Debris jets in continental phreatomagmatic volcanoes: A field study of their subterranean deposits in the Coombs Hills vent complex, Antarctica

Abstract

The Ferrar large igneous province of Antarctica contains significant mafic volcaniclastic deposits, some of which are interpreted to fill large vent complexes. Such a complex was re-examined at Coombs Hills to map individual steepsided cross-cutting bodies in detail, and we found several contrasting types, two of which are interpreted to have filled subterranean passageways forcefully opened from below into existing, non-consolidated debris. These transient conduits were opened because of the propagation of debris jets -upward-moving streams of volcaniclastic debris, steam, magmatic gases +/-liquid water droplets -following explosive magma-aquifer interaction. Some debris jets probably remained wholly subterranean, whereas other made it to the surface, but the studied outcrops do not allow us to differentiate between these cases. The pipes filled with country rock-rich lapilli-tuff or tuff-breccia are interpreted to have formed following phreatomagmatic explosions occurring near the walls or floor of the vent complex, causing fragmentation of both magma and abundant country rock material. In contrast, some of the cross-cutting zones filled with basalt-rich tuff-breccia or lapilli-tuff could have been generated following explosions taking place within preexisting basalt-bearing debris, well away from the complex walls or floor. We infer that once focused jets were formed, they did not incorporate significant amounts of existing Mawson debris while travelling through them; instead, incorporation of fragments from the granular host took place near explosion sites. Other basalt-rich tuff-breccia zones, accompanied by domains of in situ peperite and coherent basalt pods, are inferred to have originated by less violent processes.

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

“…The general aims of the experiments were to (a) study the upward propagation of gas-particle dispersions into a clastic host, and (b) learn about how injected particles could form cylindrical or conical bodies analogous to those inferred from fieldwork to exist in some volcanic vent fills. Ross and White (2006) deduced three end-member cases of debris-jet behaviour. It is interesting to compare these scenarios with our experiments.…”

“…Additionally, especially at the shallow depths investigated in these experiments, the host deforms by (i) upward doming because it is pushed by the expanding and buoyant 'bubble'; (ii) outward lateral flow of the domed material; (iii) expansion of the domed material because of upward momentum from the bubble and inter-particle air flow in the host; and (iv) inward granular flow at the base of the model, following the passage of the 'bubble'. The propagation of debris jets through granular hosts therefore seems much more complex than Ross and White (2006) envisaged, fully justifying the need for experimentation, even if imperfectly scaled.…”

“…3, Table 3), without producing subaerial eruptions (see White and McClintock 2001;Ross and White 2006). An expanding 'bubble' of air + red beads eventually collapses because the gas escapes through the permeable roof; meanwhile the red beads sediment into the transient cavity, which is also closing laterally because of inward-directed granular flow of the host at the base of the container.…”

“…1). Ross and White (2006) described the shape and size of some of these bodies from a vent complex in Antarctica and proposed theoretical end-member possibilities for the behaviour of debris jets travelling through a granular host. The manner in which debris jets propagate through clastic hosts is still poorly constrained, and so is the manner in which debris are deposited from the jet, or following the jet's passage, to produce the roughly cylindrical or conical body.…”

Geological evolution of the Coombs–Allan Hills area, Ferrar large igneous province, Antarctica: Debris avalanches, mafic pyroclastic density currents, phreatocauldrons

“…First, there are suggestions that major igneous intrusions were emplaced before the start of explosive mafic eruptions (to trigger the m 1 debris avalanche, and preserved as globular 'megablocks' of 'basalt' in m 1 ). Second, we know that igneous intrusions were emplaced during 'Mawson time' in the Coombs Hills vent complex; some were partially fragmented to form peperite and 'basalt'-rich tuff-breccia zones, with leftover 'basalt' pods Ross and White, 2005a). Third, various igneous intrusions cross-cut Mawson Formation volcaniclastic rocks: (a) thick sills at Allan Hills (Grapes et al, 1974), and perhaps at Coombs Hills (unexposed, but could exist at depth if the model of Ross and White, 2005b for the creation of clastic dikes via elutriation of fine particles above dolerite sills is correct); (ii) plugs, dikes, and thin sills at both Coombs and Allan Hills.…”