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Authors: Corsaro, R. A.* 
Metrich, N.* 
Allard, P.* 
Andronico, D.* 
Miraglia, L.* 
Fourmentraux, C.* 
Title: The 1974 flank eruption of Mount Etna: an archetype for sub-volcano dyke (SVD) eruptions at basaltic strato-volcanoes and a benchmark in recent Etna’s history
Issue Date: 10-Dec-2008
Keywords: Mt. Etna
Subject Classification04. Solid Earth::04.04. Geology::04.04.07. Rock geochemistry 
Abstract: Flank eruptions of Mt. Etna mostly result from drainage of plagioclase-rich magma from the central volcano conduits, but rarer ones emitting plagioclase-free aphyric magma, are driven by dyke propagation from beneath the volcano, bypassing the central conduits. An archetype of such events is the January-March 1974 eruption that developed in two explosive phases. New data for the mineralogy and geochemistry of the 1974 products and the coeval earthquake distribution, indicate that the eruption was likely triggered by deep tectonic fracturing that allowed a dyke to propagate from ~11 km depth below the summit at a minimal rate of 10-2 m s-1. We point to initially fast, closed system ascent of the volatile-rich magma, leading to lava fountains during the very first stage of the two eruption phases, followed by slower magma rise and increasing gas-melt separation (strombolian explosions) within the upper portion of the feeder dyke. The anomalous K-rich composition but also distinct Rb/Th, Cl/K and S/Cl ratios of the 1974 trachybasalts, tracked a recent mixing of 25% pre-existing low K-trachybasalts with a 75% new alkali-rich magma whose almost pure extrusion occurred in 2001-2003. This new input was first revealed by the 1974 event. We propose an updated terminology - sub-volcano dyke (SVD) eruptions - for 1974-type flank eruptions on Etna and other basaltic strato-volcanoes, which focuses on their driving mechanism and thereby allows us to distinguish them from the more common central conduit dyke (CCD) flank eruptions. We ultimately discuss possible criteria to forecast SVD-type flank eruptions on Etna.
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