INSIGHTS INTO MAGMA FRAGMENTATION FROM ACOUSTIC EMISSION MONITORING OF LABORATORY EXPERIMENTS

Rapid decompression of magma during ascent leads to fragmentation of the magma and gas propelled ejection of the fragments. These volcanic explosions generate distinctive earthquakes, whilst propelling hazardous ash particles into the atmosphere. The fragmentation process can be studied in the laboratory through pressurisation of a magma sample in a shock-tube apparatus, which is then rapidly depressurised to atmospheric pressure. The initial applied pressure required to induce fragmentation of different magmas during this rapid depressurisation reveals the pressure drop required to fragment them. The size distribution of the fragmented particles, which is analysed after these experiments, is related to the surface energy used to generate these fragments.

Here, we conduct rapid depressurisation experiments on phonolitic magmas whilst recording acoustic emissions (AE). AE are elastic waves transmitted through solid media; a laboratory analogue to earthquakes. Complete waveforms of the whole fragmentation process were recorded using an array of transducers connected to the sample via metal waveguides. This allowed us to locate the initiation of the fragmentation process, and to determine the energy, frequency content, and amplitude of different stages of the process. Experiments were conducted with pressure drops that induced full fragmentation, partial fragmentation, and no fragmentation of the magma samples; in addition to experiments with excess pressure to that required for fragmentation. The energies of the recorded AE correlate with the energy stored in the pore space prior to fragmentation and to the fragmentation energy calculated from the resulting particle sizes. These results are compared to volcanic explosion earthquakes, revealing details of how characteristics of these earthquakes are linked to different aspects of the fragmentation process and ash generation.