Multiphase flow above explosion sites in debris-filled volcanic vents: Insights from analogue experiments

Abstract

Discrete explosive bursts 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 surface. Field studies of diatreme deposits reveal cross-cutting, steep-sided zones of non-bedded volcaniclastic material that have been inferred to result from sedimentation of material transported by "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 analogue experiments. Specific volumes of compressed (0.5-2.5 MPa) air were released in bursts that drove gas-particle dispersions through a granular host. The air expanded into and entrained coloured particles in a small crucible before moving upward into the host (white particles). Each burst drove into the host an expanding cavity containing air and coloured particles. Total duration of each run, recorded with high-speed video, was approximately 0.5-1 second. The coloured beads sedimented into the transient cavity. This same behaviour was observed even in runs where there was no breaching of the surface, and no coloured beads ejected. A steep-sided body of coloured 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 vapour. A key conclusion from these experiments is that the geometry of cross-cutting volcaniclastic deposits in volcanic vents is not directly informative of the geometry of the "intrusions" that formed them. An additional conclusion is that complex structures can form quickly from discrete events.

The importance of the transport system in shaping the growth and form of kimberlite volcanoes

“…In combination, the characteristics of the polymict, poorly sorted, non-bedded deposits in diatremes are inferred to be most consistent with transport by downward mass movement and upward moving debris jets (McClintock and White, 2006;Ross and White, 2006;Ross et al, 2008bRoss et al, , 2008c. Because eruption bursts are episodic and mostly happen toward the base of the diatreme, transport of debris follows a cycle from upward-dominated transport to downward-dominated transport.…”

Maar-diatreme volcanoes: A review

“…Debris jets propagate within the existing diatreme fill and may or may not reach the surface. Experimental studies can be used to illustrate the processes at work (Ross et al, 2008b(Ross et al, , 2008c. With time and repetition, debris jets due to episodic bursts will not only create columnar bodies, but will also contribute to homogenizing the vent fill and destroying any bedding features in the lower diatreme.…”

“…25 Debris jet An upward moving mixture of pyroclastic debris and gasses, ±liquid water, enclosed by existing non-fluidized nonconsolidated diatreme fill, generated by an explosion within the diatreme or root zone (Ross and White, 2006;Ross et al, 2008bRoss et al, , 2008c During. pg.…”

Neogene-Quaternary Volcanic forms in the Carpathian-Pannonian Region: a review

“…Diatremes "store" potentially the most valuable information on how the magma and water interacted and how the resulting fragmented rocks are transported away from their fragmentation site to the deposition sites surrounding the craters. The mode of transportation of pyroclast from the explosion site is much under debate and involves various processes generating fluidisation, debris jet formation and various vertically moving pyroclastic density current activities [78,[172][173][174][175][176][177].…”