Why is columnar jointing not perfectly hexagonal?

Columnar Jointing (CJ) is a frequent phenomenon in solidified lavas. While the general pattern is considered to be hexagonal, there are no overall statistics that describe the geometry of naturally occurring CJ sites. Moreover, the factors controlling the characteristic sizes of columns remain debated. We undertake a series of measurements to characterize the geometry of columns in terms of side length, cross-sectional area, and order of polygonality. Based on 49 sites, we find that the variation in characteristic size of columns between different sites exceeds one order of magnitude, and that the column bounding polygons’ average order is less than 6. For sites where the column size distribution is Gaussian (i.e. single geologic event), a correlation is found between the relative area variation and the standard deviation of the polygon order distribution. The “column area - side length” relationship as a function of the order of polygons suggests that the network of fractures is longer than required by a minimum-energy hexagonal configuration. This deviation from the most efficient relief of thermal stresses during cooling indicates a dynamic, relatively quick process. In a further step, we investigate which processes may control the size of CJ. Mechanical stress balance, cooling rate, physical properties of the lava and the geometry of the emplacement are assessed based on field observation and the literature. These are traced back to the geological setting and the chemical composition of the lava. We find that both factors play an important role in sizing CJ, and their relative importance is site-dependent. Site-related boundary and initial conditions are also likely factors that constrain the order of polygons in solidifying lavas.