Phys. Rev. Lett. 115, 154301 (2015)

Credit: © MARSHALL IKONOGRAPHY / ALAMY

Basalt columns, as seen in the staggering rock formations of the Giant's Causeway in Northern Ireland (pictured), are produced by volcanic activity. As lava solidifies, it cools down from the top, generating patterns of cracks due to thermal shrinkage. Secondary fractures that appear perpendicular to primary cracks yield the highest energy release per crack face — so the initial rupture network consists of T-junctions forming a pattern of squares or rectangles. But as the cracks propagate downwards, their contact angles change from 90° to 120° and T-junctions morph into Y-junctions — resulting in basalt columns' well-known hexagonal prism shape.

Martin Hofmann and colleagues have analysed the T- to Y-junction transformation by means of finite-element simulations. Their starting point was a periodic rectangular structure, featuring T-junctions. For realistic material parameters of basalt, the calculated interplay between stress, strain and displacement fields, driven by a temperature gradient from 100 °C to approximately 1,000 °C, reproduced the transition from rectangular to hexagonal crack patterns over a short distance (roughly half of the initial crack length). During further downward crack propagation, the hexagonal configuration remained stable.