A Study of Cyclic Variation in the Properties of the Shallow Conduit Melt at Stromboli Volcano in

Stromboli volcano (Aeolian Islands, Italy) is generally considered a steady-state volcano among scientists and the public, with the current pattern of activity dating to at least the 7th century A.D. This reputation conceals the fact that typical activity at Stromboli fluctuates among ashing/puffing, weak explosions, and moderate/high intensity Strombolian explosions on time scales spanning minutes to months. Although Stromboli volcano has been intensely studied in recent years, the cause of such fluctuation remains largely unexplained. Our study compliments current knowledge by considering that evolving properties of the shallow melt phase are likely to influence eruptive activity. We present data on the microtextural characteristics of fresh ejecta from Stromboli volcano during 3 phases of variable eruption intensity in 2002. Activity shifted from mild, dominantly ash-charged explosions (January-May) into a phase of intense explosions (June-December), followed by an abrupt onset of the 2002/2003 effusive eruption (28 December). Our sets of lapilli from May, September/October, and December track changes in the shallow melt over this period. The observed textures indicate that this melt was a dynamic mixture of varying proportions of actively vesiculating magma, and stagnant, partly outgassed magma. Explosions in Sept/Oct and Dec ejected clasts composed of either type of melt independently, in addition to clasts formed from fine-scale mingling of both the actively vesiculating and stagnant magmas. In May only stagnant melt was ejected. We present observational and quantitative (vesicle size, chemistry, proportions) data for the ejecta, which highlight changes in properties of the shallow melt with time. We link this information with the variable explosive patterns witnessed at Stromboli in 2002.