Discussion
Columnar jointing in basalt is produced by
Answer: B
Correct answer: contraction on cooling
A cooling lava flow or sill contracts, and the tension that develops is relieved by fractures that propagate inward from the cooling surface. Where cooling is uniform the fractures organise themselves into a polygonal, commonly hexagonal, pattern perpendicular to the isotherms, giving columns normal to the cooling surface - as at the Giant’s Causeway and Devils Tower. It is a primary igneous feature, not a tectonic one.
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