‘Basalt fingers’ – origin of columnar joints?

Abstract

SummaryThermal stresses due to contraction during cooling have been widely accepted as the origin of columnar structures in certain igneous rocks. It is proposed that these highly ordered structures could be the result of a double-diffusive convective process in the melt during cooling, a process analogous to salt fingers observed in the laboratory and the oceans. Salt fingers are driven by appropriate differences in heat and salt content at the top and bottom of a stably stratified water column. In molten basalts, temperature and chemical-compositional differences between the top and bottom of a stagnant melt could drive highly regular columnar ‘finger’ motions in the melt. When solidification takes place eventually, cracks due to contraction during cooling would then have preferred paths of propagation into the interior from the surface along the junctions of adjacent ‘basalt fingers’ giving rise to columnar jointing. Morphological evidence supporting the finger hypothesis is cited and useful ways of gathering further field data that would shed some light on the validity of the hypothesis are suggested. For the benefit of geologists not familiar with double-diffusive processes, an up-to-date review of these flow phenomena is included.