For centuries, eyewitnesses have occasionally reported seeing an inexplicable phenomenon minutes before, during or after an earthquake: strange bright lights in the sky.
[A] team of scientists [led] by Robert Thériault analyzed the geologic circumstances of 65 earthquakes starting in the year 1600 that produced reports of light to see what these events had in common.
“The process starts deep in the crust, where rocks are subjected to high stress levels, prior to the stress being released to produce an earthquake,” [Robert] Thériault says. In certain types of rock, [study co-author Friedemann] Freund has shown in lab experiments, this stress can break apart pairs of negatively-charged oxygen atoms that are linked together in peroxy bonds.
When this happens, each of the oxygen ions are released, and these can flow through cracks in the rock, towards the surface. At that point, the thinking goes, high-density groups of these charged atoms will ionize pockets of air, forming a charged gas (a plasma) that emits light.
It’s useful as well to note that these “earthquake lights” only appear during – or before – an earthquake, meaning they represent a possible early warning in those cases where this phenomenon occurs.
Via Smithsonian for the full article.