Application of fractal fragmentation theory to natural pyroclastic deposits: Insights into volcanic explosivity of the Valentano scoria cone (Italy)

The extent of fragmentation triggered by basaltic volcanic eruptions has been studied by analyzing the grain size distribution of natural pyroclastic deposits and applying the fractal fragmentation theory. For this purpose, 75 samples have been collected from individual layers along horizontal section of the well-exposed Valentano scoria cone (Italy). The cone is constituted by well-defined layers of fall deposits of up to 180 cm in thickness. An important feature of the stratigraphic sequence is the occurrence of several intercalated layers (called breccias) with clasts of different characteristics (angular shape, low porosity, and high crystallinity) in respect to the “normal layers”. Results indicate that all achieved grain size distributions show fractal behavior and can be clearly correlated with a single fragmentation event, namely the magmatic fragmentation during the eruption. The fractal dimension of fragmentation (D) increases as fragmentation efficiency increases.Values of D are found to vary significantly along the pyroclastic sequence defining two major trends: (1) a general, long-range increase of D during the course of the eruption and (2) a superimposed high-frequency oscillating variation of fragmentation efficiency. Noteworthy is the fact that the highest values of D are measured for pyroclastic deposits following breccia deposits. Application of a conceptual model of fractal fragmentation, as claimed by natural data, indicates that larger values of D have to be associated with a higher probability of fragmentation, thus suggesting that the D value can be used as a proxy for the “fragility” (i.e. ability to fragment) of samples. Fractal dimension of fragmentation of single layers is positively correlated with the vesicularity of clasts. This indicates that the bubble content in the ascending magma prior to and at fragmentation likely was the key factor modulating the variable fragmentation efficiency observed in the studied pyroclastic sequence. We hypothesize that the long-range variation of D may be associated with magma fragmentation triggered by expansion of gas bubbles progressively coalescing below solid plugs during later stages of eruptive activity, whereas high-frequency variations of D are interpreted as due to magma fragmentation during the “slug flow” regime characterizing the strombolian activity of the scoria cone.

Research highlights
► Scoria cone pyroclastic deposits have fractal grain size distributions
► Efficiency of fragmentation is quantified by fractal dimension of fragmentation
► Fractal dimension of fragmentation is positively correlated with sample porosity
► Fractal analysis provides information on the time evolution of the eruption