Preliminary results from textural studies on tephra deposits erupted during the 2021 eruption at Cumbre Vieja volcano

On September 19, 2021, the Cumbre Vieja volcano (La Palma, Canary Islands) erupted after 50 years dormant; the last eruption occurred in 1971, forming the Teneguía cone. Historical volcanism on La Palma typically produces simultaneous explosive/effusive eruptions producing cinder cones, tephra deposits and lava flow fields. The 2021 eruption was characterized by almost continuous tephra emission along a ~ 1 km long fissure, with only a few phases of quiescence lasting no more than few hours. Up to ten explosive vents were active at different times, with eruptive styles ranging from ash-venting, powerful Strombolian activity and lava fountaining. These formed volcanic plumes occasionally reaching 6-7 km above the growing composite cone. Coeval lava flows with variable, but mostly high effusion rates accompanied the explosive activity. The eruption stopped after 85 days on December 13, 2021.

Studying the textures and morphology of tephra deposits can help define and classify the explosive processes that dominated the rise of magma and its fragmentation. For this reason, from the onset of the eruption, tephra samples were collected daily. These samples represent airfall from the plume, and were collected at distances between 1 and 13 km from the new Cumbre Vieja cone. Samples were used to evaluate the mass load per square meter unit, together with grain-size analysis. During the study period, 87 samples were weighed, giving tephra mass loads between 0.10 kg/m² and 79 kg/m². These values can be used for estimating the total erupted tephra mass within the first period of the eruption. The grain-size distribution of samples was measured at half-phi steps by CAMSIZER (Retsch), and shows significant temporal variation in magma fragmentation and dispersal. This can be correlated to variations in plume height and eruption style.

Componentry analysis on ash samples was performed to assess both style and changes in the explosive activity. Seventeen ash samples (22 September – 1 November) were sieved to separate the 0.25-0.5 mm fraction, which allows easier distinction of particle components. Four types of components were recognized: sideromelane, tachylite, lithics and free crystals, each one characterized by different morphological and textural features. During the opening phase, a high percentage of sideromelane was erupted, then over the following weeks an increase in both lithic particles and variations in morphological features of sideromelane fragments were observed. Fragmentation-related broken crystals within intact particles are also found.

These preliminary results match the variability in eruption styles observed. They confirm that tephra studies may constitute a powerful tool for monitoring ongoing intense eruptions by helping to comprehend, together with other techniques, the evolution of eruption dynamics, magma processes, and magma level in volcanic conduits.