![]() ![]() A wider examination of the outcrop also revealed a partially moss-covered fulgurite surrounding a shallowly-dipping fracture that intersects both the main sub-horizontal outcrop surface and several sub-vertical rock faces nearby (Fig. One fresh surface occurrence of fulgurite was identified, which occurs as a roughly 1-m diameter patch of brown-black glassy crust on the surface of granitic gneiss (Fig. Following the 7 th May storm, we traveled to Mt. Taiwushan, the island’s highest mountain (253 m). The strike with the highest current intensity (~ 162 kiloamperes) came to ground on Mt. About 14% of these were cloud-to-ground lightning (circles in Fig. On 7th May 2018 more than 3000 lightning events were detected during a storm on Kinmen Island. Some of the microstructural and petrographic characteristics of the fulgurite samples described here are reminiscent of those reported to form during low-level shock metamorphism, and thus our observations also have consequences for the interpretation and distinction of extreme metamorphic events. Overall, our observations suggest that cloud-to-ground lightning can result in the formation of microstructures at high-temperature (> 1700 ☌) and high-pressure (gigapascal-scale residual stress), both at the surface and along fractures to a depth of several meters in the granitic host rocks. This allows us to recognize and report the first known occurrence of rock fulgurite formed within fractures extending from the surface to a depth of several meters, suggesting that the extreme metamorphic effects of lightning are not restricted to the present-day topographic surface. Here, we document in detail the composition and microstructural features of rock fulgurites that were formed by recent cloud-to-ground lightning events on Kinmen Island, Taiwan. To date, there have been few comprehensive studies of the microstructural features formed in rock fulgurites during the transient high-temperature and high-pressure conditions experienced during a lightning strike 8, 10, 11, 12, 13, 14. However, the glassy rind of rock fulgurites is commonly weathered and altered, and thus they may be under-reported in comparison to sand fulgurites 6. Based on this, it is typically assumed that the energy dissipated during a lightning strike is insufficient to develop lightning-induced features at distances of more than a few centimeters into a solid rock body 6. Rock fulgurites are characterized by surface melting, and thin layers of a glassy surface crust are often described in the immediate vicinity of the landing point 8. ![]() In the vicinity of the landing point, cloud-to-ground lightning can cause shockwave pressures in excess of 10 GPa and temperatures above 1700 ☌, resulting in high-temperature and high-pressure features being formed in the target material, which is sometimes converted into a sand or rock fulgurite (e.g., 3, 4, 5, 6, 7, 8, 9). Lightning is a ubiquitous phenomenon on Earth (44 events per second on average 1) which can dissipate up to 10 9 J per flash 2. ![]() Our work broadens the near-surface environments in which rock fulgurite has been reported, and provides a detailed description of microstructures that can be compared to those formed during other types of extreme metamorphic events. The model shows that the electric current density in fractures up to 40 m from the landing point can be as high as that on the surface, providing an explanation for the occurrence of fracture-related fulgurites. Since this is the first time that fracture-related fulgurite has been described, we also carried out a 1-D numerical model to investigate the processes by which these can form. In comparison to a granite reference sample collected from a borehole at a depth of 138 m, microstructures in both the surface and fracture fulgurite are characterized by: (i) the presence of glass, (ii) a phase transformation in K-feldspar with the presence of exsolution lamellae of plagioclase, and (iii) high residual stresses up to 1.5 GPa. We demonstrate that a range of microstructural features indicative of high temperatures and pressures can form in fulgurites at the surface and in fractures up to several meters below the surface. Cloud-to-ground lightning causes both high-temperature and high-pressure metamorphism of rocks, forming rock fulgurite. ![]()
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