The terrain east of Mumbai, India, clearly shows the thick layers of lava laid down by massive eruptions before and after the asteroid impact that triggered the mass extinctions that claimed the dinosaurs.

The terrain east of Mumbai, India, clearly shows the thick layers of lava laid down by massive eruptions before and after the asteroid impact that triggered the mass extinctions that claimed the dinosaurs.

Mark Richards, UC Berkeley

Volcano-asteroid combo may have done in the dinosaurs

Scientists have for decades hotly debated what killed the dinosaurs. One long-held hypothesis blames immense and long-lasting volcanic eruptions that drastically altered Earth’s climate. Another more recent hypothesis suggests that the dino die-offs occurred after a massive asteroid hit the planet near the Yucatán Peninsula. Now, research finds that the extraterrestrial impact may have led to increased volcanism in the Indian subcontinent, providing a double whammy that took out Tyrannosaurus rex and his kin.

The impact took place 66 million years ago—the so-called K-T boundary that separates the dinosaur-rife Cretaceous period from the dino-free Tertiary period. Altogether, an estimated three-fourths of the species on Earth disappeared, including major groups of sea-dwelling reptiles like mosasaurs and plesiosaurs. But what caused this planetary holocaust? Previous studies suggest the asteroid struck just before the die-off. But others point to ecosystem-altering volcanism that had been taking place on the Indian subcontinent for hundreds of thousands of years, both during and after the reign of the dinosaurs.

The new study finally provides dates for those eruptions. Using an argon-argon radioactive dating technique, a team led by geochronologist Paul Renne of the University of California, Berkeley, sampled materials from ancient lava layers at sites in the Deccan Plateau of central and western India. They found that the Deccan eruptions started at least 173,000 years before the asteroid hit and continued for at least 500,000 years after the impact. What’s more, the researchers were able to determine the size and strength of each major eruption, based on lava flow estimates. Before the impact, the eruptions produced about 71,000 cubic kilometers of lava—an average rate of about 400 million cubic meters each year. But starting about 50,000 years after the asteroid impact, Deccan volcanoes and fissures began spewing lava at an average rate of about 900 million cubic meters per year, the researchers report online today in Science.

“The team’s findings are more evidence that volcanism and the impact may be linked," says Jeff Wilson, a vertebrate paleontologist at the University of Michigan, Ann Arbor.

Other changes in volcanism occurred at the same time, the researchers say. Even though the rate of lava flow more than doubled, the frequency of eruptions—as measured by the number of lava and soil layers—dropped. Although eruptions happened less often, they were—on average—much larger than preimpact eruptions, Renne says, with possibly disastrous consequences for Earth’s ecosystems.

How the asteroid impact half a world away from India bumped up lava production is a mystery, Renne says. He speculates that its effects rippled along the boundaries of nearby tectonic plates until they reached the volcanoes, expanding the size of subterranean magma chambers and thus increasing the volume of magma they could spew during any given eruption.

Not all scientists are convinced. “This is a wonderful piece of work, but I don’t think it will solve the problem” of what killed the dinosaurs, says Jay Melosh, an impact crater expert at Purdue University in West Lafayette, Indiana. According to Melosh, several studies have suggested that ecosystems largely remained intact until the time of impact. To him, that diminishes the effect of Deccan volcanism, which had been chugging along at that point for well over 100,000 years. 

Because dinosaur fossils in rocks laid down just before the K-T boundary are sparse, it’s been tough to know whether these beasts were thriving right up until the asteroid impact. But that’s not the case with phytoplankton, tiny organisms that make up the base of the ocean’s food chain. These small creatures show no die-off coinciding with Deccan volcanism in the lengthy period before the impact, says Brian Huber, a micropaleontologist at the Smithsonian Institution’s National Museum of Natural History in Washington, D.C. “I cannot be convinced that volcanism triggered the mass extinctions,” he says, adding that the eruptions may have nevertheless contributed to the enormity of the impact’s aftermath.

Ironically, by more closely linking the date of the impact with the increase in Deccan volcanism, Renne and his team may have made it more difficult to tease out the relative contribution of each phenomenon to the die-offs, Melosh says. “These findings will add greatly to the controversy of volcanism versus impact.”

*Correction, 5 October, 1:15 p.m.: The quote from paleontologist Jeff Wilson has been corrected and moved within the story.