Despite the prominent role of physical weathering in arid and semi-arid landscapes, there has been little study of the specific processes responsible for the rapid breakdown of subaerially exposed rocks. For example, many boulders and cobbles in deserts exhibit fine near-vertical cracks. Although workers have hypothesized that these and other cracks are initiated by diurnal heating and cooling, no convincing specific mechanism for their formation has been proposed. We have characterized these cracks at eight sites on surfaces of different ages in the Mojave, Sonoran, and Chihuahuan Deserts, and the high desert of central New Mexico. Our data reveal four basic types of cracks: longitudinal, surface-parallel, fabric-related, and meridional. The orientations of the first three types are associated with clast shape and rock fabric. The azimuths of meridional cracks, however, are preferentially aligned north-south, typically with a nonrandom multimodal distribution. We propose that these cracks are caused by tensile stresses that arise in the interior of clasts due to strong radial gradients in temperature that evolve and rotate in alignment with the sun's rays. We suggest that the multimodal nature of crack orientations may be in part attributable to the seasonally varying, latitude-dependent solar elevation angle. Over millennial time scales, we suggest that this thermal cracking is an efficient weathering process that, together with cumulic soil epipedon development, creates the key attributes of most desert pavements. In addition to individual clasts exposed on desert surfaces, this mechanism of cracking is potentially significant in other climates and on other planets, as well as for rock outcrops and for man-made structures.