Paradox of peroxy defects and positive holes in rocks Part II: Outflow of electric currents from stressed rocks

Understanding the electrical properties of rocks is of fundamental interest. We report on currents generated when stresses are applied. Loading the center of gabbro tiles, 30 × 30 × 0.9 cm3, across a 5 cm diameter piston, leads to positive currents flowing from the center to the unstressed edges. Changing the constant rate of loading over 5 orders of magnitude from 0.2 kPa/s to 20 MPa/s produces positive currents, which start to flow already at low stress levels, <5 MPa. The currents increase as long as stresses increase. At constant load they flow for hours, days, even weeks and months, slowly decreasing with time. When stresses are removed, they rapidly disappear but can be made to reappear upon reloading. These currents are consistent with the stress-activation of peroxy defects, such as O3Si-OO-SiO3, in the matrix of rock-forming minerals. The peroxy break-up leads to positive holes h, i.e. electronic states associated with O− in a matrix of O2−, plus electrons, e′. Propagating along the upper edge of the valence band, the h are able to flow from stressed to unstressed rock, traveling fast and far by way of a phonon-assisted electron hopping mechanism using energy levels at the upper edge of the valence band. Impacting the tile center leads to h pulses, 4-6 ms long, flowing outward at ∼100 m/s at a current equivalent to 1-2 × 109 A/km3. Electrons, trapped in the broken peroxy bonds, are also mobile, but only within the stressed volume.