The skies above Earth are constantly crackling with electric activity, from aurorae that materialize at the poles to sprites that burst upwards from thunderclouds. For the most part, we know how these phenomena work. But another mysterious light source has puzzled scientists for centuries. It doesn’t appear when clouds clash or solar rays pelt the atmosphere, but when the ground rumbles underneath our feet.
During some earthquakes, people have reported glowing lights that happen just before or during the shaking. Scientists weren’t certain the phenomenon was real, let alone explainable. But now they might have an answer. In a new study published in Seismological Research Letters, scientists from Quebec’s Ministry of Natural Resources and NASA propose that grinding rock can cause stress in deep in the crust, which in some types of rock can break apart pairs of negatively-charged oxygen atoms.
Joseph Stromberg, writing for Smithsonian:
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.
The study’s lead author, geologist Robert Thériault, says this behavior typically happens around the steep vertical faults common in rift zones. He and his team studied 65 historical earthquakes and noted how many reports of glows they could find.
Alexandra Witze, writing for Nature:
Thériault’s team decided to compile all the reliable reports they could find, from the year 1600 to today. They focused on 27 earthquakes from the Americas and 38 from Europe, and sorted through many bizarre tales. Off the Peruvian coast in August 2007, a fisherman reported the sky turning violet a few minutes before the sea began shaking. Near Ebingen, Germany, in November 1911, a woman reported seeing glows that moved along the ground “like snakes” as a quake began.
The number of reported occurrences is relatively low, making further study challenging. Still, scientists hope to test alternative theories as to how earthquake lights form. For one, they plan to measure soil’s electrical conductivity immediately before or during a quake, to see if that plays a role. No matter what their explanation, though, appearance of the lights may lead to an early warning system for earthquakes.
For more on another type of mysterious electrical phenomenon, watch “At the Edge of Space.”