Anyone who has read my articles for even a short time knows that I have a few ruts that I run around in. Topics that I repeatedly return to. I guess one should write about what they are interested in so they can share that excitement with the reader. So once again I’m writing on an occurrence of natural glass that I hope some of you will find of interest.
At two localities in South Australia; separated by about 30 km Edeowie Glass is found. This enigmatic material has many of the indicators that researchers look for in impact formed glass. Though origin by lightning can not at present be completely ruled out. The glass never occurs with the characteristic tubular form of fulgurites. It is found instead as slaglike vesicular masses. The occurrences of this glass appear to be restricted to this defined area with relative abundance in two regions of the locality. Lightning formed glass would reasonably be expected to follow the random and generalized nature of the lightning strikes themselves. It would be found over the rest of the region of similar geology and topography, but the Edeowie Glass is not found except between Flinders Ranges and Lake Torrens.
The case for impact melting of the material is quite strong however. The glass is sometime found on the baked sediment, showing a continuum of changing structure from bottom to top. The top of a piece will be well fused and very glassy while the middle will be only poorly fused. Since the material is only completed melted in portions of a piece, there are clasts of a variety of other rocks incorporated in the structure. Bits of quartzite and sandstone as well as individual quartz grains are found in the slabs of glass. These fragments of rock contain the most striking evidence for impact origin. Shock metamorphism features are seen indicating that great pressure was exerted on the rocks that the clasts originated from. Pressures of this magnitude is generally excepted to exist only in cosmic impact events.
As I examined the specimens we have of Edeowie Glass I was struck by the similarity a couple of the pieces have to Indochinite tektites in their smooth surface contours. However, the pieces quickly become only poorly melted just under the surface becoming scoriaceous. They show some similarity in appearance to Muong Nong type tektites. Mainly in the banded layers of different colored glass. For the most part Edeowie Glass is closer to an impactite material than a tektite glass. This brings up what May to this writer the most interesting aspect of the glass. It shows strong visual evidence of being the remnants of a melt sheet from some aerial explosion. There is no crater and this material does have its closest cousin in my collection in Trinitite. As the following photographs show the similarity is quite striking. Surface melting and bubbling with a quick grading to incorporated fractured native rock below are the most obvious similarities.
This would be by no means the first proposed melt sheet glass. Libyan Desert Glass has been considered to have such an origin for several years by some. There are also reports of glass from South America with the same type features as Edeowie Glass.
If the mechanics of tektite forming impacts have been hard to work out then there is the extra challenge in the case of an air-burst explosion and the glasses it forms to resolve. This type of event May be like a giant Tunguska event, where little or none of the impactor itself reaches the Earth’s surface. No great craters are formed, however tremendous energy is release nonetheless. Where LDG is very well fused and often very clear. Edeowie Glass is poorly fused and black. As with tektites it May be that the material being melted is important to the color of the resulting glass. Or it May be the magnitude of the event that determines the quality of the glass. I feel most likely it is a combination of both these factors. A larger event will certainly do more melting and fine pure sand or sandstone will certainly produced a lighter colored glass than melting granite.
I think there is a good case for Edeowie Glass being from an impact event. I think that more such glasses are likely to be found.
Proving that impact melt sheets can form without the creation of a crater will be an important discovery. It will also give us one more type of impact scenario to think about. Tunguska was apparently too small to produce any melting of rock in the area. But, there was tremendous heat. Guesses and estimates have been offered for the size of cratering impacts that are needed to be tektite forming events. Perhaps there will be good estimates soon regarding melt sheet forming events.