What Caused Argentina's Craters?

For more than a decade, planetary scientists have been puzzling over a mixed bag of meteorite evidence scarring Argentina's plains. They gradually pieced together clues to reconstruct what seemed to be a rough-hewn but generally accurate account of a prehistoric meteorite impact. A mere 10,000 years ago, scientists deduced in the original theory, a sizable meteorite came hurtling through the atmosphere at a bizarrely low angle, smacked the ground with a glancing blow, and broke into numerous pieces that gouged separate, miles-long scars in the Argentine earth.

A low-angle meteorite impact—the only one known to science, now that Argentina's craters have been reinterpreted—gouged a 16.5-kilometer depression in the surface of the Moon.

But now a fresh analysis has turned that theory on its head. The mysterious craters in Argentina may not have been caused by meteorites at all, but rather by the wind, sculpting the ground over a long time. Discovered in some of these crater-like trenches, ironically, were the remains of real meteorites that crashed into Earth over widely separated time periods. They struck at different angles and produced spectacularly different results—including, in the case of one, a widespread shower of molten glass.

The evolving interpretation of Argentina's mysterious craters, University of Arizona planetary scientist Jay Melosh writes in the May 10 issue of the journal Science, "is both less and much more than [its] discoverers originally believed."

Citing evidence presented by Philip A. Bland in the same issue of the journal, Melosh describes a newly emerging picture, in which a much older meteorite collision blasted tons of sandy, local soil into the air—melting it instantly and peppering a vast swath of country with glowing, glassy debris.

Bland's discovery of so many glass fragments over such a wide area adds a startling twist to a young but already storied saga in planetary science. One implication, the researcher from England's Open University says, is the possibility that a "well-preserved complex crater remains to be discovered beneath the Pampean Plain of Argentina."

The buzz about the nature of Argentina's extraplanetary legacy began in 1991, when Brown University geologist Peter Schultz and his Argentine pilot, Ruben Lianza, took a flight over Rio Cuarto, a small city in a region of Argentina known as the Pampas or the Pampean Plains.

From the air, Schultz and Lianza observed ten elongated depressions in the ground that resembled the craters that might form if many different objects had impacted the ground at oblique, almost level angles. What's more, it seemed, the craters all ran parallel to each other, as though a low-flying herd of Dumbos had dragged their feet along the ground beneath them. The parallel orientations suggested that they were carved by multiple meteorite fragments coming from the same direction.

Back on terra firma, Schultz and Lianza explored some of the craters on foot. In one, they found a pair of meteorite fragments and pieces of glass that had been forged by the high temperatures of an impact.

Putting those and other clues together, they hypothesized that a large meteorite, perhaps 500 to 1,000 feet (150 to 300 meters) across and traveling at about 56,000 miles (90,000 kilometers) per hour grazed the ground at an angle of less than 7 degrees, falling like horizontal rain.

The powerful impact, they believed, occurred as recently as 10,000 years ago, when early Native Americans were already living in the region.

That original story, published in 1992, faced immediate challenges from skeptical scientists. The low angle and large size of the meteorite were improbable, they argued. On the moon, which preserves a thorough record of its past meteorite impacts, only one example of such an oblique impact is known to exist.

But, comments Melosh, "mere improbability is not proof that such an event did not occur."

That's where Bland enters the picture. He and his team of ten researchers from six different countries analyzed satellite images of thousands of square miles of Argentine Pampas, including the region that Schultz and Lianza studied. What the scientists found was a more complex picture than they had imagined.

Elongated depressions like those previously described around Rio Cuarto exist across the entire region. In any given locale, all depressions run parallel to one another. But from the perspective of the satellite it became clear that the "craters" weren't craters at all—like sand dunes, they had been produced by the action of wind on soil and vary in their orientation according to the direction of local prevailing winds.

The meteorites that Schultz found inside the bogus craters, Bland's team further concludes, are once-buried remnants of ancient impacts that became exposed when the wind carved out the depressions. That notion was bolstered by their discovery of two additional meteorites—of different types and ages, and therefore not from the same impact—in nearby depressions.

Nevertheless, one piece of evidence suggests a highly unusual impact. Bland and his researchers found numerous pieces of glassy rock that could only have been formed under the intense natural heat and pressure produced at the instant of a meteorite impact.

These bits of glass, called tektites, appear to be scattered across the entire region that the researchers examined, suggesting that much of the Pampean Plain is a vast tektite field.

Tektite fields are few and far between. Only four such fields are known to exist on Earth, one each in North America, Africa, and Europe, and one—gouged from a crater that has not yet been identified—that stretches for more than 6,000 miles (10,000 kilometers) across Australia and Southeast Asia. The Argentine tektite field, if that's what it is, would be the fifth ever discovered.

The age of the tektites is greater than that of the formerly proposed oblique impact. Whatever cataclysmic collision forged the glassy rocks occurred about half a million years ago, Bland says. But Bland and his researchers have yet to identify a crater produced by that impact, and they don't even know the full scale of the tektite field, since they haven't yet found its edge.

Nevertheless, the researchers now know enough to imagine the fallout from a stunning event, which must have melted and ejected silty material from the ground, showering molten glass for hundreds of miles around.

"As terrifying as the original picture of an oblique impact that scarred the Pampas…was, the present view of a shower of hot glass over a region as large as Texas suggests a far more lethal event," says Melosh.

"Interesting info re the Carolina Bays."

Yup. I once had an underwater map that showed 3-4 larger depressions underwater that appeared to line up with the flow of these 'bays.'

a widespread shower of molten glass.

I have pieces of what appears to be molten glass that I found on the shore in Marblehead Massachusetts that are similar to flint glass substances from Illinois.

All the pieces are small, under 150 grams, and appear oxidized (orange or brown) on the surface, yet black as the blackest obsidian inside, and unusually pure. The area is heavy with granite, which may indicate it is a form of granitic glass... but no nodules of such material appear in the granite formations, and the stuff is flawless as if it were a man made black silica.

They are a strange but not rare find, I can get some tomorrow if necessary. I also found micro quarts points on the same shore.

"I forgot to mention that my wife thinks I am a lunatic and promises to smash me one if I bring home more rocks."

LOL. I hired a PhD out of Berkeley once and he would only accept the job and move to Houston if I shipped his 26,000 pound rock collection with his move. I did.

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