Morphosedimentary characteristics and formation mechanisms of new beaches generated after the Tajogaite volcano eruption of 2021 (La Palma, Spain)

Abstract

Only in very special circumstances can new beaches develop due to natural processes in areas where they did not exist before. One such circumstance is related to volcanic eruptions, when a lava flow reaches the ocean. The sudden formation of new beaches is examined in this paper. Following the Tajogaite volcano eruption in September 2021, two lava deltas formed after the arrival of several lava flows to the coast. Attached to the flanks and front of these lava deltas, thirteen gravel beaches formed in just a few months. Detailed topographic and sedimentologic information was collected both on the beaches and adjacent submarine areas. The volume of sediments accumulated on these beaches was obtained after comparison of the actual topography with a previous one, yielding a total net accumulation of about 79,000 m3 of volcanoclastic pebbles and cobbles. This material comes from two major source areas: extensive offshore deposits of volcanic clasts within the depth of closure which are moved onshore by low steepness swell waves, and rock fragments derived from erosion of the lava delta front by high energy waves. Two types of beaches were identified. Beaches that already existed prior to the eruption showed significant increases in cross-shore length and height, with a net accumulation of 67,000 m3. In contrast, the dismantling of the lava delta front led to the formation of totally new beaches in certain locations, accounting for about 12,000 m3 of sediments. Several features, such as a steep foreshore slope, a well-developed storm berm and the presence of wood debris in many of these beaches at several meters height and tens of meters inland, are examined conjointly with the sediment characteristics, showing how textural maturity can change in response to the forcing agents acting in the different parts of the beach profile. The future evolution of these beaches is also considered. This study can enrich the knowledge regarding how new gravel beaches can form in volcanic settings.