Millaa Millaa's Columnar Basalts
During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops 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 erroneously described as being predominantly hexagonal.
The size of the columns depends somewhat on the rate of cooling; very rapid cooling may result in very small columns, while slow cooling is more likely to produce large columns.
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