Molten basalt shrinks as it cools; that contraction produces tensile stress. Cracks relieve that stress and propagate where the temperature gradient is steepest. Because the steepest thermal gradient in a cooling lava flow is normal to the exposed surface, fractures advance roughly perpendicular to the flow surface. This orientation minimizes the work done by the fracture: cracks follow the direction in which contraction is largest, producing pillars whose long axes are aligned with the cooling direction. Experimental and theoretical studies of fracture-driven pattern formation by Nicholas Goehring University of Cambridge and Lakshminarayanan Mahadevan Harvard University support this mechanistic view, showing that cooling-directed stresses control crack orientation and spacing. Historical analysis of optimal tiling by William Thomson University of Glasgow explains why cracks tend toward polygonal, often hexagonal, cross-sections: hexagons are an efficient way to partition space and reduce total crack length under roughly uniform conditions.
How cooling controls joint orientation
The key physical driver is thermal contraction of a solidifying layer. The surface of a lava flow loses heat fastest, so a cooling front moves downward. Fractures nucleate at or near that front where tensile stresses exceed rock strength, then propagate in the direction perpendicular to the front. Crack tips locally relieve stress and interact with neighbors; the crack network evolves until a largely uniform pattern of vertical column edges forms. Local heterogeneities in composition, cooling rate, or preexisting fractures produce variations in column diameter and occasional irregular polygons.
Pattern geometry and broader consequences
The typical polygonal cross-section, often near hexagonal, reflects an energy-minimizing balance between fracture propagation and the spacing required to relax stresses. This has practical consequences: columnar joints create cliffs, steps, and talus that shape drainage, soil development, and microhabitats. Famous cultural and tourist sites such as Giant’s Causeway in Northern Ireland and Devil’s Postpile in California illustrate how these geomorphic features intersect human use, stewardship, and hazard perception. From an engineering perspective, the presence and orientation of columns influence rock-slope stability and quarry behavior; from an ecological perspective, columns create sheltered niches that affect vegetation and wildlife. Understanding the perpendicular orientation of columns therefore links basic fracture mechanics to landscape evolution, cultural landscapes, and environmental management. Small-scale variations in cooling and chemistry mean natural examples always depart somewhat from the idealized hexagonal pattern.