Work

Shallow magma degassing and crystallisation trigger Subplinian basaltic eruption

Year: 2025

Type: article

Abstract: La Vache et Lassolas are two monogenetic cones located in the south of the Chaîne des Puys (France). They formed during a complex trachybasaltic eruption (8.6 ka) marked by a Subplinian, two violent Strombolian and an effusive phase. We combine physical and textural analyses of pyroclasts from the explosive phases to investigate the effect of shallow magmatic processes on the intensity of this eruption. The results highlight the coexistence of four different textural facies: the crystal poor golden facies (Fg, 59 vol% vesicles; 12 vol% crystals), and three crystals rich facies (14 to 21 vol% vesicles; 38 to 56 vol% crystals), the micro-crystallised facies (Fmc), the crystallised facies (Fc) and the dense facies (Fd). Permeability analyses show that Fmc, Fc, Fd are characterised by a low permeability (< 10 −12 m 2 ). Conversely, Fg is characterised by a high permeability (> 10 −12 m 2 ). Major elements analyses show that the bulk chemical composition of all the facies is identical. We propose that a degassed, crystal-rich magma is mobilised by a gas-rich and crystal-poor magma, from which it derived at shallow depth. This latter acts as an impermeable cap, preventing degassing in the basement. This causes the overpressure of the system, leading to its opening by a violent decompression (0.87 MPa s −1 ), followed by a Subplinian phase. Overall, the variation in emissions of degassed and crystal-rich magma during the four eruptive phases proves that the formation of this magma can be a cause and/or consequence of multi-phase eruptions.

Source: Journal of Volcanology and Geothermal Research

DOI: 10.1016/j.jvolgeores.2025.108517

Authors: Lisa Corrotti, Lucia Gurioli, Carole Berthod, Etienne Médard