Samples of Westerly granite, Frederick diabase, and Oak Hall limestone were thermally cracked at room pressure to various peak temperatures. Scanning electron microscopy (SEM) was performed on ion-thinned samples. The thermally induced crack density is dependent upon the temperature, thermal expansion mismatch, thermal expansion anisotropy, initial crack porosity, and grain size. The mode of propagation is different for grain boundary and intragranular cracks. Crack densities in the granite were quantified using stereological techniques. The thermally induced crack surface area per unit volume apparently has a quadratic dependence on the temperature increase, a physical interpretation for which can be formulated on the basis of energetic balance. Fracture mechanics models are developed to interpret thermal cracking. The predictions concerning thermal crack initiation temperature and crack propagation and arrest behavior agree well with observations for the granite and the diabase. The model predicts significant thermal cracking for the Oak Hall limestone, which contradicts the SEM observation. A possible explanation for this discrepancy is that the internal stresses due to thermal expansion anisotropy are relaxed by plastic flow in this relatively fine grained limestone.