July 18, 2017 | David F. Coppedge

Lightning Fries Impact Theory

A signature in rock that was thought to come from extraterrestrial impacts could have been caused by plain old lightning.

“Impact Geologists, Beware!”

Manicouagan Crater in Quebec, believed to be an impact crater

That’s the headline of a short paper in Geophysical Research Letters by H. J. Melosh of Purdue. His subtitle is, “A Cautionary Tale for Impact Geologists.”  Why the alarm? Another “proxy” for an unobserved event has come under fire – literally. Lightning fire.

For decades, geologists have looked at shocked quartz as an unambiguous sign of an asteroid impact. Only the pressure and heat of an impact could shock quartz sufficiently to produce what was seen. Geologists clung to this notion even when there was no evidence of an impact. There are places in Argentina and in Australia like that, Melosh says. But since the shocked quartz was an “infallible” sign of impact, what else could they conclude?

On Earth, meteorite impacts are among the most rare of geologic events. On our planet, lightning strikes more often than meteorites. 

Things just got tougher for impact geologists.

There are now evidences that ordinary lightning can produce pressure waves and temperatures sufficient to shock quartz. Fulgurites are peculiar “fossils” of lightning that form when bolts strike the ground. Studies of fulgurites show that they, too, can exhibit “planar deformation features” (PDFs) that were considered diagnostic of impacts. Both pressures and temperatures achievable by lightning overlap into the ranges thought only possible from impacts. So what’s the upshot? Melosh concludes,

The overall consequence of this important paper will be to increase the workload on impact geologists: In the future, it will not be enough to find PDFs in quartz to demonstrate the presence of an impact event, but impact proponents will also have to rule out lightning strikes as well. Given the other peculiarities of fulgurites, this should not be impossible, but it will serve as a reminder that on Earth, meteorite impacts are among the most rare of geologic events. On our planet, lightning strikes more often than meteorites. 

Lightning storms from Earth orbit (NASA)

Astronauts commonly see lightning from earth orbit, but there are no accounts of astronauts witnessing a large impactor coming in. Even if few bolts actually create shocked quartz crystals, the sheer number of lightning storms taking place all the time on the planet seems a much more likely source for the phenomenon.

Geologists may develop new, unequivocal methods for distinguishing lightning-caused shocked quartz to that from impacts. What’s instructive here is another case of misplaced trust in an “infallible” proxy for unobserved past events. For decades we have heard geologists invoke impacts to explain everything from climate changes to the death of the dinosaurs. What other proxy measurements have you run across? (See “Geology Fail: The Problem with Proxies” from 6/25/2015). Proxies are extremely common in climate science.

Another noteworthy take-home lesson from this article is the quote that “meteorite impacts are among the most rare of geologic events.” Thank God for that. Earth would not be the beautiful, habitable place it is if extinction-level events were common. Scars of impacts do exist (e.g., Barringer Crater in Arizona), but most impactors burn up in the atmosphere. Descriptions in the book of Revelation, though, seem to suggest some big ones are coming in the last days of judgment (Revelation 8).

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