1. Left: This large grey fulgurite has partially collapsed under its own weight, or the weight of the host material. The top of the tube was weathered rapidly to yield an orange oxidized rind.

2. Right: A cross-section of the tube, showing dense black glass containing angular white mm-scale shards. The writer suspects that the white masses are shocked quartz from the margin of the tube, entrained in the black, totally melted glassy core zone. Igneous petrologists and volcanologists will probably find an analogy with wallrock fragments (xenoliths), spalled into a dynamic magma chamber, quite irresistible! Most certainly a fulgurite at the melt stage would be a dynamic environment, par excellence.

"Rock of the Month #124, posted for October 2011" --- This sample, and the second sample shown below, was obtained by Mark Stanley of Sumar Minerals (Norwood, Ontario: mark.stanley@bellnet.ca). The find was made at Sidekick Road, Florence, Arizona. A small section from this Arizona find was donated by Mark for a forthcoming petrographic examination. Stanley sample 1727.

Tube-like sand fulgurites and associated massive and vesicular glasses have been well-documented over the years. Julien (1901) noted that a high feldspar content would promote melting in target sand. He cited the example of a rock fulgurite formed on granite gneiss near Split Rock, Lake Champlain, New York, in which the fulgurite crust was formed largely by fusion of feldspar rather than quartz.

The nature of fulgurites has been examined in three previous "Rock of the Month" entries. While examining two more spectacular examples of fulgurites, this month's contribution may be a good time to contemplate the nature of glassy substances found in unfamiliar circumstances.

The identification of mysterious glasses is an interesting exercise in forensic science. If practiced from a classic microscopist's viewpoint, then textures and relict mineral grains (which can be characterized more precisely than glass) assume the role of important clues. In brief, natural glass can form from straw and forest fires, lightning strikes, and extraterrestrial impact (Baker and Gaskin, 1946), as well as in volcanic processes and localized frictional melting along fault planes. Impact glasses include the discrete bodies known as "tektites". Rapid cooling leads to quenching of a melt, with insufficient time for the nucleation and subsequent growth of visible crystals. Obsidian may be the best-known product, but many volcanic rocks may preserve a glassy matrix to crystals and entrained fragments of pre-existing rocks. Glass also forms the groundmass ("mesostasis") in the chondrules in many chondritic meteorites.

In archaeological contexts, glassy materials may be formed by equally diverse processes, including metal refining, glass and pottery manufacture, incidental melting in kilns, accidental fires, and acts of war. Resolution of the various possibilities can be aided by a combination of bulk chemical analyses, petrography and the microanalysis of mineral grains and glasses (e.g., Pavlish et al., 2002). In the oldest examples, such as modification of stone tools, it may be hard to determine whether or not conscious application of fire has affected an artefact (Pavlish, 1996).

The formation of fulgurites in "conventional" (bolt) lightning strikes offers a possible explanation of the curious phenomenon of ball lightning. One model involves formation of pure Si vapour in a strike, which condenses to nm-scale particles, which then glow as the Si oxidizes back to stable silica. The possible emission of Si "nanostrings" from the cavity of a freshly-formed fulgurite could generate the peculiar glowing balls (Matthews, 2000).

Lightning may be attracted by both natural and manmade structures, including conductive rock formations, trees, poles and towers. In some cases, however, structural failure of high-tension power lines may send high currents to ground and generate molten rock pools with some of the same characteristics as fulgurites. An example generated in quartzite in south India in 1998 is described by Manimaran et al. (2001), with melt generated as much as 7 metres from an 11-kV power pole. A similar example from northeast India is discussed by Bhattacharyya et al. (2002), on amphibolite bedrock. In the absence of field evidence, it is not yet clear to the author how this material may be systematically different from fulgurites, though the "accidental melts", the power line examples, formed under less extreme conditions and for a longer duration than fulgurites.

The following specimen is a stocky grey glass-filled melt tube, a fulgurite from Oswego, New York, U.S.A. Stanley sample 1735.

Two views of the thick grey Oswego fulgurite, the interior filled by spectacular bubbly grey glass. Some of the ~2-cm bubbles are separated by knife-sharp glass walls.

References

Baker,G and Gaskin,AJ (1946) Natural glass from Macedon, Victoria, and its relationships to other natural glasses. J.Geol. 54, 88-104.

Bhattacharyya,C, Das,S, Banerjee,J and Pal,SP (2002) Rock melt extrusion at Puruliya, West Bengal. J.Geol.Soc.India 60, 323-327.

Julien,AA (1901) A study of the structure of fulgurites. J.Geol. 9, 673-693.

Manimaran,G, Sivasubramanian,P and Senthiappan,M (2001) Rock melt extrusion at Abishekapatti, Tirunelveli district, Tamil Nadu. J.Geol.Soc.India 57, 464-466.

Matthews,R (2000) Great balls of fire. New Scientist 2233, 22-26, 08 April.

Pavlish,LA (1996) Aspects of the Role of Temperature Manipulation in Ancient Technologies: Testing a Model. PhD Thesis, Graduate Department of Anthropology, University of Toronto, 343pp.

Pavlish,LA, Wilson,GC and Nijagunappa,R (2002) Toranagallu mound: provenance of unusual glassy materials. J.Geol.Soc.India 59, 503-515.