NSRRC Activity Report 2022

072 NSRRC ACTIVITY REPORT 2022 Scars of Cloud-to-Ground Lightning The rarity of cloud-to-ground lightning-produced remnants in the geological record is due to the processes through which they are generated and the difficulties of preservation. C loud-to-ground lightning is a common natural phenomenon. When cloud-to-ground lightning strikes the ground, its powerful electric current generates tremendous heat that melts rock. Amorphous material forms on top of the scorched target rock, namely fulgurite ( Fig. 1(a) ). Fulgurite constitutes an ideal research subject for the study of materials subjected to extremely high temperature and pressure. However, despite the frequent occurrence of cloud-to-ground lightning, discoveries of fulgurite are rare. Li-Wei Kuo’s team (National Central University) traced a cloud-to-ground lightning strike with a peak current of 162.2 kA on Kinmen Island, Taiwan, in 2018; they discovered a resultant rock fulgurite on granitic gneiss. 1 Kuo collaborated with the NSRRC to conduct in situ micro-scale characterizations of the extremely thin, glassy layer of rock fulgurite, including the microstructure, chemical composition, and mineralogical phases. An examination of the petrographic thin sections by optical microscopy revealed that the rock fulgurite was composed of fractured quartz and feldspar grains with planar features and an overlying opaque glassy layer. Determined by Laue diffraction conducted at TPS 21A , 2 the presence of exsolution lamellae in the feldspar revealed high residual stress, indicative of high pressure (up to GPa) caused by cloud-to-ground lightning. The three-dimensional inner structure of the rock fulgurite in the in situ sample was determined using projection X-ray microscopy (PXM) at TPS 31A ( Fig. 2(a) ). The glassy layer on the fractured grains was identified to be a distinct layer with voids ( i.e. , dark areas inside the glassy layer) and mineral clasts. The researchers subsequently employed scanning electron microscopy (SEM). The microstructure of the glassy layer was characterized as having wavy lamination and intertwined flow structures mixed with mineral clasts, including quartz, feldspar, and irregularly shaped voids ( Figs. 2(b) and 2(c) ). Fig. 1 : (a) Rock fulgurite on granitic gneiss on Kinmen Island, Taiwan. (b) Jarosite group minerals on the glassy layer. [Reproduced from Ref. 1] Fig. 2 : (a) PXM image of rock fulgurite; inset of the sample and analysis area. (b,c) Backscattered electron imaging by SEM of the glassy layer; mineral clasts, voids, and flow structure. [Reproduced from Ref. 1]