Conductive cooling of lava: columnar joint diameter and stria width as functions of cooling rate and thermal gradient

Abstract

Much evidence exists that column diameter and stria width of columnar joints in lava flows vary inversely with cooling rate (∂T/∂t) and thermal gradient (∂T/∂x) at the point of fracture as the flow cools. Some evidence suggests that lava flows cool conductively. A conductive cooling model yields cooling rate and thermal gradient through space and time for cooling igneous bodies, both for extrusive and intrusive settings. Thermal gradient and cooling rate are then linked to stria width and column diameter, with faster cooling leading to narrower columns, and steeper gradients to narrower striae. This model shows large regions within a body where the ratio of thermal gradient to cooling rate is nearly constant, and predicts that the ratio of stria width to column diameter is also nearly constant. This prediction is consistent with field observations, both in this and other works. The model also suggests that if conductive cooling dominates, then joint spacing (i.e., column diameter) should increase inward from the margins of the body, as should stria width.