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 tinder 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 natur e, and the magnitude of stresses increases with increasing rates of cooling. In order to understand the effects of cooling rate oil the mode of jointing , analogue model experiments were performed by quenching melt films of orga nic, crystalline materials tinder varying cooling conditions. In experiment s, non-branching linear joints developed at relatively lower rates of cooli ng by a process of sub-critical crack propagation. Oil the other hand, at h igher rates of cooling the mechanism of crack propagation was essentially s upercritical leading to the development of branching joints.