Shear rate dependence of the pāhoehoe-to-‘a‘ā transition: Analog experiments

Abstract

Hawaiian lava flows typically erupt as pāhoehoe and transform to 'a'ā as they travel away from eruptive vents, although the exact conditions that govern this transformation are not well constrained. Here we describe a set of laboratory experiments that use corn syrup and ellipsoidal rice grains as an analog to lava with crystals to examine the dependence of the transition on shear rate and particle concentration. At a particle volume fraction of 0.3, increasing the shear rate produces a sequence of deformation regimes defined by increasing amounts of shear localization. The same regimes may be traversed at constant shear rate by varying the particle concentration. At high shear rates, the onset of nonlaminar deformation corresponds to the percolation threshold, thus providing a link between suspension microstructure and deformation behavior. Together these results highlight the combined importance of shear rate and crystallinity in determining the flow behavior of basaltic lava.

Clinker formation in basaltic and trachybasaltic lava flows

“…The diagram (Fig. 1a) has also been used by as the basis for analogue experiments of the pāhoehoe-'a'ā transition (Soule and Cashman 2005). We use this diagram qualitatively to explain clinker formation, as clinker will be generated by crossing the transition threshold zone, from the pāhoehoe field to the 'a'ā field.…”

A review of laboratory and numerical modelling in volcanology

“…These experiments model variations in heat flux, thermal gradients, and cooling on the temporal and spatial variation in lava flow viscosity, extrapolating on the impact these factors have on run-out length and flow morphology, for example. Lavas have also been modelled in the laboratory as a particle suspension, with experiments showing that increasing particle volume fraction (Soule and Cashman, 2005;Castruccio et al, 2014) and particle size (Del Gaudio et al, 2013) increases lava viscosity and can affect lava flow morphology. High-concentration particle suspensions produce low flow velocities, shear localisation, and subsequent break-up of the flow surface, causing transition from pahoehoe-like to aa-like morphologies that are reminiscent of natural flows (Soule and Cashman, 2005).…”

Benchmarking computational fluid dynamics models of lava flow simulation for hazard assessment, forecasting, and risk management

“…Loss of bubbles and growth of crystals combine to alter the bulk rheology of the flow (e.g., Soule and Cashman, 2005;Castruccio et al, 2010) and the heat budget through latent heat of crystallization (e.g., Harris and Rowland, 2001). The surface crust also plays a role in resisting the flow, controlling heat loss, and the transition between pāhoehoe and 'a'ā lavas (Griffiths et al, 2003;Lyman et al, 2005;Cashman et al, 2006).…”

“…Three-dimensional models like OpenFOAM and FLOW-3D have the potential to form a crust and simulate an evolution in bulk rheology (e.g., Vakhrushev et al, 2014), but the current implementation of our lava flow OpenFOAM solver includes only a cooling-induced increase in viscosity, and we have not tested FLOW-3D's solidification solver for a lava flow scenario. Employing analogue experiments with surface crust growth (e.g., Griffiths et al, 2003;Garel et al, 2014) as benchmarks could help guide development of appropriate models, although most existing analogue materials are not scaled appropriately to quantify crustal growth effects (e.g., Soule and Cashman, 2005).…”

“…Near the vent, lava flows have been considered isoviscous Newtonian fluids (e.g., Takagi and Huppert, 2010). Cooling, however, leads to the growth of a surface crust (Griffiths and Fink, 1993) and to lava crystallization, which introduces non-Newtonian rheology (Hulme, 1974;Lyman et al, 2005;Soule and Cashman, 2005;Castruccio et al 2013). Topography also exerts a major control on flow behavior, as the underlying slope drives advance rates, while topographic features may split and slow flows or confine and lengthen them (Dietterich and Cashman, 2014;Dietterich et al, 2015).…”