Abstract Thermal cracking experiments have been conducted to investigate the role of anisotropic thermal expansion of the constituent minerals of a basalt, a microgabbro and a microgranite in changing the physical properties of the rocks. Samples of an Icelandic basalt, a microgabbro from Sweden, and a fine-grained microgranite from Ailsa Craig, Scotland, have been selected for these experiments. Samples were heated to a range of peak temperatures up to 800°C at ambient pressure. Thermal cracking was monitored through acoustic emission and elastic wave velocity measurements. Fluid permeability of the basalt was measured under hydrostatic conditions at elevated confining pressure using the steady state flow technique.