But there has long been an alternate theory, espoused by a rump caucus of researchers who think they’ve never been given a fair hearing. They believe the extinction was caused, at least in part, by an extraordinary volcanic eruption in India.
This eruption created the Deccan Traps, a geological formation that covers nearly 200,000 square miles of western India. It was created by a flood of basaltic lava, the kind of eruption seen today on the Big Island in Hawaii. But the eruption that formed the Deccan Traps was unusually prolonged and prodigious. All told, the eruption produced about 1.3 million cubic kilometers of lava, which is about 1.3 million times as much material produced by the 1980 eruption of Mount St. Helens. The eruption pumped enormous, climate-changing quantities of carbon dioxide and sulfur dioxide into the atmosphere.
Now scientists have found a way to date more precisely the Deccan Traps eruption, and the results are a boost, potentially, for the volcano-did-it camp.
The main pulse of the lava flow began about 250,000 years before the mass extinction event, and ended about 500,000 years after, according to a paper published online Thursday in the journal Science. Thus if the eruption is not a significant factor in the mass extinction, it’s a remarkable coincidence. Earlier attempts to date the Deccan Traps, using less precise methods, had a much larger margin of error, on the order of plus-or-minus one million years.
The lead author of the paper, Blair Schoene, a professor of geosciences at Princeton, said the results indicate that both the catastrophic impact and the more gradual, but extraordinary, volcanic eruption could have been factors in the end-Cretaceous mass extinction.
"Both are potentially really important," Schoene said. "I don’t know if we can say the extinction would have or would not have happened without both of them.”
One obvious scenario: Climate change caused by the eruptions could have stressed the biosphere and set the conditions for a greater die-off when the asteroid smashed the planet.
“I sort of favor the one-two punch idea,” Schoene said.
Stronger words come from one of the paper's co-authors, Gerta Keller, another Princeton professor who has been among the most outspoken proponents of the hypothesis that the Deccan Traps caused the mass extinction. Keller has long maintained that the asteroid impact happened earlier than the die-off and couldn't have been the trigger. Her work has been controversial, and she has long been viewed as a maverick in the scientific community.
"I think this is a game-changer," she said of the new results. She said the new dating of the Deccan Traps strengthens the case for volcanism as the "primary cause" of the mass extinction.
“The data is so strong at this point that the momentum is entirely on my side," she said.
The researchers took rock samples in India and scrutinized them for crystals containing uranium and lead. Crystals of zircon form in magma with trace amounts of uranium inside. The uranium gradually decays into lead. Because the rate at which uranium decays is well known, the ratio of uranium and lead isotopes in the crystals serves as a kind of clock, revealing how long it has been since the crystals formed.
Another co-author of the Science paper, Sam Bowring of M.I.T., said what's important is that everyone now knows more accurately when the Deccan Traps eruptions began and ended.
"I do not think for a minute that this denigrates the role of the impact in the extinction, and I don’t think it proves that the impact didn’t have anything to do with the extinction. But what I think it does, for the first time, is establish with high precision and accuracy that the Deccan Traps began just prior to the extinction and continued throughout the extinction," Bowring said.
“Whether or not you can say it played a role in the extinction is quite another matter. But at least now we can have a conversation on good temporal grounds," Bowring said.
The fundamental problem for the hypothesis that volcanism caused the mass extinction is that it doesn't seem to be necessary. The mass extinction, including the demise of the non-avian dinosaurs, was for a long time among the greatest mysteries in science, but few people view it as a mystery anymore.
Until 1980, scientists struggled to understand why so many life forms abruptly went extinct. Not only did the tyrannosaurs disappear, so did many species in the seas, including most of the species of tiny marine organisms called foraminifera. The Cretaceous/Tertiary boundary, usually referred to as the K/T boundary (the K is from the German word for Cretaceous), is clearly marked in marine sediments by the sudden drop in the size and diversity of foraminifera. Below the boundary, in older sediments, the forams are relatively large. Above, there are only small forams. The evidence points to a sudden, planet-wide extinction event.
Historical explanations included some kind of disease, or the drying up of inland seas putting stress on the dinosaur population, or a supernova irradiating the Earth. But then came the stunning discovery by Luis and Walter Alvarez, father and son scientists, who while studying a clay layer marking the K/T boundary in Italy found striking amounts of iridium.
That iridium, the Alvarezes concluded, came from an object from space that collided with the planet. In the decade that followed, scientists zeroed in on the "crater of doom" at the tip of the Yucatan. Residue from the impacting asteroid, which is believed to have been about seven miles in diameter, has been found in roughly 300 locations around the planet. There are also signs of a tsunami that inundated coastal Texas.
Scientists prefer parsimonious explanations -- the simpler the better. Thus many appear reluctant to add the Deccan Traps to a tidy K/T extinction narrative. The asteroid impact appears to be the smoking gun, and any additional element to the narrative could potentially have a grassy-knoll quality to it.
“It’s a remarkable event, it’s a huge impact, and it lines up exactly with the extinction,” said Kirk Johnson, director of the Smithsonian’s National Museum of Natural History and an expert on the mass extinction. “You don’t need anything else.”
David Fastovsky, a geoscientist at the University of Rhode Island, said a coincidence in the fossil record doesn't imply causation. Asked whether the volcano-plus-impact could have been a one-two punch, he said, “Possibly, but why do we need to do that? We have enough already with this asteroid.”
But volcanoes will not be ignored: The largest extinction event in the planet's history, about 250 million years ago at the end of the Permian, appears to have happened at the same time as another massive volcanic eruption in Siberia -- the Siberian Traps. There was also a huge volcanic eruption in what is now the northern Atlantic Ocean that was coincident with a mass extinction at the end of the Triassic, Bowring said.
“The idea that flood basalts and extinctions are related is not a new idea," Bowring said. "Easy to say, difficult to prove.”
Another conjecture circulating among geophysicists is that the impact of the asteroid might possibly have sent such a powerful jolt of energy through the planet that it exacerbated the already-occurring Deccan Traps eruption on the far side of the world, and incited volcanic activity in other locations as well. That idea remains in the category of speculation.
The broader lesson here is that science is not a business in which anyone can ever safely say "case closed." Knowledge is always contingent upon the next batch of data, which could be coming along any day now.