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

¹ Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
² Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272, USA

Hawaiian lava flows typically erupt as phoehoe and transform to ‘a‘ 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.

Key Words: basaltic lava flows • rheology • shear localization • flow dynamics

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