Rapid fluid disruption: A source for self-potential anomalies on volcanoes

Self-potential (SP) anomalies observed above suspected magma reservoirs, di kes, etc., on various volcanoes (Kilauea, Hawaii; Mount Unzen, Japan; Piton de la Fournaise, Reunion Island, Miyake Jima, Japan) result from transient surface electric fields of tens of millivolts per kilometer and generally have a positive polarity. These SP anomalies are usually attributed to elec trokinetic effects where properties controlling this process are poorly con strained. We propose an alternate explanation that contributions to electri c fields of correct polarity should be expected from charge generation by f luid vaporization/disruption. As liquids are vaporized or removed as drople ts by gas transport away from hot dike intrusions, both charge generation a nd local. increase in electrical resistivity by removal of fluids should oc cur. We report laboratory observations of electric fields in hot rock sampl es generated by pulses of fluid (water) through the rock at atmospheric pre ssure. These indicate the relative amplitudes of rapid fluid disruption (RF D) potentials and electrokinetic potentials to be dramatically different an d the signals are opposite in sign. Above vaporization temperatures, RFD ef fects of positive sign in the direction of gas flow dominate, whereas below these temperatures, effects of negative sign dominate. This suggests that the primary contribution to observed self-potential. anomalies arises from gas-related charge transport processes at temperatures high enough to produ ce vigorous boiling and vapor transport. At lower temperatures, the primary contribution is from electrokinetic effects modulated perhaps by changing electrical resistivity and RFD effects from high-pressure but low-temperatu re CO2 and SO2 gas flow ripping water molecules from saturated crustal rock s. If charge generation is continuous, as could well occur above a newly em placed dike, positive static potentials will be set up that could be sustai ned for many years, and the simplest method for identifying these hot, acti ve regions would be to identify the SP anomalies they generate.