Geobatteries (also known as 'Self Potential' (SP) anomalies), are widespread measurable electrical differences in naturally-occurring geological formations. There have been several proposals to explain how they might come about - it's likely that there are numerous factors at work according to the prevailing chemical and geophysical conditions.
“According to a theory by Sato and Mooney, SP anomalies may be caused by electrochemical processes in the Earth crust similar to a galvanic cell: a steeply inclined electronic conductive mineralization connects regions of different redox potential. Such a configuration forms a giant electrochemical cell called ‘geobattery’.” source:
The measured electrical differences can be localised to just a few metres, or can stretch over many kilometres. Voltage differences can be in the range of millivolts, or in some cases up to a volt or two. See: A large self-potential anomaly and its changes on the quiet Mt. Fuji, Japan
Given the substantial scale of some of the (known) anomalies, it's perhaps surprising that they have received comparatively little academic attention. Although the voltages are low, the volumes of rock and soil can be very large, meaning that the number of electrons involved (and therefore the available current flows) imply that a geobattery might have the power to dramatically affect the way local geology develops.
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