Abstract
Fragmentation dynamics can be described by the size distribution of the fragments generated during an explosion. We approach the problem of magma fragmentation during a Strombolian explosion using the grain size distribution (GSD) of a ballistically-emplaced lapilli-and-bomb deposit. We present a novel method of collecting juvenile fragments landing in a well-defined near-vent zone (around 50 m from the source). The method uses four plastic sheets (tarpaulins or tarps) deployed below the SW crater rim of Stromboli volcano (Aeolian Islands, Italy). Due to the vicinity of the collection site to the vent, the fragments were still hot when they landed and created a map of melt holes on the sheets. This map was processed to calculate cumulative fragment size distributions (FSD) and grain size distributions (GSD). The cumulative FSD (number based) and GSD (mass-based) are best described by unique power laws with fractal dimensions of 1.40 and 2.03, respectively, testifying to the fractal nature of fragmentation. The fractal dimension is consistent with low energy fragmentation of a brittle material. This method represents a new way of measuring fragmentation dynamics and classifying volcanic eruptions according to their fragmentation energy.
Highlights
Fragmentation is a physical process that occurs commonly in everyday life
Rocks fragmented by an artificial explosion show a power law relationship between the size of the fragments and their frequency distribution, where the conditions of fragmentation can be quantified by measuring the fractal dimension Df following (Mandelbrot, 1982): N (r > R) ∼ C·R−Df
Rare data, to answer three questions: (i) what is the fragmentation mechanism for this common type of volcanic explosion, (ii) how does the energy compare with other volcanic, and non-volcanic, fragmentation events, and (iii) can the fractal dimension be used as a means for enhanced classification of explosive eruptions?
Summary
Fragmentation is a physical process that occurs commonly in everyday life. It applies to many processes in science and technology. We here present a unique methodology for sampling the deposits of such small explosions, and discuss the resulting fragment size distribution in terms of the explosive process, where the approach is tested at the most famous strombolian system: Stromboli (Aeolian Islands, Italy) We use these new, and rare data, to answer three questions: (i) what is the fragmentation mechanism for this common type of volcanic explosion, (ii) how does the energy compare with other volcanic, and non-volcanic, fragmentation events, and (iii) can the fractal dimension be used as a means for enhanced classification of explosive eruptions?. This linear relationship is valid only for very small fragments, having an area no larger than 3.2 × 10−3 m2, which corresponds to an equivalent diameter of 64 mm and a mass of 23 g This allows reconstruction of the grain size distribution from the holes in the tarps for areas
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