Desert glass: is it baked Australia?

Science News, Nov 24, 2001

A profusion of fused, glassy material found on the desert plain of southern Australia might be the result of the intense heat from an extraterrestrial impact.

Scientists have found irregular hunks and flat slabs of glass at two locations in the Edeowie region about 400 kilometers north of Adelaide. At these places, separated by about 30 km, the glass is locally concentrated but otherwise sparsely distributed over several square kilometers, says Peter W. Haines, a geochemist at the University of Tasmania in Hobart. He and his colleagues describe the mysterious melted material in the October GEOLOGY.

The so-called Edeowie glass is most common around slightly elevated areas of hardened, reddish-brown sediments. Some of these outcrops, which look like fire-baked mud, are several meters wide and contain large circular holes that the researchers believe mark the sites of former tree trunks. In rare cases, slabs of the glass have been found still attached to the underlying sediments, which share the glass' chemical composition.

Grains of quartz trapped in the glass show signs that they had partially melted, which could have happened only at temperatures above 1,710 [degrees] C. That means the glass probably didn't come from natural fires, Haines notes.

The composition of the material also is inconsistent with volcanic processes, which produce more-familiar glassy materials, such as obsidian. Nor is lightning a likely explanation, since it would have led to a more even distribution of glass over a wider area. The topography of the region argues against frictional heat due to landslides as the glass-making heat source.

The quartz grains hold another clue. They show signs of having experienced intense pressure along several different planes, a hallmark of impacts from space. Although scientists haven't found a crater of the right age in the area, Haines and his team suggest that the intense heat that fused the sediments into glass could have come from a meteor. It might have burst in the air above the ancient landscape, they say, or simply heated the air as it passed close over the area.

--S.P.