Columnar joints? Definition

What is the definition of columnar joints? (Geology)

Columnar jointing is a term used to describe the joint set pattern produced by the cooling of lava, although it can also be observed in thin layers of volcanic ash as well. It is produced (in lava) b the cooling and contraction of the rock around centers of nucleation. The joint sets are typically hexagonal (less commonly 4 or 5 sided) like a honeycomb structure which is associated with the closest packing pattern. The joint sets form perpendicular to the maximum cooling surface. Devil's Tower in Wyoming is a great example.

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