I recently visited a cliff on the south coast of Ulva in the Inner Hebrides where there are ~10m thick flows of columnar basalt. The columnar basalt forming most of the cliff is sandwiched between flows of non-columnar basalt in the same location. What factors could therefore trigger the formation of columns in some basalt flows, but not in others? At the base of the columns there was evidence of "red bole", suggesting that there was some time between the flow of the lower non-columnar basalt and the columnar basalt above.

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    $\begingroup$ The physics is a bit complex. You can find the details in Goehring and Moris's paper: See physics.utoronto.ca/~nonlin/preprints/GM_JGR08.pdf $\endgroup$ May 10 '16 at 19:45
  • $\begingroup$ Thanks. Although this is a very interesting paper, I don't think that it actually addresses my particular question. The starting point of the analysis section is the heat equation, which appears to be solved for boundary conditions corresponding to a flat slab with a constant temperature sink on both outside surfaces. As the initial heat equation solution could apply equally to a slab (lava flow) with or without columns, it would seem that the initial presence of columns, whose development is then considered, is therefore simply assumed as one of the initial conditions in the analysis. $\endgroup$
    – bobg1756
    May 16 '16 at 10:05
  • $\begingroup$ Could it be possible that columns are more likely to be formed in flows which become stationary whilst an outer surface is still above the glass transition temperature? If the liquid surface was initially relatively smooth, this could allow an initial 2D pattern of polygonal shrinkage cracks to form on the surface as it freezes, providing a surface pattern which could then propagate inwards. Such a 2D surface pattern would be more likely to be disrupted or destroyed in flows which were still moving after the surface had cooled below the glass transition temperature? Is this plausible? $\endgroup$
    – bobg1756
    May 16 '16 at 10:06

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