Experimenting in the Kitchen

Abstract

GEOPHYSICS The surfaces of basaltic lavas commonly exhibit two kinds of textures: Pāhoehoe flows form a ropy and relatively smooth surface, and ‘a'ā flows look like jumbled, sharp, angular blocks. It is generally thought that these types reflect an interaction between the viscosity of the lava, which varies as it cools and crystals form, and the shear rate of the flow. Many flows change their morphology from pa−hoehoe to ‘a'ā, and a few change back. To investigate this transition, Soule and Cashman carried out a series of laboratory experiments using corn syrup (diluted to the viscosity of hot basaltic magma) and rice (which has the same density as the diluted syrup and represents the lava crystals). They observed four different regimes: With increasing amounts of rice (corresponding to increasing viscosity), flow is laminar; the rice grains aggregate into clumps; shear zones form between the clumps; and finally, a thin film of rice-free syrup appears along the flow boundary, perhaps by cavitation, and the main flow is thus detached. This evolution and the abrupt transitions between these regimes are consistent with field measurements of the pā transition. — BH Geology 33 , 361 (2005).