Transient phenomena in vesicular lava flows based on laboratory experiments with analogue materials
Introduction
Reliable rheological data are essential for modelling eruptive volcanic processes (Marsh, 1987). However, different rheological models are commonly used in simulations. For example, in some lava flow simulations, lavas are assumed to behave as Newtonian fluids (Keszthelyi and Self, 1998). In others, e.g. Pinkerton and Wilson (1994), Dragoni (1993) and Miyamoto and Sasaki (1998), Bingham behaviour is assumed, and viscoelastic Maxwell behaviour has been used by Dragoni and Tallarico (1996). While there are many circumstances where such approximations can prove useful, there is clearly a need for a reassessment of the appropriateness of different rheological models in volcanology. In this contribution we present evidence that suggests that there are many circumstances where none of the above rheological models are appropriate. We argue that realistic modelling of many volcanological processes needs to take into account changes in rheological properties during flow caused by reversible structural changes in vesicular or crystalline lavas. The question is how to parameterise their time dependent viscosity and what kind of parameter may be used to quantify the build-up and breakdown of transient structures in magmas.
The temporal variations in lava flow rheology can be caused by one or more of the following: thixotropic effects arising from vesicle deformation (elongation and subsequent change of orientation); viscoelastic properties of the melt matrix itself; and the onset of a dynamic yield strength. We investigated the relative importance of these experimentally. In the first set of experiments we studied the thixotropic properties of bubbly liquids using a rotating shear vane, and in the second set of rotating shear vane experiment we measured the shear-thinning behaviour of a viscoelastic foam. We also used oscillatory torque experiment to investigate the importance of the viscoelastic properties of a bubble-free polymer that is commonly used to simulate lava flows.
As a theoretical introduction to the subject we begin with a brief review of the rheological properties of lavas. This highlights the need for additional experimental measurements on low temperature analogue fluids.
Section snippets
Transient rheology of lavas
Rheological properties of lava are a function of the chemistry, temperature, crystallinity and vesicularity of silicate melts of which they are composed. They are also related in a complex way to the degassing and thermal history of a flow (Sparks and Pinkerton, 1978, Marsh, 1981, Marsh, 1987, Lipman et al., 1985). At temperatures above the liquidus, the viscosity of lava at any given temperature is readily estimated from its chemical composition (e.g. Shaw, 1972). However, most lavas are
Thixotropy of lava flows and bubble deformation
Thixotropic properties may be important when the shear strain-rate varies in time or across a lava flow section. As a starting point we consider a lava flow as a suspension of bubbles in a melt with thickness h situated on a sloping plane and subjected to a shear stress due to the gravity (Fig. 1). The well known solution of the problem for a velocity v(z) and stress profile σzx in viscous flow with constant viscosity μ and variable thickness h along the slope length L is:
Thixotropy of bubbly liquid
For many fluids, the apparent viscosity varies with time as well as with applied shearing stress. There are two possible explanations for this behaviour. One relates to bubble deformation; the other to crystal interaction. During the initial flow of multiphase viscous fluids, elongate particles may form a bridging framework tending to align along the flow lines. This will decrease the bulk viscosity, resulting in thixotropic (shear thinning) behaviour as we demonstrated in Section 3. For
Discussion and conclusions
(1) Our experiments using analogue fluids have shown that bubbly suspensions are thixotropic, viscoelastic fluids with yield strengths. Previous measurements have revealed that vesicular lavas and magmas have similar properties. The effective parameter that may describe the temporal changes in structure is the deformation parameter of vesicles during flow λ. In a lava flow that moves from a shallow to a steep slope, thixotropy causes a decrease in apparent viscosity. The response of a lava to a
Acknowledgements
The authors thank G. Ryan for undertaking the gum rosin torsion experiments and S. Lane for the discussion of the shock tube decompression experiments. Comments of C. Kilburn and an anonymous reviewer helped to improve the manuscript. The authors are grateful to DAAD for the travel grants of exchange programme between Lancaster University and the University of Frankfurt.
References (53)
Thixotropy – a review
J. Non-Newtonian Fluid Mech.
(1997)The yield strength – a review or ‘παντα ρϵι’ – everything flows?
J. Non-Newtonian Fluid Mech.
(1999)- et al.
Understanding thixotropic and antithixotropic behaviour of viscoelastic micellar solutions and liquid crystalline dispersions, I. The model
J. Non-Newtonian Fluid Mech.
(1999) - et al.
A model for the opening of ephemeral vents in a stationary lava flow
J. Volcanol. Geotherm. Res.
(1996) - et al.
The yield strength of subliquidus basalts – experimental results
J. Volcanol. Geotherm. Res.
(2001) - et al.
Viscosity of magmas containing highly deformable bubbles
J. Volcanol. Geotherm. Res.
(2001) - et al.
Rheology of bubble-bearing magmas
J. Volcanol. Geotherm. Res.
(1998) - et al.
Transient phenomena in thixotropic systems
J. Non-Newton. Fluid Mech.
(2002) - et al.
Rheological properties of basaltic lavas at sub-liquidus temperatures: Laboratory and field measurements on lavas from Mount Etna
J. Volcanol. Geotherm. Res.
(1995) - et al.
Methods of determining the rheological properties of magmas at sub-solidus temperatures
J. Volcanol. Geotherm. Res.
(1992)
Rheology of foams and highly concentrated emulsions, III. Static shear modulus
J. Coll. Interface Sci.
Effects of bubble concentration on the viscosity of dilute suspension
J. Non-Newtonian Fluid Mech.
Bubble shapes and orientations on low Re simple shear flow
J. Colloid. Interface Sci.
Numerical models of the onset of yield strength in crystal-melt suspensions
Earth Planet. Sci. Lett.
Shear thickening dilatancy in crystal-rich flows
J. Volcanol. Geotherm. Res.
Rheology and microstructure of magmatic emulsions: Theory and experiments
J. Volcanol. Geotherm. Res.
Shear viscosity of rhyolite-vapor emulsions at magmatic temperatures by concentric cylinder rheometry
J. Volcanol. Geotherm. Res.
Vesiculation in a water-rich calc-alkaline obsidian
Earth Planet. Sci. Lett.
Characterisation, settling, and rheology of concentrated fine particulate mineral slurries
Powder Technol.
Yield stress of laterite suspensions
J. Coll. Interface Sci.
Deformation of foamed rhyolites under internal and external stresses: Experimental investigation
Bull. Volcanol.
Effect of alkalis, phosphorus, and water on the surface tension of haplogranite melt
Am. Mineral.
Time-dependent and flow properties of foams
Mech. Time-Depend. Mater.
On the behaviour of thixotropic fluids with a distribution of structure
J. Phys. D
Cited by (37)
Morphological and textural analysis of basaltic pyroclasts (Atexcac maar, central Mexico): Implications for fragmentation and conduit processes
2022, Journal of South American Earth SciencesThe effect of bubbles on the rheology of basaltic lava flows: Insights from large-scale two-phase experiments
2020, Earth and Planetary Science LettersRheological tests of polyurethane foam undergoing vesiculation-deformation-solidification as a magma analogue
2020, Journal of Volcanology and Geothermal ResearchCitation Excerpt :Moreover, knowledge of rheology of GR solution is limited to the steady-state viscosity as a function of the solvent concentration (Lane et al., 2008; Phillips et al., 1995) and the dynamic modulus of GR without a solvent (Bagdassarov and Pinkerton, 2004). The rheology of magma during vesiculation and solidification may change with flow, and it may depend on the decompression, deformation, and cooling histories (Bagdassarov and Pinkerton, 2004; Kolzenburg et al., 2016). However, most previous studies targeted rheology in equilibrium states (constant temperature, composition, vesicularity, crystallinity, etc).
The use of a shear-thinning polymer as a bubbly magma analogue for scaled laboratory experiments
2020, Journal of Volcanology and Geothermal ResearchCitation Excerpt :Therefore, to better replicate the natural system, it would be useful to perform experiments using a fluid that has a non-Newtonian rheology under the shear-rates attained. The simplest approach is to add bubbles to a Newtonian analogue fluid (Bagdassarov and Pinkerton, 2004; Rust and Manga, 2002b; Soule and Cashman, 2005). However, the addition of bubbles poses several experimental challenges.
The contribution of experimental volcanology to the study of the physics of eruptive processes, and related scaling issues: A review
2019, Journal of Volcanology and Geothermal ResearchCitation Excerpt :The analytic solution agreed well with the laboratory measurements, leading the authors to apply their model to several submarine and subaerial terrestrial and extraterrestrial channeled lava flows in order to estimate the viscosity from the measured channel dimensions and assumed lava properties. The effect of bubbles concentration on lava rheology was investigated experimentally by Bagdassarov and Pinkerton (2004) who used a rotational van-viscometer to measure the viscosity of aerated golden syrup suspensions. The stress-strain curves determined experimentally revealed that bubbly suspensions were thixotropic, visco-elastic fluids with yield strengths (i.e., σ0 > 0 and n < 1 in Eq. (84)).
Measuring the viscosity of lava in the field: A review
2019, Earth-Science Reviews