Posted on Feb 11, 2005

Robert L. Fleischer, research professor of geology, wrote an
article on ancient impact cratering in the December
2004 issue of Meteoritics and Planetary Science, the journal of
the Meteoritical Society. Under the shock of an
incoming meteorite arriving at a planetary surface, crystals in rocks can be
converted into glass, a distinct, disordered material. For the 250-million-year-old
Bedout crater near Australia, such glass (from a feldspar mineral) has been cited as evidence that impact
produced the crater. Fleischer notes, however, that feldspar glass can also be
produced by ordinary heating and cooling – one evidence being a two-pound
feldspar glass, produced at General Electric, that he
has been using for 40 years as a paper weight. Thus, he argues, the presence of
such glass at a puzzling geological structure is not evidence of an origin by
impact.