The elastic moduli of crystalline rocks show marked reductions when heated to high temperatures (T), even when subjected to confining pressure (P). This degradation of moduli has been attributed to the creation of new microcracks or opening of preexisting microcracks from thermal stresses due to differential thermal expansion between mineral grains. A similar lowering of moduli is observed on depressurization of crystalline rock and is also thought to be due to microcrack opening. A spherical inclusion model has been used to predict whether a given change in T or P will introduce a tensile crack between a mineral constituent and the rock matrix. By using the principal thermal expansions and compressibilities of individual minerals, and simplifying assumptions, we are able to predict crack densities as a model rock is subjected to changes of P and T. Experimental crack densities for three granitic rocks, a gabbro and two basalts are calculated from high temperature, high pressure Young's modulus data. The predicted and inferred crack densities are then compared for these six rocks. The results are dependent upon the P, T path to which the rock is subjected, and comparisons are for the case in which the rocks are heated only at maximum P, and then decompressed isothermally. Considering the simplicity of the model, we believe the comparisons are quite close for the three granitic rocks. The model underestimates the crack densities of the quartz-free gabbro and basalts.