When rocks are subjected to mechanical stress, dormant electronic defects become activated. This activation produces electron-hole pairs, which increase the electrical conductivity of rocks by releasing highly mobile defects electrons, equivalent to O− in a matrix of O2−, called positive holes and symbolized by h*. The h* charge carriers can spread from the stressed rock into surrounding unstressed rocks. Preventing the outflowof h* alters the mechanical properties of the rocks: they become softer andweaker. Ongoing studies point to a delocalization of the wave function associated with the h* charge carriers, which is far-reaching and affects many neighboring O2–. Although the number density of positive holes may be as low as 1 in 1000, essentially all O2– in the rock subvolume lose some of their electron density. This loss weakens the interatomic bonds between anions and cations, thus affecting the mechanical properties of rocks.
Rock deformation and rock failure are widely studied from the viewpoint of rock mechanics (Anderson 2005, Atkinson 1987). However, stressing a rock also creates an electronic component. Since most rocks are good insulators, changes in electrical conductivity during application of stress have not received much attention. It has long been reported that the conductivity of rocks increases with increasing load. This increase is generally believed to be due to better grainto- grain contacts during compaction (Glover & Vine 1994, Nover et al. 1995) or to a reduction in the electrochemical potential for the formation of vacancies corresponding to the diffusion of the rate-controlling ion in the space-charge at the grain boundary (Conrad & Yang 2010). Postnikov (1978) reported that during cutting, electrophysical and electrochemical properties of rocks are markedly affected. This effect reportedly leads to a reduction in strength of up to 50%.