The Giant's Causeway is, according to the Wikipedia page was formed during
during the Paleogene Period, Antrim was subject to intense volcanic activity, when highly fluid molten basalt intruded through chalk beds to form an extensive lava plateau.
Of particular note is the distinct columnar pattern (as seen in the picture below):
Formed, according to the Wikipedia page by
As the lava cooled, contraction occurred. Horizontal contraction fractured in a similar way to drying mud, with the cracks propagating down as the mass cooled, leaving pillarlike structures
What was the geochemical/petrology process(es) cause the contraction features that resulted in the columns being formed in the Giant's Causeway?