Much of what you know about how dinosaurs died is wrong

PRINCETON, N.J. — Ask any T. Rex–obsessed 5-year-old boy how the dinosaurs died, and he will tell you a giant asteroid killed them. He might describe a scene with a brontosaurus or two peacefully grazing, oblivious to the space rock whizzing past that will crash to Earth and ignite an explosion with the strength of a billion atomic bombs going off at once. A heat wave engulfs the globe, vaporizing most dinosaurs, with the rest starving to death as an ash cloud blocks the sunlight.

The by now iconic scene comes from a scientific paper first published on June 6, 1980, in Science magazine by a young geologist at the University of California at Berkeley, Walter Alvarez, and his Nobel Prize–winning father, Luis Alvarez. In the 35 years since, the impact hypothesis has become the most widely accepted explanation for the sudden disappearance of large nonavian dinosaurs (those weighing more than 40 pounds) and the demise of six out of every 10 species in a mass extinction that happened about 66 million years ago.

But according to geologist Gerta Keller this is simply not what happened. For nearly three decades she has been arguing that volcanic eruptions were the dinosaurs’ true killer. “We were in the midst of the Cold War, and the world was terrified of a nuclear explosion wiping us off the planet,” she said recently in her office at Princeton University, where she is a professor of geosciences. “Isn’t it sexy to think that this happened to the dinosaurs?”

Keller’s research indicates that sulfur and carbon dioxide were released by a multitude of volcanoes erupting on Earth over millions of years, which trapped heat in the atmosphere and caused climate change. The warming temperatures made land and marine ecosystems inhospitable to large animals. In a recent paper based on research conducted with Sam Bowring, a professor of geology at MIT, she narrowed down the timeline between ancient volcanism and dinosaur extinction to a mere 27,000 years — the blink of an eye in geologic time. But her research faces tough criticism, with some scientists openly calling her a crackpot and rallying behind the asteroid theory. The controversy, tens of millions of years after the T. Rex met its end, has excavated new details about the death of the dinosaurs and it will likely uncover more before the dust fully settles. 

When the Alvarezes put forward their idea three and a half decades ago, it was met with opposition. At the time, scientists generally accepted that dinosaurs died from rising sea levels and regarded anything that suggested sudden catastrophic change with wariness. Bowring remembers, “We joked that nobody would believe them until they found a dinosaur with a piece of asteroid lodged in it.”

Walter Alvarez’s key finding was the presence of an unusually high amount of the heavy metal iridium in a layer of the Earth that dates back to about the same time as the mass extinction. Since there’s very little iridium lying around on the ground, finding so much of it made Alvarez suspect it had come from an extraterrestrial source.

His first excavation was in Italy in the late ’70s, but he and his team visited 30 sites all over the world, from China to Siberia and Australia, finding similar levels of the heavy metal at each site. In interviews, Alvarez has said that the wide distribution of the iridium was like a lightbulb moment. It is what made him realize how much force the rock had hit Earth with. His team surmised that a giant ash cloud enveloped the planet and blocked out the sun, similar to the 1883 Krakatoa volcano eruption in present-day Indonesia, in which a dust cloud circled the globe for more than two years. Such a mass of smoke would shut down photosynthesis and stop vegetation from growing.

But the impact hypothesis had few supporters until 1990, when a giant crater about 110 miles wide and 9 miles deep was found in Chicxulub, Mexico. It too was recently dated to 66 million years ago and could have been formed only by a huge rock, about 6 miles wide: the killer space rock. When the crater was found, Robert Speijer a geologist at the University of Leuven in Belgium said, “Many, including myself, were finally converted.”

But Keller and the other supporters of the volcanism theory continued to make the case that while the asteroid may well have made a smash landing, it was the fumes from volcanoes that warmed the climate by 7 degrees and ocean temperatures by 4 degrees Celsius and killed the dinosaurs. The volcanoes also released toxic gases containing sulfides that raised the level of acidity in oceans, which killed off marine life and led to the mass extinction of life on earth. “If you raise the ocean temperature by even 1 degree today, you will kill off coral reefs and the marine ecosystem. Just imagine what would happen if you warmed it by 4 degrees Celsius,” she said.

MIT’s Bowring said their research shows “that volcanism started at this time, and it just predates the extinction.” He sees the volcanoes as contributing to the mass extinction but not being the sole cause. Keller, however, has staked out a much more extreme position, arguing that the asteroid was not responsible for the extinction of even “a single species.”

Her stance has alienated other geologists and prompted vociferous arguments between her and asteroid supporters at conferences. And in 2010, a group of 41 scientists published a paper that threw support behind the asteroid theory. As the research grows to disprove volcanism, it is providing a deeper and more textured image of what Earth looked like at the end of the Cretaceous era, even as disagreement persists among scientists about the details.

Earth physicist Jay Melosh of Purdue University, who considers the impact “the sole cause of dinosaur extinction,” has created computer simulations that provide more detail about what he believes happened. According to him, the impact must have had the force of “ten thousand suns at noon. Imagine being in a pizza oven for 20 minutes.”

One of his collaborators, Jan Smit, came up with an elegant model to explain how the iridium that Alvarez found got all over the planet after the asteroid hit in North America. His research suggests that the force of the impact condensed the soil and the asteroid into tiny balls smaller than 5 millimeters. They flew up thousands of miles into space. “Some scientists think they reached the moon,” Smit says. “And then they came down over the whole world — New Zealand, Turkey, Italy and in the deep sea — precisely at the moment of extinction. They came down like shooting stars, billions of them,” spreading iridium everywhere.

Smit is gathering evidence for a paper arguing that the meteor impact caused a giant tsunami that swept from the Gulf of Mexico across the United States all the way to Canada, while Keller is conducting research on the effect of ocean acidification during that time. Which means that 35 years after the impact hypothesis, the dispute will continue.

And this misses the point, according to American Museum of Natural History curator Mark Norell. “There is really so little data, it’s hard to tell exactly how the species perished,” he says. “What’s interesting is who survived the mass extinction. Somewhere in those species were small rodents who can hide in water and escape the blast. One of them was a mammal that became our ancestor. Perhaps we should be focusing on that.”