When cracks form in a thin contracting layer, they sequentially break the
layer into smaller and smaller pieces. A rectilinear crack pattern encodes
information about the order of crack formation, as later cracks tend to
intersect with earlier cracks at right angles. In a hexagonal pattern, in
contrast, the angles between all cracks at a vertex are near 120∘.
However, hexagonal crack patterns are typically only seen when a crack network
opens and heals repeatedly, in a thin layer, or advances by many intermittent
steps into a thick layer. Here it is shown how both types of pattern can arise
from identical forces, and how a rectilinear crack pattern evolves towards a
hexagonal one. Such an evolution is expected when cracks undergo many opening
cycles, where the cracks in any cycle are guided by the positions of cracks in
the previous cycle, but when they can slightly vary their position, and order
of opening. The general features of this evolution are outlined, and compared
to a review of the specific patterns of contraction cracks in dried mud,
polygonal terrain, columnar joints, and eroding gypsum-sand cementsComment: 19 pages, 9 figures, accepted for publication in Phil. Trans. R. Soc.
A; theme issue on Geophysical Pattern Formation (to appear 2013
