Visualization of Subsurface Magma Fragmentation in Explosive Volcanic Eruption by Analogue Experiments

Abstract

Laboratory experiments help visualizing subsurface volcanic processes of which direct observation is difficult. We have explored "fragmentation wave" that has been believed to occur in magma during an explosive eruption. Shock-tube apparatuses are used to apply rapid decompression to specimens made of silicone compound and volcanic rocks containing bubbles. We successfully visualized propagating fragmentation fronts, which were exactly like the image of the volcanic processes. However, applying the results to magma fragmentation was found difficult, because of significant effects of adhesion of the specimen to the tube wall and difference in viscoelastic properties between flowing magma and the specimens. The difficulties have overcome by the experiments using porous syrup specimens. We have found a particular fragmentation behavior characterized by significant time delay after decompression and by successive explosions. Combining the fragmentation experiments with X-ray radiography and computed tomography, we are now approaching ductile processes inside the specimen that may control the speed of fragmentation.