Model experiment on magma fragmentation in explosive volcanic eruption

Abstract

A new experimental facility has been developed to investigate the fragmentation of vesicular, magma undergoing rapid decompression. The facility based on a vertical shock tube was designed and constructed to produce starch syrup foams of high void fraction but low permeability, high liquid viscosity, and at high pressure as alternative to foamy magma exposed to expansion waves. The highly viscous material (starch syrup) of dynamic shear viscosities ranging from 5 to 1012 Pa s were intermixed with nitrogen gas bubbles under 1.6 to 2.6 MPa and were subsequently loaded into an acrylic test section. The foamy starch syrup was pressurized up to 2.6 MPa and then rapidly decompressed to 0.1 MPa, which caused its fragmentation and simultaneous ejection of the fragmentation products into a large-volume chamber. In ejecting high-pressure foam into low-pressure chamber, the foam's fragmentation process was characterized with the help of pressure measurements and high-speed video recording. Prior to decompression experiments, we examined the viscoelastic properties of the foam specimens by using a rheometer. The rapid decompression generated wide varieties of fragment shapes similar to those observable in volcanic explosions, such as elongated to blocky pumice and flat, pointed shards with smooth or conchoidal fracture. The results prove that a shock dynamic approach is effective to model magma fragmentations.