Depósitos de tefra de la erupción del volcán Tajogaite en Cumbre Vieja (2021) (La Palma): Resultados iniciales de magnetismo de rocas

Abstract

The recent eruption of the Tajogaite volcano (La Palma) has allowed the study of basaltic and tephra materials shortly after their accumulation, something generally unusual in volcanic eruptions. The eruption lasted 85 days (September-December 2021), and it is estimated that it generated a volume of about 20 million cubic meters of tephra, although it only represents 15% of the total volume of material emitted. Several studies carried out during the eruption indicate variations in the seismic energy released, lava emission, viscosity, and composition over time, reflecting changes in the eruptive dynamics and conditions of the magma system. It is foreseeable that in a similar way, the lapilli and ash deposits also reflect variations in time, through their magnetic properties. During the 85 days of volcanic emission, tephra deposits accumulated continuously, producing a blanket on the flanks of the Cumbre Vieja edifice and reaching several meters near the SE flank of the volcano. For our study, we took tephra samples during and just after the eruption, in order to carry out rock magnetism analyses of the emitted material. Samples were collected from a tephra profile about 350 cm thick, located 700 meters SW of the emission center. The tephra deposit under study is characterized by the size of lapilli, with intercalations of fine to thick ash layers, barely cemented, which allowed sampling by driving a plastic tube in order to obtain about 20 cubic cm. of tephra per horizon or sampling level. A total of 77 samples were collected, at an interval of about 5 cm. For remanent magnetization (RM, IRM) studies, a 1:1 solution of sodium silicate was used to consolidate the tephra and thus prevent particle movement within small plastic containers. These samples were also used to obtain hysteresis loops and FORC diagrams (“First Order Reversal Curve”). In addition, thermomagnetic curves and susceptibility measurements to variable field and double frequency were obtained. Initial magnetic results show a time variation of properties such as frequency-dependent magnetic susceptibility, suggesting a variation of fine-grained magnetite throughout the eruption. This decrease in susceptibility is compatible with the variation of the hysteresis cycles, which reveal, throughout the eruption, a change from “wasp-waisted” type curves to cycles with coercivities close to zero, consistent with the presence of particles of magnetite in the SP state. The presence of nanometric magnetite crystals could be related to temporary variations in magma viscosity, all together suggesting that rock magnetism can be a valuable tool in the study of the mechanisms of explosive volcanic eruptions.