Rapid injection of particles and gas into non-fluidized granular material, and some volcanological implications

Abstract

In diatremes and other volcanic vents, steep bodies of volcaniclastic material having differing properties (particle size distribution, proportion of lithic fragments, etc.) from those of the surrounding vent-filling volcaniclastic material are often found. It has been proposed that cylindrical or cone-shaped bodies result from the passage of "debris jets" generated after phreatomagmatic explosions or other discrete subterranean bursts. To learn more about such phenomena, we model experimentally the injection of gas-particulate dispersions through other particles. Analogue materials (glass beads or sand) and a finite amount of compressed air are used in the laboratory. The gas is made available by rapidly opening a valvetherefore the injection of gas and coloured particles into a granular host is a brief (<1 s), discrete event, comparable to what occurs in nature following subterranean explosions. The injection assumes a bubble shape while expanding and propagating upwards. In reaction, the upper part of the clastic host moves upward and outward above the 'bubble', forming a 'dome'. The doming effect is much more pronounced for shallow injection depths (thin hosts), with dome angles reaching more than 45°. Significant surface doming is also observed for some full-scale subterranean blasts (e.g. buried nuclear explosions), so it is not an artefact of our setup. What happens next in the experiments depends on the depth of injection and the nature of the host material. With shallow injection into a permeable host (glass beads), the compressed air in the "bubble' is able to diffuse rapidly through the roof. Meanwhile the coloured beads sediment into the transient cavity, which is also closing laterally because of inward-directed granular flow of the host. Depending on the initial gas pressure in the reservoir, the two-phase flow can "erupt" or not; non-erupting injections produce cylindrical bodies of coloured beads whereas erupting runs produce flaring upward or conical deposits. Changing the particle size of the host glass beads does not have a large effect under the size range investigated (100-200 to 300-400 µm). Doubling the host thickness (injection depth) requires a doubling of the initial gas pressure to produce similar phenomena. Such injections -whether erupting or wholly subterranean -provide a compelling explanation for the origin and characteristics of multiple cross-cutting bodies that have been documented for diatreme and other vent deposits.

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.…”

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].…”

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.…”