Experimental vs. natural fulgurite: A comparison and implications for the formation process

A. Zeynep Çalışkanoğlu, Corrado Cimarelli, Donald B. Dingwell und Alessandra S.B. Camara

Abtract

Fulgurites are glassy structures formed when lightning strikes the ground, causing ground material (e.g., rocks, sediments, or soil) to melt and fuse. While fulgurites are relatively rare, they provide valuable insights into paleoecology and may play a key role in prebiotic chemistry. Despite their significance in nature, understanding the conditions underlying the formation of fulgurites poses severe challenges, as the physical parameters and timing of the fulgurite-generating lightning event still need to be discovered.
Here, we use a unique opportunity from the recent in situ discovery of a natural fulgurite still embedded in its protolith. (The natural fulgurite-generating event is visible in the World Wide Lightning Network data.) Using a high-voltage setup, we further compare this natural fulgurite with the experimentally generated fulgurite obtained from the original protolith. The natural and experimental fulgurites exhibit evidence of similar melting sequences and post-melting recrystallization structures. Using Raman spectroscopy applied to the quartz phase transition, we estimate the thermal gradient present in the fulgurite during formation to be a minimum of 1600 °C at the inner wall of the fulgurite and ca. 600 °C at the outer wall of the fulgurite. Those findings suggest that the current responsible for the cloud-to-ground lightning discharges that generated the natural fulgurite lay in the range of 11 960 to 14 473 kA. The state of the experimental fulgurites matched that of the natural fulgurite, validating the experimental option for studying fulgurite generation.