Find a map of Egypt and put a pin right in the center. You’ll find yourself in one of the most remote human settlements in Egypt, way out in the Western Desert about 400 kilometers west of Luxor and the Valley of the Kings. Dakhleh Oasis occupies the basin of an ancient lake, the site of human habitation going back some 200,000 years or more.
In 1987, University of Toronto anthropologist Maxine Kleindienst reported the discovery of peculiar lumps of vesiculated glass in the lake sediments of the Dakhleh Basin. Subsequent investigations have shown this material to be meteorite-related impact glass, possibly the result of a monster aerial burst, something like Tunguska, but much more intense (The Tunguska event did not vitrify the underlying ground surface at all). Dakhleh glass is highly similar to Argentine Escoria. Some specimens are also very similar to the impact melt glass bombs of Nordlinger Ries (figure 1).
The airburst suggestion arises from the distribution of the glass, scattered over an area of about 400 square kilometers (too extensive for typical cratering events) and the absence of any recognized impact crater. The best radiometric date for the event is 145,000 years +/- 19,000. Evidence of human habitation is found in layers stratigraphically above and below the glass-bearing strata, showing that humans were on hand to see and experience this event (certainly not a good thing for anyone that was too close!).
Dakhleh glass is quite variable. Smaller pieces tend to be highly vesiculated while bigger lumps can be mostly crystalline and dense. The glass is chemically distinctive, being unusually rich in calcium and aluminum (up to 25% and 18 wt% respectively). This composition is consistent with derivation from calcarious and clay-rich lake sediment target material. It contains small enclaves of very high silica glass (lechatelierite) which forms at temperatures greater than 1700 degrees C. Interestingly, about one third of the pieces show imprints of plant matter (marsh reeds?) on their basal surface (see figure 2), indicating that the original blobs of bubbly glass rained from above and were still of sufficiently low viscosity to mold over the plants they landed on. There is also sometimes seen a flow-stretching to the vesicles and a tendency for them to be present in greater abundance near the upper surface of a given specimen, again indicating fluid behavior that allowed the bubbles to stretch and rise.
The material resembles coal clinkers with smooth ropy surfaces and a highly vesiculated interior. (The presence of lechatelierite distinguishes it from some sort of ancient clinker). The interior color is brownish gray to black. Rare specimens show spherical beads up to 5 mm in diameter (see figure 3). While it is tempting to compare these with microtektites or larger impact spherules, studies to date indicate that they are mono-mineralic pyrrhotite or calcite which most likely formed as immiscible droplets in the molten glass. Prismatic cavities in the basal surface appear to be spots where lake-bottom mud-chips have weathered out (remnants are locally present).
Recent investigations of Trinitite glasses from the initial atom bomb test in New Mexico suggest that the glass there did not form by in situ fusion of the crater wall as has been commonly supposed, but instead rained down as vitrified blobs of target material that was initially drawn up into the mushroom cloud (Eby et al, 2010). The Dakhleh glass and Argentine escorias are remarkably similar in morphology to Trinitite, and indeed, the basal plant imprints directly imply that the glass rained down from above. One is led to picture an immense mushroom-cloud aerial burst that literally vacuumed material from the ground beneath, melted it in the heart of the turbulent fireball, and rained it back to earth.
References
Eby, N., Hermes, R., Charnley, N. and Smoliga, J. A., 2010, Trinitite-the atomic rock. Geology Today, v. 26: pp. 180-185.
Osinski, G.R., Haldemann, A.F.C., Schwarcz, H.P., Smith, J.R., Kleindienst, M.R., Kieniewicz, J., and Churcher, C.S., 2007, Impact Glass at the Dakhleh Oasis, Egypt: Evidence for a Cratering Event or Large Aerial Burst?, Lunar & Planetary Sci., v. 38.
