At the turn of the Mesozoic Era some 66 million years ago, two of prehistory’s greatest catastrophes struck almost in unison. We’ve all grown familiar with the cosmic collision that dispatched the non-avian dinosaurs, ending their 135-million-year run as the world's overlords. But over the past four decades, while the asteroid narrative settled into the collective conscience, a vigorous debate raged as scientists continued to amplify another character in the story: volcanism.
Despite being the most studied of the “Big Five” mass extinctions, the Cretaceous-Paleogene — which, in addition to the dinos, wiped out three-quarters of all species — still holds its secrets. The case is muddled, with two suspects confirmed at the scene of the crime. “I don’t want to be too morbid,” says University of Edinburgh paleontologist Stephen Brusatte, “but a body disappears, and you know that both John Wayne Gacy and Jeffrey Dahmer were in the neighborhood the day before.” Which killer is to blame? Did they act in concert?
The impact crater and its debris coincide, more or less, perfectly with the extinction. Across the globe, a thin layer of iridium — a chemical element that’s rare on Earth but common in extraterrestrial objects — entombs the dinosaurs like a coffin lid. Meanwhile, the ancient lava flows of India’s Deccan Traps, one of the largest volcanic provinces of all time, straddle the end of the Cretaceous Period with a few hundred thousand years on either side.
During the late 20th century, the asteroid theory became an all but uncontested fact, and it's still the more widely accepted culprit. But these days, many researchers seriously consider the possibility that the monstrous eruptions helped by warming the planet to a lethal degree, and much of the recent extinction literature seeks to assign proper levels of guilt. “There are fewer and fewer people who will argue strenuously that the volcanism played no role,” says Paul Renne, a geophysicist at the University of California, Berkeley. “The question now is, what’s the balance of culpability?”
The time-honored explanation for the disappearance of the dinosaurs describes perhaps the worst day in our planet’s history — a six-mile-wide space rock slammed into the ocean off the coast of the Yucatan Peninsula with such ferocity that its debris blocked out the sun and triggered a global nuclear winter. Locally, it sparked voracious wildfires and sent a tsunami roughly 200 miles inland of the Gulf of Mexico.
Renne was a graduate student when the physicist Luis Alvarez first proposed this hypothesis. “I thought it was a bunch of bull----, frankly,” he recalls. “But gradually the impact theory took hold and became very much the dogma.”
All the while, though, evidence accumulated for the role of eruptions in other major die-offs. In the 1990s, the two mass extinctions preceding the Cretaceous (the Permian and Triassic) were linked to volcanic activity, marked by layers of flood basalt just like those in India. These eruptions exuded massive quantities of carbon dioxide, which warms the planet and acidifies the oceans, as well as sulfur dioxide, which conversely filters out sunlight and cools the planet. The Permian was even deadlier than the Cretaceous, killing 95 percent of marine species. “In the face of that,” Renne says, “people started to think, ‘Gee, if flood basalts alone are capable of doing this, maybe we ought to rethink the Deccan Traps.’”
In contrast, annihilation by asteroid has no precedent, leaving some experts unconvinced. Gerta Keller, a Princeton geologist and paleontologist, has drawn attention for being especially hostile to the theory. She has likened it to a fairy tale, saying “it has all the aspects of a really nice story. It’s just not true.”
Some have tried to carve out credit for both disasters. Greg Wilson, a paleobiologist at the University of Washington and curator of vertebrate paleontology at the Burke Museum, thinks of the extinction as a one-two punch: Global warming — and, perhaps, cooling — may have destabilized the biosphere, making it more susceptible to the incoming asteroid’s devastation and less able to rebound in the aftermath. “I think the impact by itself would have had an effect on ecosystems,” Wilson says, “but the severity was amplified by ... climate change related to volcanism.”
Refining the Record
Wilson’s view is based partly on the fact that dinosaurs seem to have dwindled toward the end of the Cretaceous Period, hinting at some level of ecological disruption. In Hell Creek, the prolific Montana fossil site where he often works, their diversity declines in the run-up to the impact. “It seems to me there’s mounting evidence that it’s more subtle than just an asteroid hitting the earth ... and everything else before that was hunky-dory,” Wilson says.
Others disagree. “There’s no reason to think that the dinosaurs were in any real trouble,” Brusatte says. And, he adds, “you never want to take the fossil record at face value” — these scant remains may not reveal the whole picture. In recent years some other lines of evidence, like climate and ecological modeling, have implicated the asteroid as the main driver, though some researchers concede that eruptions could have played a bit part, perhaps delaying the recovery of life. One paper published early last year described the extinction as “impact with a dash of volcanism.”
That sums up Brusatte’s own opinion. Again, fossils can only say so much. But, he says, “It really is just a very striking and very evocative thing that you never find any dinosaur bones above the iridium. Never, ever.” Returning to the forensic metaphor: “Sometimes we just have to look at the most visceral evidence. If there’s a crime scene and there’s a business card with the killer’s name on it, don’t overthink it — that's the killer.”
That said, he and Wilson agree the fossil record is wildly undersampled. Western North America is the only region that’s been studied in depth, and even there Wilson still spends his summers filling in the gaps at Hell Creek, where collection began in the early 1970s. Those extinction patterns, sitting so close to the impact site, may not reflect what happened on the other side of the planet. Some paleontologists hope to begin investigating fossils in India — “ground zero of the volcanism and antipodal from the impact,” as Wilson put it. Other promising sites lie uncharted in the Pyrenees and South America.
A better idea of when exactly the eruptions occurred could also resolve some uncertainty, and suggest unexpected possibilities. As Renne dated the layers of the Deccan Traps, he realized that right around the time of impact, they began to issue larger, thicker lava flows. In 2015 he and his colleagues speculated that the asteroid may have triggered globe-shaking earthquakes that unleashed this potent rush of volcanism, like a humongous hand shaking an Earth-sized bottle of coke to the point of explosion. “Or beer,” Renne says, “if that’s your preference.”
After 40 years, the nuances of the Cretaceous-Paleogene cataclysm remain elusive. But with a global perspective on fossils, an enhanced timeline and new methods of modeling the likely effects of impact and volcanism, future scientists might reach more definitive answers. “As all that stuff comes together, it will allow us to have an even better understanding of the extinction,” Brusatte says. “And there could be some surprises there. I always keep an open mind.”