We present a model for mode I propagation and concentration of microcracks in brittle solids driven by remote stresses or volumetric changes, e.g., due to thermal contraction or desiccation. Using a crack density integral, we obtain an effective elastic tensor; the dynamics of the crack density integral is described by an additional differential equation assuming subcritical microcrack growth. Numerical simulations allow the investigation of the physical background of pattern evolution and provide some hints that the pattern formed by the concentration of microcracks may have a significant influence on the macroscopic crack pattern which finally occurs. ¿ 2000 American Geophysical Union