Columnar jointing occurs in cooling process of lava and sill. Hot lava contracts as it cools. If physically uniform lava with the flat surface is static and cools evenly, the lava contracts equally with many centers of contraction across the surface. The each contraction area represents a circle due to physical uniformity. As it contracts, cracks, ideally hexagonal, are formed among the circles. These cracks extend into the lava, forming parallel hexagonal columns. Actually, since the lava does not cool evenly, columns including five-sided and seven-sided columns are created as well as six-sided columns.