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Fire and Ice
Big rocks, raging volcanoes, and a big chill

As mass extinctions go, the Cretaceous-Tertiary (K-T) die-off 65 million years ago was only moderately horrendous. Far worse, of course, was the Permian-Triassic extinction 250 million years ago, which obliterated about 90 percent of the planet's animal species. But the K-T extinction (never mind the peculiar acronym) is surely the most famous. It wiped out the dinosaurs and spawned a remarkably enduring scientific debate.

At least half of the animal species on Earth vanished at or around the K-T boundary. There was a time when scientists blamed volcanoes, falling sea levels, or a nearby supernova—or they threw up their hands and declared it a mystery.

In recent decades, however, an Earth-shattering idea has gained acceptance: A big rock did it. Physicist Luis Alvarez, his geologist son Walter Alvarez, and nuclear chemists Frank Asaro and Helen Michel published a paper in 1980 arguing that a mountain-size asteroid struck the planet and triggered the K-T extinction. In 1991 scientists declared that they'd found the remnants of the impact crater near the tip of the Yucatán Peninsula.

Beyond the immediate effects (tsunamis, earthquakes, shock waves, and no doubt a general feeling of dread and anxiety) there would have been residual calamities. Global wildfires may have torched 25 percent of the planet's biomass, while dust and soot shut down photosynthesis. The planet got dark, and perhaps very cold.

Yet the story doesn't end there. Today a vocal minority is complicating the picture by suggesting that the die-off could have been more gradual. Before the K-T impact event, several hundred thousand years of intense volcanism could have poisoned the atmosphere, argues Vincent Courtillot, a French geophysicist. He claims to have found proof that all the Earth's mass extinctions were associated with powerful flood basalt eruptions.

San Diego State biologist J. David Archibald also criticizes the single-cause theory. Comparing creatures in North America that survived the K-T event and those that didn't, he finds no pattern to support the asteroid scenario. Most turtles survived; most lizards didn't. Nearly half of mammals survived, while all those dinosaurs didn't. "It should have been more of an equal opportunity killer," he says.

Meanwhile, Princeton paleontologist Gerta Keller contends that multiple impacts contributed to a prolonged extinction over several hundred thousand years. The Yucatán crater predates the K-T boundary, she says, and certainly isn't the only killer.

But let's stay calm here. Many scientists still favor the single-cause scenario. "The data are overwhelming," points out David Fastovsky, a geobiologist at the University of Rhode Island. He argues that new research at the Hell Creek formation in North Dakota shows that a great number of species died suddenly at the end of the Cretaceous.

The lesson here is that science usually doesn't deal in absolutes. Rather, we search for explanations that, for the moment, seem better than any alternative. The big-rock theory isn't doomed to extinction, but you can bet that there are still plenty of surprises ahead in our study of the distant past.

—Joel Achenbach
   Washington Post  Staff Writer

Web Links

Rough Draft
Writer Joel Achenbach's column is gaining a cult following. It takes a sometimes humorous, sometimes eye-squinting, but always intelligent look at today's headlines, personal interests, and the little life-annoyances we all live with.

Dinosaurs Come Alive
Joel Achenbach writes about a new generation of scientists using computer modeling and a better understanding of living animals to bring dinosaurs back to life—virtually.

The Extinction Files
Five times in Earth's history, mysterious events wiped out more than half the species alive. Explore mass extinctions—and find links to other resources—at this BBC website.

Museum of Paleontology—University of California at Berkeley
What killed the dinosaurs? Find the current arguments here—and examine the hypotheses that didn't make the cut. You can also click on a dinosaur and watch it fossilize before your very eyes!

The Geological Society
How did volcanism figure in the Cretaceous-Tertiary extinction? Intense flood basalt events had been going on in India for hundreds of thousands of years before the impact, says French geophysicist Vincent Cortillot. Visit this site to learn more about these powerful eruptions.

Free World Map

Achenbach, Joel. "Flesh and Bone: A New Generation of Scientists Bring Dinosaurs Back to Life," National Geographic (March 2003), 2-33.

Archibald, J. David. Dinosaur Extinction and the End of an Era: What the Fossils Say. Columbia University Press, 1996.

Courtillot, Vincent. Evolutionary Catastrophes: The Science of Mass Extinctions.
Trans. Joe McClinton. Cambridge University Press, 1999.

Fastovsky, David E., and David B. Weishampel. The Evolution and Extinction of the Dinosaurs. Cambridge University Press, 1996.

Hoffmann, Hillel. "When Life Nearly Came to an End: The Permian Extinction," National Geographic (September 2000), 100-13.

Johnson, Claudia C. "The Rise and Fall of Rudist Reefs," American Scientist (March-April 2002), 148-53.

Keller, Gerta. "The End-Cretaceous Mass Extinction in the Marine Realm: Year 2000 Assessment," Planetary and Space Science (July 2001), 817-30. Available online at


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