Insights into magmatic fragmentation: a revised fragmentation model based on high-speed video analysis of rapid decompression of magma

It is the fragmentation of magma which distinguishes explosive volcanic eruptions from the more quiet effusive form of volcanic activity. Hence the conditions leading to magma fragmentation as well as the fragmentation process itself are key points in order to better our understanding of the dynamics of volcanic eruptions. Better understanding of eruption dynamics will lead in return to improved hazard mitigation. Fragmentation experiments have been performed on abroad variety of natural magma and varying analogue materials, pointing out important correlations between magma properties and fragmentation behavior but also posed new questions. We performed rapid decompression experiments of vesicular natural magma and analyzed the fragmentation process from high-speed video recordings of the experiments. In the classical understanding of magmatic fragmentation one fragmentation wave travels through the magma causing “layer-by-layer” fragmentation. Our observation showed that this model had to be revised: Within the investigated energy range the process of layer-by-layer fragmentation due to vesicle bursting is overriding, but it does not propagate as one fragmentation front through the sample, it rather occurs in several generations. The number of such generations depends in first order on the applied energy, but is also influenced by sample properties as i.e. permeability. The secondary internal fracturing of already fragmented parts in the same manner as the primary fracture could not be explained by previous models. We build a theoretical model to describe the experimental results, and show that it is capable of describing both, the primary sequence of fracturing, and the secondary intra-block fracturing. The model allows us to suggest a practical criterion for when such explosions occur.