Coupling of viscous and diffusive controls on bubble growth during explosive volcanic eruptions

Citation
Jd. Blower et al., Coupling of viscous and diffusive controls on bubble growth during explosive volcanic eruptions, EARTH PLAN, 193(1-2), 2001, pp. 47-56
Citations number
24
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
EARTH AND PLANETARY SCIENCE LETTERS
ISSN journal
0012-821X → ACNP
Volume
193
Issue
1-2
Year of publication
2001
Pages
47 - 56
Database
ISI
SICI code
0012-821X(20011130)193:1-2<47:COVADC>2.0.ZU;2-Y
Abstract
The coupling of viscosity and diffusivity during explosive volcanic degassi ng is investigated using a numerical model of bubble growth in rhyolitic me lts. The model allows melt viscosity and water diffusivity to vary spatiall y and temporally with water content. We find that the system is highly sens itive to the distribution of volatiles around the bubble, primarily as a co nsequence of the great sensitivity of melt viscosity to water content at lo w water concentrations. The dehydrated portion of the magma near the bubble exerts the major control on the effective viscosity of the melt; however t he values of effective viscosity are lower than previously reported values calculated using an approximated concentration profile. Degassing is found to be highly sensitive to the choice of the solubility law, which controls the volatile concentration near the bubble, but insensitive to the equation of state. The form of the concentration profile in the case of concentrati on-dependent diffusivity is such that the effective viscosity is substantia lly lower (by as much as an order of magnitude) than for constant diffusivi ties of similar magnitude. This leads to unexpectedly high bubble growth ra tes, particularly for cases of highly non-equilibrium degassing. This is be cause low values of diffusivity are more than compensated by low melt visco sity due to higher volatile concentrations. This complex interplay between viscosity and diffusivity means that models of bubble growth must take into account the concentration-dependent nature of both these parameters; appro ximating either of these with a constant value will lead to significant err ors in estimations of bubble growth rates. (C) 2001 Elsevier Science B.V. A ll rights reserved.