The complete fragmentation history of a strombolian eruption revealed using new bomb mapping method

During June 2008, a multidisciplinary experiment was deployed at Stromboli volcano (Aeolian Islands, Italy). Our aim was to better define the magma fragmentation associated with the mildly explosive activity for which Stromboli is famous and to improve our ability to place Stromboli in a granulometric classification of explosion products. Four tarps were deployed just below the rim of Stromboli’s SW crater to collect ash, lapilli and scoria from a single explosive event. Clasts landing on the tarps burnt holes in the plastic, effectively creating a map for the clast distribution across each area. These “maps” could be paired with geophysical data, such as seismic tremor, infrasonic pressure and thermal images, for the same events. After retrieval, digital photo mosaics were created for each tarp. The images were processed to obtain scoria size distributions based on the size of the holes left by scoriae. The scoria size distribution followed a power law revealing that the magma fragmentation process was not random, but was controlled by a single mechanism. In addition, the distributions must be the result of a single explosion, or derived from multiple explosions with identical mechanisms. Small scoria clasts that remained stuck to the tarps were measured and weighed, allowing us to obtain a reliable weight-area relationship. Using this relationship, cumulative area-distribution data were converted in cumulative mass data with the aim of obtaining mass balances.