HR: 17:45h
AN: T14A-08 INVITED    [Abstracts]
TI: The dynamic similarity between columnar joints in basalt and starch
AU: Goehring, L
EM: goehring@physics.utoronto.ca
AF: University of Toronto, 60 St. George St., Toronto, ON M5S 1A7, Canada
AU: Mahadevan, L
EM: lm@seas.harvard.edu
AF: Harvard University, 29 Oxford Street, Cambridge, MA 02138, United States
AU: * Morris, S W
EM: smorris@physics.utoronto.ca
AF: Harvard University, 29 Oxford Street, Cambridge, MA 02138, United States
AB: We compared the scales of columnar joints due to thermal contraction in basalt to the scales of columnar joints due to desiccation in corn starch slurries. We made extensive field measurements of basalt columns at several sites including the island of Staffa, the Giant's Causeway and the Columbia Plateau. At each site, we examined in detail the scaling relationship between the column width and the height of the striae. Striae are the surface traces left on the sides of the columns by individual fracture advances. These data can be analyzed using thermal and fracture models in such a way that the average fracture advance speed and cooling rate can be determined. We found that the column radius and striae size are proportional to each other, and inversely proportional to the cooling rate. Controlled laboratory experiments in desiccating corn 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 Peclet 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. The steady advance speed of the cracks, due to ground water boiloff in basalt joints or the controlled evaporation rate in the starch experiments, corresponds to a non-equilibrium steady state of these equations and hence naturally scales with the Peclet number. We show that this scaling holds for a wide range of column widths in both cases.
DE: 4460 Pattern formation
DE: 5104 Fracture and flow
SC: Tectonophysics [T]
MN: 2007 Fall Meeting