Common igneous and high-grade metamorphic rocks contain dormant defects, which release electronic charge carriers when stressed. Rocks thereby behave like a battery. The charge carriers of interest are defect electrons h(center dot), e.g. electronic states associated with O- in a matrix of O2-. Known as "positive holes" or pholes for short, the h(center dot) travel along stress gradients over distances on the order of meters in the laboratory and kilometers in the field. At rock-water interfaces the h(center dot) turn into center dot O radicals, e.g. highly reactive oxygen species, which oxidize H2O to H2O2. For every two h(center dot) charge carriers one H2O2 molecule is formed. In the laboratory the battery circuit is closed by running a Cu wire from the stressed to the unstressed rock In the field closure of the circuit may be provided through the electrolytical conductivity of water. The discovery of h(center dot) charge carriers, their stress-activation, and their effect on Earth's surface environment may help better understand the oxidation of the early Earth and the evolution of early life. (C) 2009 Elsevier B.V. All rights reserved.