From Cyclic to Paroxysm: the cryptic paroxysmal eruption of Etna, December 2023

Basaltic and basaltic-andesite open-conduit volcanoes offer valuable opportunities to explore the mechanisms governing transitions from Strombolian activity to more intense paroxysmal events, such as lava fountains. Strombolian explosions can escalate into lava fountaining, a process characterized by surface fragmentation under choked-flow conditions, requiring low-viscosity, rapidly ascending basaltic magma. While strongly cyclic behaviour is common in Strombolian and lava-fountaining activity, it is rarely captured in geophysical datasets.

Beginning on November 12, 2023, the South-East Crater of Etna volcano exhibited cyclic Strombolian activity that culminated in paroxysmal events on December 2, 2023. This study exploits geophysical signals recorded by the monitoring network of the Istituto Nazionale di Geofisica e Vulcanologia (INGV) to investigate the processes and timescales driving this eruptive sequence. The activity featured cyclic clusters of Strombolian explosions lasting 15–20 minutes, recurring every 50–70 minutes. Analysis of seismo-acoustic data reveals continuous, repetitive, and highly regular energy and volume emissions. Within each cycle, a systematic increase in both explosion frequency and amplitude was observed.

This behaviour is interpreted through a model involving foam collapse in the shallow conduit of the South-East Crater, which regulates degassing processes. The transition to paroxysmal activity occurs when the barrier trapping gas bubbles dissipates, enabling the choked flow conditions and driving the eruption. This study provides critical insights into the cyclic eruptive behaviour of basaltic volcanoes and contributes to a broader understanding of volcanic degassing dynamics and paroxysmal transitions.