The origin of columnar jointing, particularly in basalt flows

Certain igneous rocks have been fractured into columnar prisms by thermal stresses produced during cooling. In some flows a threefold structural division is recognized with a lower colonnade, central entablature and upper colonnade. The entablature contains various patterns of curved columns. Joints in many flows form in a definite sequence with master joints first, mega-columns next, then normal columns and finally cross-fractures. In the simplest cases columns form perpendicularly to the isotherms but in the more complex examples, the column direction is controlled by the orientation of surfaces of equal tensile stress and these may not be parallel to the isotherms.