The electrical signature of mafic explosive eruptions at Stromboli volcano, Italy
Caron E. J. Vossen, Corrado Cimarelli, Alec J. Bennett, Markus Schmid,
Ulrich Kueppers, Tullio Ricci & Jacopo Taddeucci
Volcanic lightning is commonly observed in explosive volcanic eruptions of Volcanic Explosivity Index (VEI) > 2 and can be detected remotely providing real-time volcano monitoring information. However, little is known about the electrical activity accompanying the lower-magnitude spectrum of explosive eruptions, often involving mafic magmas. We narrow this gap in knowledge by presenting the electrical signature of the explosive activity (VEI ≤ 1) of Stromboli volcano (Italy) recorded by an electrostatic thunderstorm detector. The persistent eruptive activity of mild Strombolian explosions is occasionally interrupted by larger-scale major explosions and paroxysmal events.
Here, we present electrical observations of three major explosions and unprecedented measurements of the 3 July 2019 paroxysm. The electrical signals of the major explosions show apparent similarities, with movements of charge and tens of electrical discharges, arising the question of whether these observations could be used to supplement the classification scheme of explosions on Stromboli. The electrical signals from the 3 July 2019 paroxysm exceed those from the major explosions in amplitude, discharge rate and complexity, showing characteristic variations during different phases of the eruption.
These results show that also impulsive lower-magnitude explosions generate detectable electrical activity, which holds promise for monitoring low VEI activity at mafic volcanoes.
Table 1. Eruption parameters and electrical measurements for the 25 June 2019, 19 July 2020 and 16 November 2020 major explosions and the 3 July 2019 paroxysm.
Electrical activity and lightning in volcanic plumes. Explosive volcanic eruptions generate changes in the electric field and volcanic lightning. An important component of plume electrification is the presence of silicate particles, which are considered the main carrier of electrical charge in volcanic jets and plumes. Upon explosion, the fragmentation of magma into pyroclasts (fragments of silicate melts) and their subsequent collisions generate high electrical charge in the expanding volcanic jets. Under specific conditions, also ice nucleation/riming, interaction with (sea)water and, to a lesser extent, natural radioactivity may contribute to plume electrification. Besides these external effects, the distribution of charges in the evolving plume creates the conditions for electrical discharges to occur.
Volcanic plume electrification and lightning is commonly observed at volcanoes characterized by a Volcanic Explosivity Index (VEI) >, therefore being generally associated with intermediate to high-silica magma compositions. The occurrence of volcanic lightning during explosive eruptions of basaltic composition was reported for a wide range of plume heights (1–21 km), although considerably fewer reports were found for eruptions of VEI ≤ 115, such as Strombolian explosions. Albeit experiencing brittle fragmentation, the reduced ability of such magmas to produce lightning activity is generally attributed to the low viscosity and high temperature of low-silica magmas which promotes outgassing and gas-magma decoupling, often resulting in effusive eruptions or mild explosions. However, specific processes, such as molten fuel–coolant interaction (MFCI)10, the combination of strong magma foaming and geometrical obstructions, the obstruction of the conduit through talus accumulation or the presence of a magma plug, can result in enhanced explosivity and consequently generate more volcanic lightning. High-temperature experiments have investigated the production of electrical signals of basaltic magma upon fragmentation. The experiments indicate that formation of new surface area and subsequent particle cloud expansion generate charge separation, which can be detected on a short timescale as an electrostatic field gradient.
Electrostatic field gradients comparable to those measured in the experiments were detected during mild Strombolian explosions20,21.
Additionally, an electric potential gradient accompanied by electrical discharges was recorded during an ash-rich major explosion on 7 September 2008.
To further increase our knowledge of electrical activity accompanying basaltic eruptions, we carried out
long-term electrostatic field measurements on Stromboli volcano, Italy.