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.

Computer artwork of a group of dinosaurs and flying reptiles during the meteor impact. According to a popular hypothesis, this is how dinosaurs met their end. Science Photo Library / Corbis

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.”

An artist’s rendering of a volcanic eruption wreaking havoc with the hunting grounds of a Tyrannosaurus rex. Sergey Krasovskiy / Corbis