Columnar joints form as a result of relative contraction due to cooling of a basaltic lava body (Figure 1) or drying of sediments such as mud and silt. These are generally referred to as thermal and desiccation fractures, respectively.

Figure 1. Fractures on the surface of a solidifying lava lake following the 1959 eruption of the Kilauea volcano on the Big Island of Hawaii. Note initial linear fractures with nearly orthogonal intersection angles on the surface.

Columnar joints divide rocks into prismatic columns (Figure 2) with commonly polygonal cross section geometry, varying from tetragonal or irregular patterns (Figure 1 and Figure 3) dominated by orthogonal intersections to ideally hexagonal cross sections with approximately 120 degree intersection angles (Figure 4 and Figure 5).

Figure 2. Near hexagonal columns of the Giant's Causeway, Northern Ireland, defined by a network of fractures formed during cooling and solidification of a series of basaltic lava flows. From Aydin and DeGraff (1988).

Figure 3. Plan view of mostly tetragonal columnar joint arrangement of a recent basaltic lava flow along the east rift of Kilauea Volcano, Hawaii. The joints are marked by a white sublimate. Circle 1 marks a T type intersection and Circle 2 marks curved T intersections. Scale is 30.5 cm long. From Aydin and DeGraff (1988).

Figure 4. Nearly perfect hexagonal cross sectional geometry of columnar joints on a cut and polished surface by a glacier, Devils Postpile, California. The notebook with larger dimension of about 20 cm is for scale. From Aydin and DeGraff (1988).

Figure 5. Columns of rock of about 40 cm diameter defined by joints on five or six sides in a volcanic rock exposed along the scenic coastal Northern Ireland. Note that the top morphology of the exposed columns is defined by column-normal fractures and erosion. See Aydin and DeGraff (1987) for more details.

Columnar joints are perfect for conceptualizing the temporal and spatial growth of joints because their driving stresses can be well constrained and their kinematics can be readily determined as described in the links entitled 'Growth of Columnar Joints' and 'Mechanisms and Mechanics of Thermal Fractures.'

On the practical application side, there are some radioactive contamination sites in volcanic rocks such as the Hanford site, Washington, and the Snake River Plain, near the Idaho National Engineering and Environmental Laboratory, Idaho, in which columnar joints play an important role in groundwater and contaminant flow. There are also some reservoirs in volcanic rocks. Thermal fractures are also relevant to thermal recovery in energy extraction and oil shale production.