Characterization of the tephra deposit associated with the 2021 eruption of Cumbre Vieja (La Palma)

Between 19 September and 13 December 2021 (85 days), a flank eruption took place along the Cumbre Vieja ridge (La Palma Island), one of the most active volcanic centers of Canary Islands. The last 7000 years of Cumbre Vieja activity has been characterized by a combination of effusive and explosive eruptions. These generated both cinder cones and lava flows. The previous event occurred between October 26 and November 28, 1971 (eruption of Teneguía). The 2021 eruption was characterized by the alternate (and often simultaneous) emission of lava flows, lava fountains and tephra plumes along a ~1 km-length fissure consisting of about ten vents that built a cinder cone complex. The southern vents were mostly associated with the generation of tephra plumes and lava fountains, while the northern vents were mostly associated with the generation of lava flows. The ~12 km² lava flow field on the west side of the island reached the sea on September 28, forming a new lava delta and interacting with sea water producing lava haze (i.e. laze). Even though tephra was sedimented all over La Palma and sometimes reached the neighboring islands, the cumulative tephra deposit is mostly elongated towards the southwest and the northeast due to the prevailing wind direction, reaching a maximum thickness southwest of the fissure. Both lava fountains and more explosive tephra plumes contributed to the formation of the tephra deposit. Tephra plumes were associated with variable intensity reaching a few kilometers of altitude (<10 km). Multiple tephra layers, which are associated with distinct phases of the eruption, can be identified based on grainsize, clast texture and deposit characteristics. Some of the layers are dominated by black fluidal glassy clasts mostly associated with lava fountain activity, while some others are dominated by brown, finely to coarsely vesicular clasts mostly associated with more explosive tephra plumes. Most layers, as well as the cumulative deposit, are characterized by a thinning break-in-slope between 3-4 km from the vents. A distinctive lithic-rich, reddish layer, which mostly sedimented on October 15-16 during a new vent opening phase, helps the correlation among the various layers. Grainsize on land is dominated by lapilli and coarse ash, with fine ash being mostly deposited in the ocean (beyond 6 km from the vents). Individual layers are associated with volumes that range between Volcanic Explosivity Index (VEI) 2 and 3, while the total tephra deposit is associated with a VEI 3 (excluding the volume of the cone).