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