In a fashion similar to the old chicken-or-the-egg adage, when it comes to the fifth mass extinction, scientists for the last several decades have been asking themselves which came first: devastating asteroid impact or suffocating string of volcanic eruptions? A pair of two studies published in Science set out to answer that very question. 

First, let's jump back 30 years. When researchers in the 1980s proposed that the Chixculub impact crater located on Mexico’s Yucatan Peninsula was the landing site for an asteroid responsible for wiping out the dinosaurs, the science world was turned upside down. Was it the asteroid or volcanic eruptions? A combination of the two?

Taking a more precise approach than previous methods, the new studies take us across the world to the Deccan Traps, one of the largest volcanic chains on Earth. Using two different dating methods, both studies agree that volcanic eruptions likely contributed to the mass extinction event that wiped out the non-avian dinosaurs. Results suggest that this volcanic chain erupted over the course of 1 million years, beginning some 400,000 years before the Chixculub impact and ending around 600,000 years after the Cretaceous period.

Here’s where it gets gritty. A UC Berkeley-led team propose the Deccan Traps began to increasingly erupt in the 100,000 years prior, putting already-stressed ecosystems in a tough place to recover even before the asteroid struck. Princeton researchers, however, argue that the majority of the eruptions occurred after the impact and the asteroid was indeed the demise of the dinos.

Berkeley researchers conducted argon-argon dating to measure the point in time when lava flow occurred. They found that as much as three-quarters of the lava found at the Deccan site erupted 600,000 years after the impact, indicating that an asteroid may have caused an uptick in eruptions that then further delayed any hope of ecological recovery. Central to this theory is that large amounts of gasses capable of driving climate change were released before the extinction event.

On the other hand, Princeton researchers conducted uranium-lead dating on zircon crystals found in cooled magma from nine formations. Their timeline suggests that the Deccan Traps erupted in four 100,000-year events, releasing greenhouse gases and magma all at once – like the moment an asteroid hit – further driving climate change. A limitation of this study is that the crystals could have formed before the eruption, changing the course of the timeline.

"Both the impact and Deccan volcanism can produce similar environmental effects, but these are occurring on vastly differing timescales," said Berkeley researcher Courtney Sprain in a statement. "Therefore, to understand how each agent contributed to the extinction event, assessing timing is key.

Which came first? Looks like the jury is still out on the eruption-or-the-impact question.