What caused the demise of the dinosaurs about 66 million years ago? You may think you know, but new study in Proceedings of the National Academy of Sciences now states that volcanic activity—not asteroid impacts—probably caused their extinction, as well as most other mass extinctions in our planet’s past.
Carrying out a mass extinction
Extinction of species is a normal part of the evolutionary process, but sometimes they become so numerous and so sudden that they transform into what scientists call “mass extinction.” These events occur whenever approximately 75 percent of the world’s extant species become extinct within two to three million years or less. There have already been five such extinctions. The demise of the dinosaurs is the most famous and most the new one at the Cretaceous-Tertiary boundary.
Each of the five extinctions, and especially the extinction of the dinosaurs, has inspired countless scientific explanations. One of the most accepted is the theory that collisions of comets and asteroids on the Earth’s surface threw huge amounts of ash and dust into the atmosphere. They blocked the sun and plunged the planet into several periods of extinction-causing cold. And since the 90s of the last century, this theory gained popularity only thanks to the discovery that Chicxulub Impact Crater on the Yucatan Peninsula was formed around the same time as the extinction of the dinosaurs.
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“All other theories that try to explain what killed the dinosaurs, including volcanism, collapsed when the Chicxulub impact crater was discovered,” said Brenhin Keller, study author and professor of geology and geochemistry at Dartmouth, in press release. But despite the theory’s popularity, its evidence remains elusive for many of the earlier disappearances.
Keller and a team of researchers compared the asteroid theory to a similar theory that states that volcanic activity actually produced cold blankets of atmospheric ash, leading to the five extinction events. Ultimately, their comparison found that four of the five events corresponded to huge volcanic explosions that flooded large areas of the planet in lava in just a million years or less.
These findings offer the strongest evidence yet for a causal link between eruptions and extinctions, and suggest that volcanic activity was the largest cause of past extinctions, including the decline of the dinosaurs.
The team focused on huge volcanic eruptions that formed so-called “submerged basalts,” or large areas of land covered in basaltic lava. Identified today by their stepped igneous rock structures, the researchers focused on flood basalts with more than 100,000 cubic kilometers of lava. The majority involved in their investigation contained about a million times more lava than this.
After identifying these huge eruptions, the team then used supercomputers to analyze their timing with that of past extinctions, including the five mass extinction events. To verify that any correspondence between the timing of the two represented cause-and-effect relationships rather than coincidences, the computers then tested whether volcanic activity would coordinate with 100 million randomly generated patterns at a similar level. The results revealed that the eruptions and extinctions coincided much more than could be explained by simple chance.
“The large step-like zones of igneous rock from these large volcanic eruptions appear to be time-aligned with mass extinctions and other significant climatic and environmental events,” said Theodore Green, another study author and a graduate student at Princeton, in a press release.
In addition to this overlap in time, the researchers also found that the periods with the worst eruption rates also corresponded to the periods with the worst extinction rates. This suggests that the worse the explosion, the worse the extinction.
“Although it is difficult to determine whether a particular volcanic eruption caused a particular mass extinction, our results make it difficult to ignore the role of volcanism in the extinction,” Keller concluded in a press release.
Asteroids vs Volcanoes
So does this mean that asteroid impacts have not played a role in past extinctions? Using the same process to analyze the time of asteroid impacts compared to the time of extinction, the team found that the fit was poorer and essentially disregarded the impact of Chicxulub in the data.
This, the researchers say, suggests that the asteroid contributed little to past extinction events other than the one that wiped out the dinosaurs. And even then, since volcanoes were also very active about 66 million years ago, spewing huge amounts of ash into the atmosphere on their own, the extinction of the dinosaurs probably would have happened even without the involvement of asteroids.
“Our results indicate that there was in all likelihood a mass extinction at the Cretaceous-Tertiary boundary of some significant magnitude, whether or not there was an impact,” concluded Paul Wren, another author and a professor of geology and geochronology at the University of California, San Francisco. Berkeley, in a press release. However, “the fact that there was an impact undoubtedly made things worse.”