Tektites: Where Do They Come From?

They are small, black blobs that might pass for hardened bits of asphalt but they are actually walnut-sized glassy stones. They commonly take on distinctive regular shapes like teardrops, dumbbells, and strangely flanged buttons that look like the tops of large rivets with the stems melted off. They're called tektites, and they are found strewn about on the ground in widely separated "fields" around the world, the largest of which covers most of Australia.

The one thing on which most scientists can agree about tektites is the way in which they obtained their shapes. Flow structures seen on tektites examined under the microscope tell of a very fast trip through the atmosphere while they were still molten. The flanged lip of the rivet-head type was formed by air forcing the molten glass from the nose back around the outer edges where it solidified while the tektite was still in flight. The dumbbell types were created when the molten mass was spinning like a majorette's baton, forcing material to the ends. If the connecting rod broke, the end result was two teardrop-shaped pieces, which then flew away from each other. Intermediate stages shaped like disks or rods are also found.

The earliest written reference to tektites was made in 1787, when it was thought that they were a special kind of obsidian, or volcanic glass (it was also suggested that they might be the product of some prehistoric glass factory). After all the intervening years of study, there is still disagreement as to their true origin.

Because the chemical composition of most tektites is so similar to that of the earth's crustal rocks, early theories that they may be true meteorites can almost certainly be rejected. However, it seems almost essential to include an "extraterrestrial connection" in explaining their formation.

One of the more durable theories about the origin of tektites is that they were formed from molten blobs that were splashed away from the site of impact when a meteorite struck the earth. Unfortunately for this theory, appropriate impact craters cannot be found to account for all the earth's tektite fields.

Another theory is that the meteorite impacts occurred on the moon and that tektites are droplets of rock that splashed off the moon onto the earth. Still other researchers insist that they have been propelled from the moon to the earth by titanic eruptions of lunar volcanoes that formed the mares, the stony seas of the moon. (Most astronomers strongly disagree with this, holding that these large, flat areas on the moon's surface were created by the impacts of large meteorites, with lava from the moon's interior filling the craters).

In the last few years, it has been learned that some of the meteorites found on earth are almost certainly fragments of Mars, but where tektites come from is still an unresolved question.

As the saying goes, "What on Earth...?"