The scaling of columnar joints in basalt

Lucas Goehring, Stephen W. Morris

Department of Physics, University of Toronto,

60 St. George St., Toronto, Ontario, Canada M5S 1A7.

Zhenquan Lin

Department of Physics, Wenzhou University, Wenzhou 325027, Zhejiang, China

Phys. Rev. E, 74, 036115 (2006)

Download preprint pdf: PRE2006preprint.pdf

Supplementary Movies: here, and here

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.