Why basalt and mud crack into hexagonal columns

Settled in 2005 by Lucas Goehring & Stephen W. Morris; refined with L. Mahadevan.

The polygonal basalt columns of the Giant's Causeway in Ireland puzzled 18th-century naturalists, some of whom debated whether the formations were artificial or crystalline, sparking real scientific controversy over their natural formation from cooling lava. The general cracking mechanism (columns grow from an advancing cooling or drying front, with cracks meeting near 120-degree T-junctions that mature toward hexagonal order) was understood qualitatively by the 20th century, but a quantitative account of the ordering and column-width selection was missing. Lucas Goehring and Stephen Morris used desiccating cornstarch as a lab analog for cooling lava and, controlling the drying-front speed, showed in 2005 how order versus disorder arises, following up with a 2009 PNAS paper pinpointing the nonequilibrium mechanism that sets the characteristic column width.

The same slow-motion fracture mechanics that carved Ireland's Giant's Causeway out of solidifying lava can be watched in miniature in a tray of drying cornstarch on a lab bench.

Explanations that circulated

• Early folk explanations invoked crystallization of basalt as if it were a mineral crystal (fueling Giant's Causeway legends of giants building it), rather than a fracture-pattern phenomenon.
• A purely static energy-minimization argument (hexagons tile the plane most efficiently) is incomplete - it doesn't explain the observed size selection or the frequent pentagons/heptagons; the ordering is a nonequilibrium, sequential cracking process.

References

• Order and disorder in columnar joints, L. Goehring, S. W. Morris, Europhysics Letters, 2005
• Nonequilibrium scale selection mechanism for columnar jointing, L. Goehring, L. Mahadevan, S. W. Morris, PNAS, 2009