When North America Burned

By Carl Zimmer|Saturday, February 01, 1997
Sixty-five million years ago, at the end of the Cretaceous Period, a 10-mile-wide comet or asteroid dropped out of the sky and plowed out a 120-mile-wide crater in the Caribbean near Mexico’s Yucatán Peninsula. The impact, many researchers believe, was at least partly responsible for the fifth biggest extinction of all time, with numerous species of plants, marine animals, and, most famously, dinosaurs vanishing. But finding the links between the crater, known as Chicxulub, and the pattern of extinctions has been tricky. Recently, however, two Rhode Island researchers have found a clue--the angle of the impact--that could explain a lot about the extinctions, particularly why North America suffered more than the rest of the world. The Chicxulub fist of fury, say the researchers, came streaking toward the northwest over the Atlantic at an angle of 20 to 30 degrees, creating an enormous, searing vapor cloud that within minutes incinerated large parts of the western United States.

Peter Schultz, a planetary geoscientist at Brown, applied to Chicxulub his years of experience studying craters on other worlds. Analyzing the surfaces of our moon and the planets Mercury and Venus, he figured out ways to determine the angles at which impacts occurred from the craters they left behind. If an asteroid falls straight down on its target, he found, it leaves a symmetrical ring. If it comes in more obliquely, however, the bottom of the asteroid rams into the ground while the top, its tremendous momentum unchecked, shears off, spraying debris forward. An egg- shaped crater forms, with the long, narrow end downrange. It was one of these deals where you hit yourself on the head, says Schultz. I started noticing that repeatedly there was a nice relationship between the asymmetry of the crater and the angle of impact. I saw it on the moon, I saw it on Mercury, and I saw it on Venus.

Since researchers first realized in 1991 that a crater lay hidden a mile underground in the Yucatán, they have studied the patterns of magnetism and gravity of the area to map Chicxulub. Schultz’s own recent study of the crater revealed that it had the telltale egg shape and a number of other signs of an oblique collision pointing toward the northwest.

If the Chicxulub impact had been perpendicular to the ground, much of its energy would have been absorbed by Earth, with most of the debris launched straight up into space. But the oblique trajectory of the impactor must have produced a very different aftermath, says Schultz. The impact shaved off a vast acreage of surface rock and launched a huge low cloud of hot vapor that hugged the ground as it continued along the impactor’s path. Within three minutes the cloud overran much of the western United States. Only after this local cataclysm did the impact’s debris begin to spread more evenly around the world.

Schultz’s scenario makes sense to paleobiologist Steven D’Hondt of the University of Rhode Island. Such a regional apocalypse, says D’Hondt, explains some puzzles of the fossil record. The extinction rate of plants in North America, for example, was at least triple that found in the rest of the world. The fossil record in North America also shows an odd abundance of ferns after the extinction event. D’Hondt and Schultz point out that even 2,000 miles northwest of the Yucatán, the heat of the cloud would have ignited plants. After the conflagration, opportunistic ferns would have invaded and dominated for a time.

The theory may also explain another anomaly: studies of late Cretaceous fossils in Montana suggest that a species that lived on land was nine times more likely to go extinct than a freshwater resident. That seems logical if a scorching vapor cloud raced across North America at six miles a second: a turtle submerged in a pond might survive, while a Tyrannosaurus rex stalking in a glade would fry.

If Schultz and D’Hondt are right, further study should continue to show that extinction patterns in North America were unique, providing mute testimony to a time when the continent burned.
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