Giant's Causeway (not)

The scale at the top is in mm, and the material is starch rather than basalt. 

Found all over the world, [similar geologic formations] 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. There are many mysteries: 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?

Amazingly, you can make columnar joints in your kitchen! Just mix 50:50 corn starch and water and put it into a coffee cup. Dry the mixture with a bright light above it --- this might take up to a week. When the mixture is dry, carefully break it apart and you will see that the interior is broken up into small “starch columns” a few mm wide [pix at the link]. In this experiment, water replaces the heat in the lava, and the columns are 1000X smaller.

More details (and more photos) at the Proceedings of the National Academy of Science, via Fresh Photons.