Uniaxial compression of plates of brittle materials containing pre-existing planar cracks oriented at certain angles with respect to the direction of overall compression has revealed that the relative frictional sliding of the faces of the pre-existing cracks may produce, at their tips, tension cracks which deviate at sharp angles from the sliding plane. These tension cracks then continue to grow in a stable manner with increasing axial compression, curving toward an orientation parallel to the direction of axial compression. Within the framework of linear fracture mechanics, the out-of-plane extension of a pre-existing straight crack, induced by overall far-field compression, is analyzed, and various parameters which characterize the growth process are quantified. It is shown analytically that, for a wide range of pre-existing crack of orientations, the out-of-plane crack extension initiates at an angle close to 70¿ from the direction of the pre-existing crack; the exact value of this angle, of course, depends on the friction factor and the orientation of pre-existing crack. It is found that the growth process is stable initially, but the rate of increase of the length of the extended portion with respect to the increasing axial compression dramatically increases after a certain extension length is attained, and in fact, this length becomes unbounded if a small lateral tension also exists. Various limiting cases are examined and the corresponding analytical estimates are compared with the numerical results, arriving at good correlations. A series of qualitative experiments is performed on thin plates of Columbia Resin CR 39, arriving at excellent agreement with the analytical results. In light of the analysis, the phenomena of axial splitting, exfoliation ( or sheet fracture), and reckburst are examined, and it is suggested that they may all be the results of the out-of-plane (tensile) extension of pre-existing cracks, induced by large overall far-field compressions. This assertion is then supported by a series of experiments which show that the relative frictional sliding of the faces of one or even an array of prexisting cracks does not result in coplanar (sliding mode) crack growth, but rather leads to the formation of tension cracks which grow in the direction of maximum compression. Moreover, a pre-existing crack close to a free boundary grow in a similar manner under compression parallel to the boundary, and shows no tendency to move toward the free surface. Possible lateral buckling which may result, and which may cause further unstable crack extension, is illustrated experimentally, and discussed in an effort to shed light on the phenomena of rockburst and surface spalling.