Around 66 million years ago, a giant asteroid smashed into our planet, unleashing a terrible firestorm that blotted out the sun and killed the dinosaurs.
Or did it? A new study has thrown doubt on the theory that the dinosaurs were wiped out solely by a mountain-sized asteroid - instead pointing the finger at volcanoes.
Researchers believe that huge, continent-spanning 'flood basalt' eruptions are what caused the mass extinction – and others in Earth's history.
The presence of an asteroid just made things worse, they said.
Their study, published in the Proceedings of the National Academy of Sciences (PNAS), claims volcanic activity appears to have been the key driver of mass extinctions.
In fact, a particular kind of volcanic activity may also explain other mass extinctions in history, the researchers said.
Co-author Brenhin Keller, an assistant professor of earth sciences at Dartmouth College in New Hampshire, said: "All other theories that attempted to explain what killed the dinosaurs, including volcanism, got steamrolled when the Chicxulub impact crater was discovered."
But, he added, there is very little evidence of similar impact events that coincided with the other mass extinctions despite decades of exploration.
Keller said: "While it is difficult to determine if a particular volcanic outburst caused one particular mass extinction, our results make it hard to ignore the role of volcanism in extinction."
The researchers found that four out of five mass extinctions happened at the same time as a type of volcanic outpouring called a flood basalt.
These eruptions flooded vast areas – even an entire continent – with lava in a mere million years, the blink of a geological eye.
They left behind giant fingerprints as evidence – extensive regions of step-like, igneous rock (solidified from the erupted lava) that geologists call "large igneous provinces".
To count as "large", an igneous province must contain at least 100,000 cubic kilometres of magma.
For context, the 1980 eruption of Mount St. Helens involved less than one cubic kilometre of magma.
A series of eruptions in present-day Siberia triggered the most destructive of the mass extinctions about 252 million years ago, releasing a gigantic pulse of carbon dioxide into the atmosphere and nearly choking off all life.
Bearing witness are the Siberian Traps, a large region of volcanic rock roughly the size of Australia.
Volcanic eruptions also rocked the Indian subcontinent around the time of the great dinosaur die-off, creating what is known today as the Deccan plateau. This, much like the asteroid strike, would have had far-reaching global effects, blanketing the atmosphere in dust and toxic fumes, asphyxiating dinosaurs and other life in addition to altering the climate on long time scales.
The researchers compared the best available estimates of flood basalt eruptions with periods of drastic species kill-off in the geological timescale, including but not limited to the five mass extinctions.
Paul Renne, professor-in-residence of earth and planetary science at University of California, Berkeley, said: "Our results indicate that in all likelihood there would have been a mass extinction at the Cretaceous tertiary boundary of some significant magnitude, regardless of whether there was an impact or not, which can be shown more quantitatively now.
"The fact that there was an impact undoubtedly made things worse."
The eruption rate of the Deccan Traps in India suggests that the stage was set for widespread extinction even without the asteroid, lead author Theodore Green said.
Green, who conducted this research as part of the senior fellowship program at Dartmouth and is now a graduate student at Princeton, added that the impact was the double-whammy that loudly sounded the death knell for the dinosaurs.
Flood basalt eruptions aren't common in the geologic record, Green said. The last one of comparable but significantly smaller scale happened about 16 million years ago in the US Pacific Northwest.
"While the total amount of carbon dioxide being released into the atmosphere in modern climate change is still very much smaller than the amount emitted by a large igneous province, thankfully," Keller said, "we're emitting it very fast, which is reason to be concerned."
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