A study of 3D crack patterns and columnar jointing in corn starch M.Sc. thesis, Unpublished, 2003. Lucas Goehring Department of Physics, University of Toronto, 60 St. George St., Toronto, Ontario, Canada M5S 1A7.

The nature of columnar jointing has remained an enticing mystery since the basaltic columns of the Giant's Causeway in N. Ireland were first reported to science, over three hundred years ago. More recently, this phenomena, which causes shrinkage cracks to form into a quasi-hexagonal arrangement, has been shown to produce columns in a wide variety of situations and media. This report will focus on experiments investigating the nature of columnar jointing in corn starch, which has been dried using an overhead heat lamp. A study of the 3D nature of this pattern produces the first qualitative description of the ordering process, whereby a disorganized superficial crack pattern arranges itself into a quasihexagonal crack pattern at depth. Experiments probing the nature of the pattern in deep samples show that two distinct types of coarsening can increase the pattern scale. The difference between a slow, gradual shift in scale, and a sudden catastrophic jump in scale is explained by assuming that a resistance to scale change is inherent to this pattern. Such a hysteretic pattern may answer a fundamental question of columnar jointing -- why the columns are so regular in the direction of their growth. This theory may be tested in other media, notably in a number of suggested measurements on basaltic colonnades. Computer control over the evaporation rate in a growing sample continues to be worked on, as do experiments revealing the dynamics of this pattern in an actively drying starch sample.

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