Explosive injection of gas-particle dispersions into a non-fluidized granular host: volcanological implications

Discrete explosive bursts, or explosions, are known from many volcanic eruptions. In maar-diatreme eruptions, they have occurred in debris-filled volcanic vents when magma interacted with groundwater, implying that material mobilized by such explosions passed through the overlying and enclosing debris to reach the surface. Although other studies have addressed the form and characteristics of craters formed by discrete explosions in unconsolidated material, no details are available regarding the structure of the disturbed debris between the explosion site and the crater floor. Field studies of diatreme deposits reveal cross-cutting, steep- sided zones of non-bedded volcaniclastic material that have been inferred to result from "debris jets" driven by explosions. In order to determine the general processes and deposit geometry resulting from discrete, explosive injections of entrained particles through a particulate host, we ran a series of analog experiments in which specific volumes of compressed (5-20 MPa) air were released in bursts that drove gas-particle dispersions through a particulate host. The air expanded into and entrained red particles in a small crucible before moving upward into the white beads used for the host. Injected particles and the overlying host material were glass beads of identical size, shape and density. The experiments revealed that each burst drove into the host an expanding cavity containing air and colored particles, which rapidly collapsed as the driving gas escaped through the permeable roof. Total duration of each run, recorded with high-speed video, was approximately 0.5-1 second. As gas slowed and escaped the cavity the colored beads sedimented into the transient cavity, which was also closing laterally because of inward-directed granular flow of the host. This same behavior was observed even in runs where there was no explosive breaching of the surface, and no colored beads ejected. A columnar body of colored beads was left that is similar to the cross-cutting pipes observed in deposits filling real volcanic vents, in which cavity collapse can result not only from gas escape through a granular host as in the experiments, but also through condensation of water vapor formed in phreatomagmatic explosions. The key conclusion from these experiments is that the geometry of crosscutting granular deposits is not directly informative of the geometry of "intrusion" that formed them. An additional conclusion is that complex structures can form quickly from discrete events.