LIGHTNING STRIKE GLASSES: THERMODYNAMICAL EFFECTS ON THE FORMATION OF ROCK FULGURITE FROM BAVENO-MOTTARONE (ITALY)

Fulgurite is a natural glass generated when lightning strikes sand, rock, or soil. Fulgurite forms with an energy density within an order of magnitude of that of meteor impacts, and displays the effects of impact-related processes owing to lightning. The aim of this study is to provide insights into the processes occurring during lightning strikes for fulgurites collected near Baveno, Italy. The samples are from a pink granite comprised of quartz, plagioclase (Ab₉₈An₁Or₁), orthoclase, and micas. Fulgurite coats the granite as a thin glassy to fine-grained porous layer. X-ray diffraction of the fulgurite reveals the presence of newly-formed cristobalite and relict quartz in a glass matrix. The presence of glass indicates that the abrupt Joule heating of the rock surface produced a thin melt layer which then cooled adiabatically. The presence of cristobalite would indicate induced temperatures above 1700 ºC and atmospheric pressure if thermodynamic equilibrium were obtained. Electron microprobe analyses revealed that fulgurite glass is mainly composed of SiO₂ and Al₂O₃ and has a porosity of about 10% in the area analyzed. The lack of Na, Ca, and K in the glass suggests that alkalis were liberated from decomposing feldspars during heating. The brown-to-black color of the fulgurite is due to 7 wt% of carbon, derived from organic matter that was present on the weathered surface of the granite prior to the lightning strike. The residual organic matter in the glass suggests that rapid quenching of the melt traps NO_x, and CO_x gases produced during heating. To better clarify the thermodynamics of fulgurite formation, idealized physical models were developed to simulate the effects of Joule heating on the granite and to estimate the area of the surface which became hot enough to burn the organic matter. Calculations suggest that a weathered layer of higher electrical conductivity on the surface of the granite results in strong Joule heating near the surface, which causes a relatively large area of burnt organic matter.