Development of micro-scale joints in volcanic rocks under thermal stress

Petrographic studies of samples of the Rajmahal basalt reveal a variety of microscopic joints within phenocrysts, which seem to have developed under the influence of thermal stresses during cooling. The theoretical analysis shows that:

–	•thermal stresses develop in a system only under unsteady state of cooling,
–	•the stresses responsible for the development of joints are tensile in nature, and
–	•the magnitude of stresses increases with increasing rates of cooling.

In order to understand the effects of cooling rate on the mode of jointing, analogue model experiments were performed by quenching melt films of organic, crystalline materials under varying cooling conditions. In experiments, non-branching linear joints developed at relatively lower rates of cooling by a process of sub-critical crack propagation. On the other hand, at higher rates of cooling the mechanism of crack propagation was essentially supercritical leading to the development of branching joints.