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The rock formations seen at Giant's Causeway in Country Antrim, Northern Ireland are an example of columnar jointing.
Such formations result when lava cools and contracts and the rock is pulled into geometric shaped columns of 5 or 6 sides.
During cooling the solidifying matter is moved sideways creating deep cracks (joints) across the surface. The outer edge cools first and then cooling proceeds towards the centre and the joints develop in depth eventually appearing in vertical columns. The slower the cooling the more pronounced the columns.
Giant's Causeway features a tightly packed series of hexagonal shaped basaltic columns lying perpendicular to the original lava flow.
