MODELING THE EVOLUTION OF COLUMNAR JOINTS

Cooling cracks, developing from the margins of solidifying lava flows or intrusive dykes or sills, often propagate inward and divide the rock into prismatic columns. It has been shown that: (1) the initial crack pattern that forms at the surface of the body is not as regular as the pseudo-hexagonal pattern which evolves later; (2) columnar structures develop by repeated, step-wise crack advances, forming cm to dm transverse bands on the column faces; (3) even within well-developed colonnades, the polygonal outlines continue to shift slightly from one crack advance to the next. We propose that each new crack should propagate parallel to the highest thermal gradient ahead of the current crack tip. When adjacent columns are of unequal size, the local asymmetry of the isotherms drives the new crack toward the biggest and hottest column. We present a geometrical model and algorithm that follows this rule and mimics the successive crack advances as the cooling front migrates into the lava sheet, or dyke. The model polygonal pattern obtained consists of convex, irregular polygons with a variable number of sides, but in which pentagons and hexagons predominate. The algorithm successfully reproduces the rapid evolution from an initial, immature crack pattern to a pseudo-hexagonal, mature one in which the average number of sides approaches six. It predicts also the predominance of Y-type crack junctions, the rapid evolution toward columns of approximately equal sizes, and the persistent changes in length and diverging orientation of growth steps observed along the sides of columns. The model polygonal patterns exhibit geometrical properties which are very similar to those observed in well-developed columnar jointed flows such as the Giant's Causeway.