Generation of volcanic ash by basaltic volcanism

The recent eruptions of Eyjafjallajökull and Grimsvötn volcanoes in Iceland demonstrate the importance of a better understanding of processes leading to the formation of volcanic ash, specifically of fine volcanic ash that poses a threat to air traffic. Continuous deformation and brittle-type experiments were carried out to better constrain these processes. The studies on short-time continuous deformation behavior of basaltic melt showed viscoelastic properties deviating from hydrodynamic Newtonian models by more than 5 orders of magnitude. High-temperature deformation experiments on basaltic rock samples revealed an increase of elastic strengths as approaching the melting regime, also pointing to a very complex behavior at the solid-ductile boundary. Understanding magma fragmentation from the "liquid" side is a challenge, but meanwhile we propose a pragmatic solution: a thermodynamic model based on fracture mechanics. This model is in agreement with experiments and observations that show that fine volcanic ash is produced by brittle-type fragmentation of magma. A critical material property was defined, characterizing the conditions for brittle fragmentation: the fracture surface energy density, which represents the critical fragmentation energy. Short-term fracture experiments using silicate glass have been performed to investigate the formation of ash-sized particles by brittle failure and to extract this critical physical property, which was found to range between 40 and 130J/m². This value is in good agreement to fragmentation energies determined from experiments using remelted volcanic rocks. Now there is a tool to define critical conditions for the production of volcanic ash of a specific magma type.