A sudden beaches formation on the coastal lava-deltas of the 2021 volcanic eruption on La Palma

Abstract

<p>On September 19, 2021, a new monogenetic volcano erupted on the island of La Palma (Canary Islands, Spain), in the northwestern sector of the Cumbre Vieja volcanic rift, at an altitude of approximately 1,050 meters above sea level. As a result of approximately 12 weeks of volcanic material emissions, a pyroclastic cone about 200 meters high and 800 meters in diameter was formed in the Cabeza de Vaca area, as well as successive lava flows that descended the western slopes of Cumbre Vieja until reaching the sea for the first time on September 29, 2021, in the coastal area of Los Guirres. As they descended the coastal cliffs and entered the sea, the lava flows formed two lava delta (locally called "islas bajas" or "fajanas") of 75 and 5 ha, respectively, on the insular shelf. This work explores the coastal transformations associated with this process through the analysis of high-resolution orthophotos, obtained between September 29 and December 27, 2021, with an average frequency of 2 weekly drone images. The observations show a sudden accumulation of fine-grained particles in the shoreline in short periods of only 24 to 72 hours, and their subsequent apparently stability. This process was verified in two similar events of lava delta progradation. The first lasted from September 29, when the first aa lava flows entered the sea; until October 8, when the lava front stabilizes. Just after, a total of 10 small beaches of fine volcanic material appeared in following hours (24-72 hours). The second episode took place between November 10, with a new lava front affecting the southern half of the lava delta, and the formation of a new, smaller one, to the north; until November 24, when both lava fronts stabilize. Once again, 16 small beaches of fine volcanic grains appeared in the following 24-72 hours. This research discusses the processes that may have given rise to the sudden formation of these new beaches, as well as their possible short-, mid- and long-term stability and evolution.</p>