Preliminary results on the rheological characterization of the 2021 lava from Cumbre Vieja volcano (La Palma, Canary Islands, Spain)

After half a century of quiescence, activity at Cumbre Vieja volcano (La Palma, Canary Islands, Spain) restarted with a spectacular flank eruption characterized by both high fire fountaining and effusive activity. The products emitted comprise tephra fall and lava flows, ranging from tephrite to basanite. Between September 19th and December 13th 2021 the lava flows covered ~ 12,5 km², affecting more than 3000 buildings and paralyzing the viability and the essential activity on the SW sector of the island. This scenario highlights the importance of rheological data deriving from experimental studies of such low viscosity magma to better understand lava flow emplacement dynamics, hazard and mitigate risk.

We performed a detailed experimental study to characterize the rheology of the basanitic lava sampled between October 3rd and 7th in a Concentric Cylinder set-up. Starting from a superliquidus state of 1400 °C, a set of isothermal deformation experiments was carried out at different target subliquidus temperatures (from 1225 to 1175 °C) and fixed shear-rate of 10 s^-1 to investigate the near equilibrium viscosity. Moreover, a series of cooling deformation experiments were performed at different cooling-rates (ranging from 0.1 to 10 °C/min) and at constant shear-rate of 10 s^-1 with the aim to mimic the dynamic evolution of natural flowing lava through controlled cooling-rate conditions. In isothermal deformation experiments, the steady state conditions (i.e., stable crystal contents) were achieved faster at increasing degree of undercooling, showing a progressive increase in the final viscosity values. In cooling deformation experiments, with increasing cooling-rate applied, the onset of crystallization took place at progressively lower temperature over shorter timescales. The experiments performed at cooling-rates from 0.1 to 1 °C/min were interrupted when viscous rupture (i.e., the transition from coherent flow to shear localization and physical separation) was observed. For the experiments conducted at higher cooling-rates (i.e., from 3 to 10 °C/min), the experimental runs were stopped at viscosity values of ~10⁴ (Pa s), when the stress limit of the device was achieved.

Preliminary results show that the thermal history plays a fundamental role on the kinetics of the crystallization hence modulating the capacity of lava to flow. The different viscosity paths observed at low and high cooling-rates lead to a rheological decoupling between the slow-cooling core and the fast-cooling external part of the lava flows. This process would be key in promoting the transition from pahoehoe to ‘a‘ā emplacement regimes, ultimately controlling the runout distance of lava flows.