Products of magma fragmentation can pose a severe threat to health, infrastructure, environment, and aviation. Systematicevaluation of the mechanisms and the consequences of volcanic fragmentation is very difficult as the adjacent processes cannot beobserved directly and their deposits undergo transport-related sorting. However, enhanced knowledge is required for hazardassessment and risk mitigation. Laboratory experiments on natural samples allow the precise characterization of the generatedpyroclasts and open the possibility for substantial advances in the quantification of fragmentation processes. They hold the promiseof precise characterization and quantification of fragmentation efficiency and its dependence on changing material properties andthe physical conditions at fragmentation.We performed a series of rapid decompression experiments on three sets of natural samples from Unzen volcano, Japan. Theanalysis comprised grain-size analysis and surface area measurements. The grain-size analysis is performed by dry sieving forparticles larger than 250 Am and wet laser refraction for smaller particles. For all three sets of samples, the grain-size of the mostabundant fraction decreases and the weight fraction of newly generated ash particles (up to 40 wt.%) increases with experimentalpressure/potential energy for fragmentation. This energy can be estimated from the volume of the gas fraction and the appliedpressure. The surface area was determined through Argon adsorption. The fragmentation efficiency is described by the degree of fineparticlegeneration. Results show that the fragmentation efficiency and the generated surface correlate positively with the appliedenergy.
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