Ross, Pierre-Simon; White, James D. L.; Zimanowski, Bernd; Büttner, Ralf
(2008).
Rapid injection of particles and gas into non-fluidized
granular material, and some volcanological implications
Bulletin of Volcanology
, vol. 70
, nº 10.
p. 1151-1168.
DOI: 10.1007/s00445-008-0230-1.
Résumé
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 valve –
therefore 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.
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