Crack patterns in laboratory experiments on thick samples of drying cornstarch are geometrically similar to columnar joints in cooling lava found at geological sites such as the Giant’s Causeway. We
present measurements of the crack spacing from both laboratory
and geological investigations of columnar jointing, and show how
these data can be collapsed onto a single master scaling curve. This
is due to the underlying mathematical similarity between theories
for the cracking of solids induced by differential drying or by cooling. We use this theory to give a simple quantitative explanation of
how these geometrically similar crack patterns arise from a single
dynamical law rooted in the nonequilibrium nature of the phenomena. We also give scaling relations for the characteristic crack
spacing in other limits consistent with our experiments and observations, and discuss the implications of our results for the control
of crack patterns in thin and thick solid films.