Symmetry in a natural fracture pattern: The origin of columnar joint networks

AbstractThe remarkable regularity of the rock columns outlined by the cooling related contraction joints in lava flows at sites such as the Giant's Causeway, Ireland, Fingal's Cave, Scotland and the Devil's Postpile, California is well known. Columnar joints are the only system of natural fractures to approach an optimal hexagonal honeycomb-like pattern. 3-connected vertices in near-surface natural-fracture networks are almost exclusively orthogonal “T-type” junctions. Columnar joints owe their unique quasihexagonal symmetry and the prevalence of six-sided columns to the presence of “Y” junctions. Initially, the cooling-related fracture network at the upper and lower surfaces of a flow are dominated by T junctions and are similar in appearance to mudcrack patterns. However, as the tips of these cracks grow towards the center of the flow to relieve the thermal strain which develops there during cooling, the poorly positioned ones are eliminated or modified to yield a more regular network dominated by Y rather than T junctions