When a thick lava flow cools it contracts vertically but cracks perpendicular to its directional flow with remarkable geometric regularity - in most cases forming a regular grid of remarkable hexagonal extrusions that almost appear to be made by man. One of the most famous such examples is the Giant's Causeway on the coast of Ireland (shown above) though the largest and most widely recognized would be Devil's Tower in Wyoming . Basalt also forms different but equally fascinating ways when eruptions are exposed to air or water.
These structures are often erroneously described as being predominantly hexagonal. In reality, the mean number of sides of all the columns in such a structure is indeed six (by geometrical definition), but polygons with three to twelve or more sides can be observed. Note that the size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, while slow cooling is more likely to produce large columns.
Perhaps the most famous basalt flow in the world is the Giant's Causeway on the northern coast of Ireland, in which the vertical joints form polygonal columns and give the impression of having been artificially constructed.
An ancient 13th century religious complex, called Nan Madol, was built on the Pacific island of Pohnpei, using columnar basalt quarried from various locations on the island. The massive ruins remain to this day.
