Unexpected hazards from tephra fallouts at Mt Etna: The 23 November 2013 lava fountain

Abstract

Hundreds of paroxysmal episodes and a few long-lasting ash-emissions eruptions make Mt. Etna, in Italy, one of the most productive basaltic volcanoes in the world over recent years. This frequent explosive activity certainly gives volcanologists plenty of stimulating scientific material for study. Volcanic hazard from tephra fallout associated with lava fountains is still an issue that has not been fully assessed, albeit having to face this scenario several times in 2013.The 23 November 2013 lava fountain was exceptionally intense despite the short duration of the paroxysmal phase (<1h). Abundant decimetric-sized bombs fell within the first 5–6km from the vent, and a macroscopically thicker and coarser tephra deposit than usual formed between 5 and 25km; in addition, ash was reported to fall up to distances of 400km. The analysis of fallout deposit provided a total erupted mass of 1.3±1.1×109kg (for a mass eruption rate of 4.5±3.6×105kg/s), in agreement with the value of 2.4×109kg estimated by modeling. Grain-size distribution of samples shows poor sorting at least up to 25km from the vent. By comparing dispersal, sedimentological features and physical parameters of the fallout deposit with other lava fountains of Etna, the 23 November 2013 episode may well be one of the largest events of the 21st Century in terms of eruption column height, total erupted mass and mass eruption rate. Furthermore, the impact of tephra on the territory was so high as to make it opportune to introduce a distinction, within the class of lava fountains, between small- and large-scale episodes. This classification can be a starting point for hazard assessment and help prevent the hazards from large-scale lava fountains at Etna in the future.