Over the past few years, various theories have been proposed for how earthquake lights form, including the disruption of the Earth's magnetic field by tectonic stress and the so-called piezoelectric effect, in which quartz-bearing rocks produce voltages when compressed in a certain way.
But Freund and colleagues now report that what causes earthquake lights appears to be an entirely different electrical process.
"When nature stresses certain rocks, electric charges are activated, as if you switched on a battery in the Earth's crust," he says.
The types of rocks that are particularly given to the phenomenon are basalts and gabbros, which have tiny defects in their crystals. When a seismic wave hits, electrical charges in the rocks may be released.
In some areas, basalts and gabbros are present in vertical structures called dikes, which formed as magma cooled along vertical faults and may reach as deep as 60 miles (97 kilometers) underground. These dikes may funnel electrical charges along, the scientists wrote. "The charges can combine and form a kind of plasma-like state, which can travel at very high velocities and burst out at the surface to make electric discharges in the air," Freund added. Those discharges are what make the colorful light shows.
The right conditions for lights exist in less than 0.5 percent of earthquakes worldwide, the scientists estimate, which explains why the phenomena are relatively rare.
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