Basalt Columns: Why Cooling Rock Cracks into Hexagons

When a lava flow pools several metres deep, it loses heat from its top and bottom surfaces first, and cooling rock takes up less room than it did as melt. The shrinking pulls the surface apart into a network of cracks, the same way mud splits in a drying pond. Those cracks then work their way inward, always at right angles to the surface that is losing heat, so a flat flow ends up as a crowd of upright posts.

The six sides are not perfect, and that is the useful part to look at. Fresh cracks often meet at right angles, but as they push deeper the junctions drift toward 120 degrees, the arrangement that relieves stress with the least total length of fracture. Count the faces along any cliff and you will find fives and sevens among the sixes. Running a hand down a column you can feel faint horizontal bands a few centimetres apart, each one a step where the crack advanced and paused.

Width records speed. Slow cooling deep inside the flow gives thick, orderly columns, while water seeping in from above can chill the upper part fast enough to produce thin, twisted ones, which is why many cliffs show a neat lower tier under a tangled upper one. Seen straight on, the vertical edges turn a screen into a set of evenly spaced lines, and in low side light every face takes a slightly different grey.