An Experimental Investigation of the Scaling of Columnar Joints

TL;DR

This study investigates the scaling behavior of columnar jointing using laboratory analogs of desiccated starch, proposing an advective-diffusive model to explain the observed patterns and their dependence on environmental conditions.

Contribution

It introduces a new experimental framework and a theoretical model linking evaporation rates to column scale in analog systems, explaining natural variations.

Findings

Columnar joint size correlates with evaporation rate.

Hysteresis influences pattern evolution.

Stable column scales exist for given conditions.

Abstract

Columnar jointing is a fracture pattern common in igneous rocks in which cracks self-organize into a roughly hexagonal arrangement, leaving behind an ordered colonnade. We report observations of columnar jointing in a laboratory analog system, desiccated corn starch slurries. Using measurements of moisture density, evaporation rates, and fracture advance rates as evidence, we suggest an advective-diffusive system is responsible for the rough scaling behavior of columnar joints. This theory explains the order of magnitude difference in scales between jointing in lavas and in starches. We investigated the scaling of average columnar cross-sectional areas due to the evaporation rate, the analog of the cooling rate of igneous columnar joints. We measured column areas in experiments where the evaporation rate depended on lamp height and time, in experiments where the evaporation rate was fixed using feedback methods, and in experiments where gelatin was added to vary the rheology of the starch. Our results suggest that the column area at a particular depth is related to both the current conditions, and hysteretically to the geometry of the pattern at previous depths. We argue that there exists a range of stable column scales allowed for any particular evaporation rate.