Reappraising columnar joints in different rock types and settings

Columnar joints comprise a network of opening-mode fractures that divide material into prismatic columns. They are known in several materials and rock types, including basalt, mud, sandstone, and starch. In all types, the resultant prismatic columns have a similar geometry, facilitating a comparison of columnar joints in different materials and processes such as cooling and drying. In this study, we provide a unique example of columnar joints in low-grade meta-chalks that are the product of spontaneous combustion metamorphism of organic-rich chalks in the “Mottled Zone Complex”, Israel. Although the geometry of the prismatic columns observed in these rocks resembles those seen in basalts, our approximate calculations, based on thermal expansion and temperature difference, show that contraction due to cooling is two orders of magnitude less than the measured contraction. Moreover, the cooling effect should be much more significant in high-grade (ca. 550–900 °C) rather than in low-grade (ca. 200 °C) rocks, but only the latter contain prismatic columns. The prismatic columns are restricted to a discrete stratigraphic level between 1 and 10 meters above a chert and phosphorite unit. We suggest that burning of organic matter in the protolith results in mass and volume losses, and is the principal process that leads to contraction. Changes in texture and pore-structure (recrystallization) accompany the contraction resulting in the rearrangement of the protolith into columnar meta-chalk. This intriguing example of columnar joints in meta-chalk indicates that the geometric similarity of prismatic columns should not serve as the sole argument for their formation during cooling.