Lightning-induced features on granitic gneiss and its implication for rare lightning scars from the geological record

Cloud-to-ground lightning can cause high-temperature and high-pressure deformation features in rock at the strike point, forming fulgurite. Although cloud-to-ground lightning is a frequent phenomenon, rock fulgurites are rarely reported. Here, we present field, optical microscope, X-ray fluorescence, scanning and transmission electron microscope, in situ synchrotron projection X-ray microscopy and X-ray diffraction, and Raman data to investigate the formation and preservation of a recently formed rock fulgurite from Kinmen Island, Taiwan. In the field, the fulgurite is asymmetrically distributed on the surface over an ~ 12 m 2 area. In detail, the fulgurite contains an opaque glassy layer comprised of quartz and feldspar clasts, voids, iron-rich spheres, and barite aggregates that overlies fractured quartz grains and feldspar grains with planar features. Secondary minerals found locally adhering to the glassy layer as greenish patches are mainly jarosite group minerals, which commonly form as a secondary hydrothermal alteration product. We interpret the jarosite to have formed during the solidification of melt in the presence of raindrops, suggesting the rapid alteration of glass and short preservation potential of rock fulgurite. Finally, we estimate the production ratio of fulgurite by calculating the energy required for fulgurite formation and that released to the ground by the lightning event. A calculated energy ratio of 0.026% is within the range of 10 –2 to 10 –4 of the lightning total energy delivered to the strike point. We suggest that the scarcity of rock fulgurite in the geological record may be due to both the low production possibility and rapid alteration.