The transition from summit to flank activity at Mt. Etna, Sicily (Italy): Inferences from the petrology of products erupted in 2007-2009

Mt. Etna is an active basaltic volcano where both flank and summit eruptions take place. In recent decades, the South-East summit crater (SEC) has in particular been characterized by 'episodic' eruptions, consisting of recurrent lava fountains associated with lava flow emissions and lasting from a few weeks to months. Recent volcanic activity shows that, if there is a transition from an episodic summit eruption to a flank eruption, then this usually takes place while the last paroxysm of the episodic eruption is still underway. By contrast, the 2007-08 episodic eruption at SEC was followed by the 2008-09 flank activity which started three days after the conclusion of the last paroxysm, coinciding with the strong earthquake (M = 7.9) in Sichuan (China), whose perturbations were recorded by the monitoring network of Mt. Etna. We therefore investigated the transition from a summit episodic eruption at SEC to flank activity utilizing a petrologic study of the products erupted from 2007 to 2009, integrated with literature data. The compositional variability of the products may largely be explained by a mixing between an evolved magma stored in the SEC reservoir and a more primitive magma which intrudes it. In the studied period, the most significant episodes of magma recharge occurred before the onset of the 2007-08 episodic eruption and during the 2008-09 flank activity, more precisely before June 2008. According to previous studies, the seven paroxysms of the 2007-08 episodic eruption at SEC have been interpreted as resulting from the disruption of a foam layer at the top of the SEC reservoir which was rebuilt before each subsequent episode. The transition from the 2007-08 episodic eruption at SEC to the 2008-09 flank activity was essentially triggered by the Sichuan earthquake which caused a variation of dynamic stress. It caused the volatile exolution, the pressurization of SEC stored magma, and the consequent fracturing of the surrounding rocks where magma intruded to then be erupted during the 2008-09 flank activity.