Abstract

An ongoing challenge in volcanology is to relate conditions of magma ascent to eruption style using information preserved in pyroclastic deposits. Verhoogen [1951] first suggested a correlation between the kinetics of bubble formation and the proportion of magma that is fragmented to ash; Walker [1973, 1981] extended this concept by linking ash formation (fragmentation efficiency) to eruptive style. However, a theoretical basis for this relationship is not well established. Dynamic fragmentation models typically predict only a threshold criterion for fragmentation and not the resulting grain size distribution. In contrast, experiments that constrain relationships between grain size characteristics and fragmentation overpressure consider initial vesicularity but not the dynamics of decompression-related vesiculation. I will examine ways by which we can relate the physical characteristics of individual clasts to the total grain size distributions from which those individual clasts are derived to extend fragmentation models.