During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. The extensive fracture network results in the formation of columns. The topology of the lateral shapes of these columns can broadly be classed as a random cellular network. These structures are often 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. The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small columns, less than 1 cm diameter, while slow cooling is more likely to produce large columns.
PHOTO: Joint Pattern of Columnar Basalt Hills to the North of Ankara by voyageAnatolia.blogspot.com