and internal structure. The north delta has followed a similar retreat history to Heimaey (Iceland), which may therefore provide a possible template for future cliff retreat.