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多相流动科学与技术
SJR: 0.183 SNIP: 0.483 CiteScore™: 0.5

ISSN 打印: 0276-1459
ISSN 在线: 1943-6181

多相流动科学与技术

DOI: 10.1615/MultScienTechn.v11.i3.10
pages 147-195

PHYSICAL PROCESSES IN EXPLOSIVE VOLCANIC ERUPTIONS

H. M. Mader
Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK

ABSTRACT

This article provides a critical overview of current research into the physical processes that operate in the most explosive volcanic eruptions, so-called ‘Plinian’ eruptions. The scope of article is restricted to processes that occur during an eruption as material moves between magma chamber and vent. Effects involving the influx and vaporisation of ground- or seawater are largely neglected.
Magmas are silicate melts that contain variable amount of crystals and dissolved volatiles (H2O as vapour, CO2, S). An explosive eruption occurs when these volatiles suddenly come out of solution in the form of many bubbles, leading to the rapid expansion of the multiple-phase material. At some point in the flow, ‘fragmentation’ occurs, i.e. the magma is disrupted into many discrete particles. The exsolution event is often precipitated by a sudden decompression. Bubbles are preserved as holes, or ‘vesicles’, in the rocks which are the solid end-products of the eruption. The main sources of information concerning the dynamics of the flows derive from textural studies of volcanic rocks, theoretical models and physical experiments of explosion dynamics.
In textural studies on volcanic rocks, the vesicle populations in rocks are studied to gain information about dynamical parameters such as nucleation density and rate and bubble growth rate. Vesicularity (the volume percentage of vesicles) is observed to vary significantly both within and between deposits. Variations in vesicularity are found to correlate most closely with changes in magmatic composition and viscosity, but not with discharge rate.
Theoretical models of the flow dynamics can be broadly grouped into first- and second-generation models; the former generally impose a pressure gradient that is linear with depth and a constant. Newtonian viscosity whereas the latter include equations for the Theological changes that take place during the gas evolution and solve for the pressure. Second-generation models derive highly non-linear 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 involving bubbly magmas that are not subject to ongoing gas generation. Which mechanism operates in explosive eruptions is not yet known.
Dynamical laboratory experiments provide observations of the physical processes operating during explosions that are driven by rapid internal gas evolution. Gas-expansion experiments have shown that it is possible to generate violent explosions by unloading even in cool maginatic 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 some pumices, have been generated in an analogue system whose rheological behaviour mimics that observed 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/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 discliarge rate grows, as frequently observed in volcanic eruptions. The fragmentation region does not propagate down into the magma chamber.


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