Littoral activity in the lava deltas of 2021 eruption on Cumbre Vieja Volcanic Rift, La Palma (Canary Islands): constraints on explosive water-magma interaction

Tephra jets are a characteristic explosive phenomenon of lava deltas built by pāhoehoe or ‘a‘ā lava flows. Field observations made during the growth of the 2021 South Lava Delta (La Palma island), emplaced under a 100–150 m high marine cliff, show tephra jets driven by penetration of seawater through the external lava breccia into the interior of ‘a‘ā lava flows entering the ocean. However, this littoral explosive activity was weak and very scarce throughout the entire period of delta emplacement, a circumstance that seem to have concur in other ‘a‘ā lava deltas also emplaced under high marine scarps. Main constraint to explosive activity in these coastal settings seem to be the slow penetration into the ocean of ‘a‘ā lava flows, which is clearly induced by coastal morphology, as lava abruptly slows and accumulates on the flatter abrasion platform after flowing on ramps of lava debris down the marine cliff.