Fissure-type vents at 2021 Cumbre Vieja volcanic eruption

Multi-vent opening in monogenetic eruptions in the Canary Islands poses a threat considering the fast population increase over the last years. On the 19^th of September of 2021 the last volcanic eruption of La Palma Island took place in the Cumbre Vieja volcanic ridge. The lava issued during the eruption affected more than 3000 buildings. The eruption was mainly concentrated along a main fissure that formed a cinder cone of approximately 200 m-high topped by several craters trending NW-SE. Although the geophysical data did not suggest the possibility of new vents opening far from the main fissure, more than 80 low effusion rate vents were opened following NW-SE, N-S and E-W directions. Effusive vents were opened as far as 3 km from the main fissure. Hawaiian style dominated in distal vents, and fast-flow pahoehoe lavas inundated new urban areas apparently out of danger to the south of the lava field. Previous historic eruptions show a similar pattern of several fissure-type vents extending over more than 4 km like Martín in 1642, El Charco in 1712 or San Juan in 1949.

In the 2021 eruption some possible precursors were observed previous to the distal vents opening such as CO₂ diffuse gas anomalies, the opening of small fractures, local deformation, fumarolic activity and related small landslides before the feeder dike reached the surface. Since distal vents opening was unnoticed by the geophysical monitoring, we propose a detailed monitoring of diffuse gas emission and local fragile and ductile deformation in a radio of several kilometres from the main fissure. The previous knowledge of the detailed volcano-tectonic structure is also essential.