# Experimental Simulation of Lightning Current Discharge on Rocks
## 4 Discussion
### Fulgurite Glass Formation  
For the first law of thermodynamics for closed systems, energy transferred across the boundary of a system in the form of heat always results from a difference in temperature between the system and its immediate surroundings and a change of internal energy of the system. Due to the mechanical imbalance between the high-temperature air around the lightning channel and the air surrounding the rock, the former expands, vaporizing the rock at its contact and producing a crater on the rock surface. These effects were observed via optical microscopy on testing sample A0 (Figure 7) and basalt samples A1-A4, which show millimeter-size craters surrounded by glass (Figures 8-10). Figure 8: Optical microscopy images of fulgurite on dry sample A1 (**a**) and wet sample A3 (**b**). Figure 7: Optical microscopy images of fulgurite on dry testing sample A0. An orange-brown spray rim was observed via optical microscopy around the crater of dry sample A2 (Figure 9) and it could be ascribed to the thermal decomposition of magnetite according to XRPD patterns on samples A1-A4. Octahedral sites in the magnetite atomic structure contain ferrous and ferric iron species. Different \(T_{f}\) and total fault energy values observed on sample A2 and A4 after multiple strikes (Table 2) could be related in part to the magnetite grains, which may increase the local conductivity of the sample owing to its electromagnetic properties. The electrons coordinated with iron species are thermally delocalized and migrate within the magnetite atomic structure [30]. The electrical conductivity of a rock at room temperature is determined by the amount of water present, the salinity of the water, and the manner in which the water is distributed through the rock [31; 32]. Therefore, the faster process involving the electric impulse discharge and the total fault energy (\(E\)) values observed on wet sample A3 and A4 (Table 2) could be ascribed to the variation of the electrical properties for water-bearing rock and the distribution of the water layer on the rock sample. The chemical interactions between the water and rock material at the pore surface can also be significant in the bubble formation [18; 33]. Optical microscopy of the wet samples (A3, A4) showed the presence of bubbles in fulgurite glass (diameter of bubbles \(<1\) um, Figure 10). These bubbles can be generated from the vaporization of water and incorporation of air during the fast cooling of melted rock on the wet sample surface. [PERSON] et al. [18] observed a diffuse porosity in the fulgurite glass of granite via environmental scanning electron microscopy (ESEM). These authors ascribed the formation of the voids and the change in the chemical composition of fulgurite compared to the bulk granitic rock to  
Figure 10: Optical microscopy images of bubbles in fulgurite glass of wet sample A3. These bubbles can be generated from the vaporization of water and incorporation of air during the fast cooling of rock melt on the wet sample surface. Figure 9: Optical microscopy images of orange-brown rims around a crater of dry sample A2. the vaporization of the wet rock during lightning strike. In samples B1 and B2, glass formation was not observed on the top of the rock because the arc passed directly to laboratory ground without evidence of melting on the surface of the granite. The felsic minerals composing the sample B1 and B2, and in particular, the large quantity of micas (micas = 7.5 wt% from XRPD quantitative analysis, Table 1), are known for their excellent electrical insulation properties [34], and can inhibit the lightning strike at the granite surface. However, fulgurite formation in granite has been previously observed, particularly in weathered samples [18]. Rock can weather via chemical decomposition or by physical disintegration. In the chemical decomposition of granite, quartz generally remains unchanged, biotite is transformed to chlorite and other clay minerals, and feldspars break down to a variety of clay minerals with plagioclase decomposing first [35].