|Soule, S.A. and Cashman, K.V., Shear rate dependence of the pahoehoe to 'a'a transition, analog experiments, Geology, submitted|
The surface morphology transition in basaltic lava flows from pahoehoe to aa is dependent on both the effective viscosity of the lava and shear rate it experiences. Recent studies have constrained the effective viscosity dependence, but the shear rate dependence remains unknown, as does the mechanism by which p?ahoehoe becomes aa. Here we describe a set of analog laboratory experiments using corn syrup and rice grains conducted to examine the shear rate dependence of the transition. We identify a sequence of deformation regimes that result from increases in the degree of shear localization with increasing rotation rate (shear rate) and particle concentration (effective viscosity) and whose boundaries agree in form with those proposed for the pahoehoe-to-aa transition. The onset of non-laminar deformation at high shear rates corresponds to the percolation threshold, thus providing a link between suspension microstructure and deformation behavior. Together these results highlight the importance of shear rate in the flow behavior of basaltic lava, allow us to identify mechanisms that lead to deformation by tearing rather than flow, and support recent suggestions that this transition may be reversible.