Columnar joints are three-dimensional fracture networks that form in cooling basaltic lava flows. The network organizes the solid flow into ordered, mostly hexagonal columns. Famous examples include the Giant's Causeway in Northern Ireland, Fingal's cave in Scotland and The Devil's Postpile in California. The same pattern can be observed on a smaller scale in desiccating corn starch, and in some other materials. We have made the first three dimensional study of the evolution of the network in corn starch and relate these observations to the mature patterns observed in field studies of basalt. The starch patterns are statistically similar to those found in the Giant's Causeway, suggesting that mature columnar joint patterns contain inherent residual disorder. Controlled laboratory experiments in desiccating starch produce uniform fracture advance rates and uniform column widths. Starch columns are 100 times smaller than their basalt counterparts. We show that both basalt and starch joints form at similar values of the dimensionless Péclet number --- the ratio of the fracture advance rate times the column width to the diffusion constant of heat or moisture. This parameter arises because the two systems can be described by essentially the same continuum advection-diffusion equations. We show that this scaling holds for a wide range of column widths in both cases.
Work done with Lucas Goehring and L. Mahadevan.
Relevant publications: http://www.physics.utoronto.ca/nonlinear/
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