The influence of geometry on the ascent of magma in open fissures

Giberti, G.; Wilson, Lionel

doi:10.1007/bf00301532

Cited by 51 publications

(25 citation statements)

References 9 publications

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“…The vertical gas velocity in the jet is a maximum on its centre-line and goes to zero at the edge of the vent. In general, there is a non-zero lateral velocity in the jet which arises due to variations in the width of the conduit system as it approaches the surface-conduits commonly flare outward as they reach surface vents (Wilson & Head 1981; Giberti & Wilson 1990). The lateral velocity is zero on the centre-line of the jet (by symmetry) and has its maximum at the edge of the vent.…”

Section: Introductionmentioning

confidence: 99%

Explosive volcanic eruptions-VIII. The role of magma recycling in controlling the behaviour of Hawaiian-style lava fountains

Wilson

Parfitt

Head

1995

Geophysical Journal International

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S U M M A R YExplosive eruptions of mafic magmas produce lava fountains whose heights are a function o f the exsolved volatile content of the magma, its erupted mass flux, and the geometry of the vent (which may be an elongate fissure or a localized. near-circular conduit). The geometry of the initial vent (and the cruptive behaviour) can be distinctly modified by lava drainback and accumulating ejecta. Hot pyroclasts landing near the vent may coalesce to form rootless flows, some of which may drain back into the vent to be recycled into the eruption products. Rootless flows may be at least partially confined by preexisting topographic features, or by spatter or cinder ramparts being built up by the eruption itself, so that they accumulate into a lava pond over and around the vent. The erupting jet of magmatic gas and pyroclasts must force its way through such a pond and will entrain some of the pond lava as it does so. The energy expended in entraining and accelerating previously crupted materials will reduce the eruption velocity and the lava fountain height by an amount which can be calculated as a function of the eruption conditions and the lava pond depth (or lava drainback rate). The results of such calculations are presented, and are used to assess the influence of this process on attempts to infer magma volatile contents from field observations of lava fountain heights.

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Section: Introductionmentioning

confidence: 99%

Explosive volcanic eruptions-VIII. The role of magma recycling in controlling the behaviour of Hawaiian-style lava fountains

Wilson

Parfitt

Head

1995

Geophysical Journal International

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“…The density of the liquid magma is specified as input.Friction factor. Following previous eruption modellers[Wilson et al, 1980;Giberti and Wilson, 1990;Dobran, 1992], I calculate the friction factor f using an equation that sums a friction term associated with wall roughness fo with one that depends on the Reynolds number for flow (Re):where D is the conduit diameter and TI is the viscosity of the erupting mixture. The variable fo is empirically derived; D is specified as input; and the velocity •, is calculated from the mass conservation equation, m -p •,`4 = const, where m is the mass flux through a given cross section.Viscosity.…”

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confidence: 99%

Evidence for water influx from a caldera lake during the explosive hydromagmatic eruption of 1790, Kilauea volcano, Hawaii

Mastin¹

1997

J. Geophys. Res.

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Abstract. In 1790 a major hydromagmatic eruption at the summit of Kilauea volcano, Hawaii, deposited up to 10 m of pyroclastic fall and surge deposits and killed several dozen Hawaiian natives who were crossing the island. Previous studies have hypothesized that the explosivity of this eruption was due to the influx of groundwater into the conduit and mixing of the groundwater with ascending magma. This study proposes that surface water, not groundwater, was the agent responsible for the explosiveness of the eruption. That is, a lake or pond may have existed in the caldera in 1790 and explosions may have taken place when magma ascended into the lake from below. That assertion is based on two lines of evidence: (1) high vesicularity (averaging 73% of more than 3000 lapilli) and high vesicle number density (105-107 cm -3 melt) of pumice clasts suggest that some phases of the eruption involved vigorous, sustained magma ascent; and (2) numerical calculations suggest that under most circumstances, hydrostatic pressure would not be sufficient to drive water into the eruptive conduit during vigorous magma ascent unless the water table were above the ground surface. These results are supported by historical data on the rate of infilling of the caldera floor during the early 1800s. When extrapolated back to 1790, they suggest that the caldera floor was below the water table.

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“…Different models relax one or more of these simplifying assumptions. Wilson et al (1980), Wilson & Head (1981) and Giberti & Wilson (1990) study the effect of variations in conduit cross-section. Supersonic two-phase flow requires growth in the conduit radius.…”

Section: Conduit Flow Modelsmentioning

confidence: 99%

Conduit flow and fragmentation

Mader

1998

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This chapter provides a critical overview of syn-eruptive processes between magma chamber and vent in magmatic, sustained, Plinian-type eruptions. Phreatomagmatic effects are largely neglected. The main sources of information derive from textural studies of ejecta, theoretical models and physical experiments of eruption dynamics. Textural studies commonly find bimodal vesicle populations, indicative of several discrete nucleation events. The degree of disorder increases with explosivity. Dynamical parameters, such as nucleation density and rate and bubble growth rate, can be inferred from studies of the moments of bubble size distributions. Vesicularity in pumices is observed to vary significantly both within and between deposits, which suggests that the vesicularity at fragmentation is affected by the flow dynamics. Vesicularity variations correlate most closely with changes in magmatic composition and viscosity, but not with discharge rate. Conduit flow models can be broadly grouped into first-and second-generation models; the former generally impose a lithostatic pressure gradient and a constant Newtonian viscosity, whereas the latter include equations for the rheological changes that take place during vesiculation and solve for the pressure. Second-generation models derive highly non-lithostatic pressure gradients with the result that most of the vesiculation occurs at a high rate over a short distance just prior to fragmentation. The mechanisms of brittle and ductile fragmentation have been investigated in separate studies in non-vesiculating magmas, but which mechanism operates in explosive eruptions is not known. Dynamical laboratory experiments provide observations of the physical processes operating in conduit flows. Gas-expansion experiments have shown that it is possible to generate violent explosions by unloading in cool magmatic materials. Expanding dusty flows are found to be stable only if the bulk density increases with height. Exsolution experiments have demonstrated that acceleration precedes fragmentation and that gas evolution is enhanced by advection and bubble deformation. Deformed vesicles similar to those found in 'woody' pumice have been generated in an analogue system that has similar rheology to that found in vesiculating magmas. Large-scale exsolution experiments suggest that explosive volcanic eruptions are inherently heterogeneous; the fluctuations in discharge rate and discrete pulses and shocks commonly observed are a consequence of the large physical scale of volcanic systems. The effect of the magma chamber and conduit geometry has also been investigated. Eruption of material from a spherical flask up a narrow cylindrical tube generates quasi-steady flow conditions after an initial transient during which the discharge rate grows, as frequently observed in volcanic eruptions. The fragmentation surface does not propagate down into the magma chamber.

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The influence of geometry on the ascent of magma in open fissures

Cited by 51 publications

References 9 publications

Explosive volcanic eruptions-VIII. The role of magma recycling in controlling the behaviour of Hawaiian-style lava fountains

Explosive volcanic eruptions-VIII. The role of magma recycling in controlling the behaviour of Hawaiian-style lava fountains

Evidence for water influx from a caldera lake during the explosive hydromagmatic eruption of 1790, Kilauea volcano, Hawaii

Conduit flow and fragmentation

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