Melosh reported in 2017 that geologists examining meteor impact sites have long assumed that shock metamorphic features, such as planar fractures and Planar Deformation Features (PDFs) in quartz are reliable indicators of an extraterrestrial impact. A new paper by Chen et al., (2017), now shows that such features might arise in terrestrial lightning strikes, thus raising the bar for identification of impact sites.
For decades now, geologists have relied on a seemingly infallible test for the pedigree of a suspected impact structure: If you can find shock metamorphosed minerals, especially quartz, then the structure was made by a forceful extraterrestrial impact because no other known process could achieve the pressures necessary to alter quartz into one of several high-pressure forms, commonly referred to as “shocked” quartz. 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 [Haines et al., 2001; Schultz et al., 2004]. Now, a group at the University of Pennsylvania [Chen et al., 2017] has challenged this canon by demonstrating that, under the right circumstances, ordinary storm-generated lightning may produce shock lamellae in quartz.
Geologists have long applied the name “fulgurite” (from Latin fulgur for lightning) to hand-sized patches of glass on exposed rock surfaces or, more commonly, to glassy hollow tubes that form in sand [Pasek et al., 2012]. Distinctive types of fulgurite may also form in clay or caliche. Fulgurites have also been made artificially by strong electrical discharges and so their origin is not controversial. They form when cloud-to-ground lightning strikes the Earth’s surface, generating air temperatures up to 100,000 K from currents that range upwards of 30 kA in centimeter-diameter channels with linear energy densities of a few MJ/m. Energies of this order can easily melt rocks on the surface, locally raising them to temperatures exceeding 2000 K.
The issue here, however, is not whether lightning can create high temperatures, but whether it can also generate pressures equal to those achieved in meteorite strikes. “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 [Stöffler and Langenhorst, 1994]. This is well above pressures that can be achieved in volcanic “explosions” (volcanic eruptions are really events in which pressure is relieved in the underlying 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). Such pressures are only achieved statically at depths of about 200 km in the Earth. “Planar Deformation Features (PDFs)” form at higher pressures (7 to 30 GPa) and mineralogical transformations to high pressure phases Stishovite (12 to 15 GPa) and Coesite (> 30 GPa) occur at still higher pressures. Shock melting requires pressures in excess of about 50 GPa [Stöffler and Langenhorst, 1994].
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