Aftermath of a cataclysm

Natural History, March, 2003 by Stephan Reebs

Most scientists agree that about 65 million years ago a catastrophic meteor impact wiped out the dinosaurs. That collision, however, is dwarfed by events that took place billions of years earlier, when the Earth was only a billion years old.

According to Gary R. Byerly, a geologist at Louisiana State University in Baton Rouge, and his colleagues, during the Earth's early history four meteors slammed into the planet with such force that they vaporized rocks for hundreds of miles around. The clouds of rock vapor quickly, condensed and fell back to Earth as a rain of small rounded particles called spherules.

Spherules occur in what are now South Africa and western Australia, embedded in layers of sedimentary rock that contain unusually abundant, and thus demonstrably extraterrestrial, chromium isotopes and iridium. The spherules are mixed up with inorganic detritus, perhaps because of a tsunami--also generated by the collision--that sloshed back and forth across the Earth. Some of the spherule beds are as much as a foot thick, so the impacts that created them must have been enormous. By comparison, the impact layer left by the meteor that did in the dinos is less than an inch thick.

Byerly and his coworkers have analyzed the lead isotopes in small zircons extracted from the lowest (hence the oldest) spherule studded layer. The relative abundance of those isotopes, which reflect the slow decay of uranium over the millions of millennia since the spherules were formed, has enabled the geologists to calculate the age of the layer: nearly 3.5 billion years. That makes it the earliest evidence discovered so far of an asteroid impact. In those days, bacteria were the Earth's principal life-forms, and they've turned out to be a lot tougher than the dinosaurs. After all, they're still with us. ("An archean impact layer from the Pilbara and Kaapvaal cratons," Science 297: 1325-27, August 23, 2002)