Dike Emplacement Triggering the 2001 Etna's Flank Eruption

On 12 July a seismic swarm in the upper southern flank of Etna, between La Montagnola and the summit craters, heralded the start of a new flank eruption. The emplacement of a new feeder dyke, occurred while the summit of the volcano was still the site of persistent eruptive activity, was confirmed by a number of data, spanning from seismicity, ground deformation, petrology, volcanology, geomagnetism, gravity, tectonics and gas geochemistry. The dike emplacement was marked by the seismic swarm composed by more than 2500 events mostly concentrated in the few days preceding the opening of the eruptive fissures. Ground deformation data from permanent GPS and tilt networks showed marked variations in concomitance with the seismic swarm. The deformation pattern inferred the dike penetration. Magnetic data revealed a sharp and intense decrease in the total geomagnetic field on the upper southern flank, and an increase on the upper northern flank. The Serra La Nave continuously-running gravity station (southern slope of the volcano; 1740 m elevation) recorded a sudden gravity increase, starting on 12 July. The result on the surface was the opening of a 6-km-long field of fractures that intersected the summit of the volcano from north (Valle del Leone: 2600 m a.s.l.) to south (Rifugio Sapienza: 2100 m a.s.l.). Four eruptive fissures opened between 17 and 19 July on the south and north flanks of the volcano, from the SE Cone down to 2600 m, producing small lava flows. These formed the upper fissure system. An additional vent opened on 18 July at 2100 m elevation on the south flank producing a large lava flow that spread south towards the village of Nicolosi and traveled 6.5 km. Explosive activity, increased by the intersection of the feeder dike with a shallow aquifer, since 19 July gave rise to a new cone just north of Montagnola at 2550 m elevation. This new cone built up as a result of alternating phreatomagmatic and magmatic explosive and effusive activity. The new cone at 2550 m and the 2100 m vent formed the lower fissure system. Petrology of tephra and lava flows, and SO2/HCl ratios of magmatic gases measured by FTIR, evidenced a marked chemical distinction between the upper and lower fissure systems related to the two distinctive plumbing systems. The eruption ended on 9 August with a gradual reduction in the effusion rate and a marked decrease of the SO2 flux from the four summit craters.