Fragmentation mechanism of vesicular magma in soild/fluid transition resime

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Magma fragmentation is a key phenomenon controlling volcanic eruptions. The fragmentation of magma, which is a viscoelastic fluid, occurs through a combination of viscoelasticity and rapid deformation. We conducted a rapid decompression experiment to clarify the viscoelastic effect on the fragmentation using a magma analogue, syrup containing gas bubbles. We demonstrated the existence of a brittle-like fragmentation, which occurred even if the response of material should be in a ductile manner. Comparing the realistic decompression time with the viscoelastic relaxation time for magma, it is probable that the fragmentation in the real volcanic system occurs in a brittle-like manner.Observation indicated that the onset of brittle-like fragmentation was triggered by the sudden release of a considerable amount of gas from a crack in the specimen. Further observation implies that the crack is initiated from the interior of the specimen due to non-uniform spatial distribution of bubbles.