Columnar joints are strange, uncanny formations in which outcroppings of rock are mysteriously broken into nearly perfect hexagonal pillars, all the same size and packed tightly together. The most famous examples have accumulated legends since ancient times: The Giant’s Causeway in Northern Ireland, Fingal’s Cave in Scotland, The Devil’s Postpile in California (picture) and The Devil’s Tower in Wyoming. The names conjure the apparently supernatural degree of organization of these columns, which appear to be the work of giants or the devil. What causes these “joints” (to use the geologist’s word for cracks)?
Since they were first discussed in the scientific literature in the 17th century, these formations have attracted the interest of geologists and tourists. Found all over the world, they are now known to be the result of cooling lava flows, in which shrinkage causes stresses that fracture the rock. The columns are formed as a sharp front of cooling moves into the lava flow, assisted by the boiling off of groundwater. As the front advances, it leaves behind a crack network which evolves into an almost hexagonal arrangement. This network carves out the columns. Many mysteries remain: what causes the ordering of the network into hexagons? What sets the size of the columns, which varies in different outcrops between a few inches and a few yards?