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Darkish fulgurites scar the lightning-prone peak of the Mount Shasta volcano in California.

Castro et al., Earth and Planetary Science Letters (2020) 10.1016/j.epsl.2020.116595

Fossilized lightning bolts reveal when ancient storms struck

When a lightning bolt strikes a mountaintop, it can melt rocks in a flash, leaving a narrow glassy scar called a fulgurite. Now, researchers have shown that these fossilized thunderbolts are geological clocks that record the passage of time. The technique offers geologists a way to date thunderstorms from tens of thousands of years ago, and could give them a window into ancient climate patterns.

“It’s a very interesting and ingenious method,” says Rafael Navarro González, a chemist from the National Autonomous University of Mexico, University City, who was not involved in the study.

When exposed to the elements, glasses like fulgurite slowly and steadily absorb moisture, “sort of like a sponge,” says Jonathan Castro, a volcanologist at the Johannes Gutenberg University of Mainz. In the past, researchers have tried to measure the amount of water in the outer layers of glassy archaeological artifacts, such as obsidian arrowheads, as a way to date how long they had been exposed. However, the method was messy, Castro says.

Part of the problem is that many of these glasses come from volcanoes and already contain water from the time they were forged. That water can muddle measurements of new water absorbed near the surface of the glass. But Castro thought that in fulgurites, the lightning strike might vaporize any residual, internal moisture.

To investigate, he and his team first made artificial fulgurites of their own. They shocked cores of volcanic rock with an arc welder, raising temperatures to 10,000°C. Zapping the rocks boiled off most of the moisture near the surfaces of the samples. That means the lightning strikes initiate a geological stopwatch the moment a fulgurite is created.

A glassy fulgurite found on top of Oregon’s South Sister volcano

Castro et al., Earth and Planetary Science Letters (2020) 10.1016/j.epsl.2020.116595

“They show for the first time that the water content completely goes down” during the lightning strike, leaving behind a clean surface in which environmental water can accumulate, says geochemist Matthew Pasek of the University of South Florida.

Next, the researchers tried to apply the dating system to natural fulgurites. They climbed volcanic peaks in Oregon to hammer out the shiny, black samples. The glassy rocks “stand out in stark relief” from the surrounding rocks, Castro says.

Close inspection of the fulgurites’ outer rinds revealed higher levels of moisture that decreased sharply further into the rock. These moisture profiles behaved very “mathematically,” Castro says—good for dating. The scientists calibrated their clock by using computer models to figure out what rate of moisture absorption best accounted for the profiles seen in the samples.

They found that some of the fulgurites (and the thunderstorms that made them) were hundreds of years old. More importantly, the method could date the rocks to within decades or centuries, Castro and colleagues report this week in Earth and Planetary Science Letters. That offers sharper time resolution than that of techniques that rely on the bombardment of rocks by cosmic rays. In addition to shedding light on historic lightning patterns, the team believes the new method could reveal when mountaintops became vulnerable to lightning strikes.

For example, fulgurites could show when ice age glaciers began to retreat. Geologists already know where: Mountainous piles of debris, called terminal moraines, record the farthest thrust of an advancing glacier. In mountainous regions, these moraines should include fulgurites. By dating them, scientists can pin down the timing of the retreat, which could help reconstruct past climate trends, Castro says. “They’re going to be a very powerful way to bracket the ages of moraines.”