Borehole breakout experiments are performed on sintered glass bead bricks and a new microscale fracturing mechanism is described that results in a macroscopic fracture plane oriented perpendicular to the uniaxial compressive loading stress. Scanning Electron Microscopy (SEM) is used to inspect the fracture surfaces and it is found that fracture propagation occurs through a process of grain debonding and ejection from the sample. A two-dimensional boundary integral simulation models the effect of grain removal on subsequent fracturing by reducing the Young's modulus of failed grains, thereby allowing the compressive loading stress to be transferred to adjoining grains and grain contacts. It is found that this stress concentration is sufficient to induce additional grain failure and macroscopic fracture growth perpendicular to the applied load.¿ 1997 American Geophysical Union