FRANK SCHWARTZ' CONTRIBUTIONS TO IDENTIFICATION AND FORMATION OF FULGURITES ON THE BARRIER ISLANDS OF NORTH CAROLINA

Fulgurite is an amorphous mineraloid that can form when lightning strikes sandy or rocky materials and transfers its thermal energy. Lightning is an enormous spark of static electricity with temperatures exceeding 50,000 °F, or five times the Sun's surface temperature. Lightning flashes usually are 1-2 inches in diameter and emanate from cumulonimbus clouds between 3-5 miles above Earth's surface. Despite being found worldwide, fulgurites are rare, especially rock fulgurites. Sand fulgurites form in beach and desert environments where lightning strikes uniform sand lacking silt and clay. A temperature of 3300° F is needed to instantly melt quartz sand to form fulgurite. Lightning strikes on the ground have temperatures near 4500° F. The heat transferred to the sand creates a downward oriented tube with a glass liner formed from fused sand. Desert fulgurites resemble branching tree roots composed of tube-like structures covered with partially melted sand grains. Complete specimens of desert fulgurites can be 2-3 feet long and 1-2 inches in diameter.

Beach fulgurites are found as pieces along the upper intertidal zone. More than 150 beach specimens were collected for this study. A few specimens are the size of a football, others are like a small plate, most are the size of an egg. The specimens are gray and brown and are smooth, gritty, or bumpy in texture. Some specimens show remnants of the original beach sand layering, whereas others show later infilling of voids. Beach fulgurites are oddly shaped, wider, and more bulbous than those formed in deserts. This is due to electrically conductive saltwater in the pores surrounding the sand grains transporting heat away from the central tube. The intense, nearly instantaneous heat causes the saltwater to volatilize and escape upward to lower pressures while forming pockets, channels, interconnected tunnels, and domed vents not seen in desert fulgurites. Petrographic thin sections show heat deformation of quartz ranging from the inner glass, to melted and toasted grains, to the outer crust. It is unlikely a complete beach fulgurite specimen will be found due to erosion by crashing waves and the daunting task of collecting it in a thunderstorm. Consequently, it may not be possible to determine the original diameter, depth, and continuity of beach fulgurites.