If we can ever hope to understand the non-seismic signals that the Earth sends out before major earthquakes, we need to understand the physics of rocks under increased levels of stress. In particular we need to understand the generation of electrical currents in the ground. We have begun to study how electrical conductivity of igneous rocks changes under stress and what types of charge carriers are involved. We show that quartz-rich granite and quartz-free anorthosite both generate electronic charge carriers when subjected to stress. The charge carriers are positive holes (p-holes), i.e., defect electrons on the oxygen anion sublattice. They spread out of the stressed rock volume, the "source volume", into the surrounding unstressed rocks. Time-varying ground currents are required to generate pre-earthquake local magnetic field anomalies and low-frequency electromagnetic emissions. We posit that stress-induced activation of p-hole charge carriers and their outflow from the source volume is the basic process by which ground currents can be generated in the Earth's crust. We propose that the arrival of p-holes at the Earth's surface leads to changes in the ground potential that may induce ionospheric perturbations. We further propose that the build-up of high electric fields at the ground surface can ionize the air, hence cause ion emission and corona discharges. When p-holes recombine at the ground surface, they are expected to form vibrationally highly excited O-O bonds. The de-excitation of these O-O bonds will lead to stimulated mid-IR emission, which may explain the reported pre-earthquake "thermal anomalies" identified in satellite images.