|Argentine Pampas lightning glass. Photos: Mike Cellinesi|
Time and time again, Earth has been pummeled by asteroids, but that ancient record is often faded and dubious. Lightning can beat up rocks like an asteroid strike, casting doubt on past impactsTo bolster the notion that rocks were beat up in an extraterrestrial impact, geologists search for a distinctive signature: microscopic bands in the mineral quartz, created when powerful pressure waves ripple through the rock. Now, a new study suggests that a different sort of shock can create the same banding patterns: a lightning bolt.
The result could cast further doubt on claims of asteroid impacts in Argentina and Australia that relied on observations of shocked quartz. The analysis should serve as a warning to geologists not to rely only on that line of evidence, says Matthew Pasek, a geochemist at the University of South Florida in Tampa who was not involved in the study. “This definitely shows that geologists need to consider the geological context of their samples.”
But lightning is powerful, too: It can heat rocks to temperatures above 2000°C within microseconds. Gierè and his colleagues set out to show that the bolts from the blue could also generate the extreme pressures needed to shock quartz.
In the team’s computer simulation, the target of the lightning bolt was a hunk of granite, which bears quartz crystals in large numbers. (It’s also common on mountain ridges and other high spots often struck by lightning.)
When a moderately strong bolt of cyber lightning struck the virtual rock, it created pressure waves that peaked at about 70,000 atmospheres, well into the range needed to produce shocked quartz, the researchers report this month in Geophysical Research Letters.
FulguritesThe simulated lightning bolt also generated a glassy veneer on the rock up to 9 centimeters or so from where the bolt struck, a so-called fulgurite (fulgur is Latin for lightning) that serves as a sign to geologists that rocks have been zapped. Just beneath this glassy layer lay a thin layer of containing shocked quartz.
Gierè says the result could help explain why his team found evidence for shocked quartz in fulgurites they collected from mountainous sites in France and Italy.F The results could also provide a more conventional explanation for strange, glassy rocks found at some sites in South Australia and on the Pampas of Argentina.
The presence of shocked quartz crystals in rocks from both regions have been cited as evidence of impacts, although the new findings suggest that the rocks could have just as easily been made by lightning strikes.
Gierè says geologists will have to be more careful when they make their impact diagnosis. But other researchers say that the scale of the effects should make that easy. Shocking from lightning is focused and limited to a thin layer near the surface, whereas impacts create shocked quartz widely throughout a target rock.
“This is a curious finding that is understandably going to fascinate a lot of people,” says David Kring, a cosmochemist at the Lunar and Planetary Institute in Houston, Texas. “But the scales of lightning strikes are small and not likely to be confused for kilometer-size or larger impact cratering events.”
This idea has continued to hold sway in the field in spite of a few cases where such quartz is found, but an impact origin seems unlikely. Now, a group at the University of Pennsylvania has challenged this canon by demonstrating that, under the right circumstances, ordinary storm-generated lightning may produce shock lamellae in quartz.
“Shocked”quartz comes in several varieties, depending upon the peak pressures involved. The lowest pressures that can produce features considered diagnostic of impact create“planar fractures”that form at pressures from 5 to 8 GPa.
This is well above pressures that can be achieved in volcanic“explosions”(volcanic eruptions are really events in which pressure is relieved in the under lying rocks, not generated, in spite of the common description of these events as explosions—they are only explosive in the sense that they can produce atmospheric shock waves).