Lightning can Beat Up Rocks like an Asteroid Strike, Casting Doubt on Past Impacts

Astrogeology, Asteroids, Robots, Space Flight & Extraterrestrial Matters

Date posted: 07/26/17

Sid Perkins reports that Earth has been pummeled by asteroids, but that ancient record often appears to be faded and difficult to unravel. To 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. In the computer simulation, the target of the lightning bolt was a hunk of granite, which bears quartz crystals in large numbers. (It is 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. The simulated lightning bolt also generated a glassy veneer on the rock up to 9 cms 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. Making shocked quartz is not easy. Previous studies suggest that it requires shock waves of at least 50,000 to 80,000 times the pressure of Earth atmosphere-extreme levels that geologists presumed only impacts could provide, says Reto Gierè, a mineralogist.