Conduit flow and fragmentation

Abstract

Abstract 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.