HR: 11:05h
AN: V51L-04    [PDF]
TI: Shear Rate Dependence of the P\={a}hoehoe to `A`\={a} Transition
AU: * Soule, A
EM: ssoule@gladstone.uoregon.edu
AF: Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403-1272 United States
AU: Cashman, K
EM: cashman@oregon.uoregon.edu
AF: Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403-1272 United States
AB: The surface morphology transition from p\={a}hoehoe-to-`a`\={a} on basaltic lava flows can be used to interpret the emplacement conditions of solidified flows and predict the behavior of active flows. Investigations of this phenomenon have emphasized either the mechanical properties of the solidified crust (e.g., Kilburn, 1981), or the rheologic properties of the liquid interior (e.g., Peterson and Tilling, 1980). In the latter, the boundary separating p\={a}hoehoe and `a`\={a} is represented qualitatively by an inverse relationship between apparent viscosity and shear rate. Recent investigations of the rheology dependence of the transition have revealed a critical crystallinity range at which p\={a}hoehoe transforms to `a`\={a} of $\phi$ = 0.18 to 0.35 that can vary between flows. Here, we extend this approach to investigate the shear rate dependence of the p\={a}hoehoe-to-`a`\={a} transition. We use a suspension of corn syrup and rice to represent lava with crystals. Suspensions of varying particle concentration ($\phi$ = 0.15 to 0.40) are sheared in a Couette rheometer over a range of constant shear rates (0.1 to 2.0 s$^{-1}$). We describe three deformation regimes, clumping, shear zone formation, and fluid failure that produce changes in the suspension microstructure and lead to shear localization. The deformation mechanisms are imaged with digital video and quantified by tracking individual particle paths. In the presence of cooling, these shear localization may be the mechanism by which `a`\={a} flow surfaces form. We find that the onset of each regime follows the expected inverse relationship between shear rate and suspension viscosity. We expect that the results of these experiments apply to the thermal boundary layer of a flow and thus bridge the distinct approaches taken to investigate this phenomenon. The results of these experiments can contribute to more detailed lava flow modeling and better assessment of flow dynamics from solidified lava flows.
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting