Columnar joints forms as volcanic rock cools and contracts and is thus a geological structure where sets of intersecting closely spaced fractures, referred to as joints, result in the formation of a regular array of polygonal prisms, or columns. Columnar jointing can occur in cooling lava flows and ashflow tuffs (ignimbrites), as well as in some shallow intrusions. Joint growth is perpendicular to the surface of the volcanic flow.
Column diameters vary from a few centimeters to three meters (10 feet), and can be as much as 30 meters (about 100 feet) in length. They are typically parallel and straight (colonnade), but can also be curved (entablature). The number of sides of the individual columns can vary from 3 to 8, with 6 sides being the most common.