There have been a number of mass extinctions in the Earth's past, and not all of them have an obvious cause. But the one that ended the dominance of dinosaurs is an embarrassment of riches: it has two viable candidates, either of which could have easily caused extinctions. It's left geologists asking, “Which was worse: The asteroid or the eruptions?”

The asteroid is familiar to most people, but a truly incredible series of eruptions were also going on in what is now India, spewing up over a million cubic kilometers of basaltic lava along with noxious and climate-changing gases. Neither one of these catastrophes represent a relaxing day at the beach for living organisms, but was their combined unpleasantness required for the mass die-off? And was their timing purely coincidental?

In order to answer those questions, we have to know the timing really well. Really, really well. Considering that we’re talking about events 66 million years in the past, it doesn’t take much in the way of error bars on your dating measurements to keep the picture just a little too blurry.

In the past few years, Paul Renne’s Berkeley lab has been dating the events in question with greatly improved precision. Worries that the Chicxulub asteroid might have fallen to Earth too early—or too late—to truly line up with the mass extinction were put to rest when new dates put the impact bang-on the extinction boundary within uncertainty of just 30,000 years.

Still, the massive Deccan Traps eruptions in India started rolling before the asteroid struck and continued for a while afterward. There are those who think the asteroid gets way too much credit for the merciless wave of death.

In a paper published earlier this year, a group led by Renne’s Berkeley colleague Mark Richards argued for the possibility that the asteroid impact actually strengthened the Deccan eruptions. Past curiosity about this sort of connection has centered on whether the focusing of seismic waves on the exact opposite side of the Earth from the impact could be violent enough to initiate eruptions.

That’s not in play here. The crater and the eruptions don’t line up, modeling has shown that there probably isn’t enough seismic energy to do something like that, and the eruptions were already underway. But most of the volume of lava erupted came toward the end of the Deccan run, and the change in intensity looked like it might have occurred shortly after the asteroid hit.

Now, the researchers have published new dates for a number of lava flows in the Deccan Traps to try to nail down that timeline and see if a correlation with the asteroid really exists.

Geologists studying the Deccan Traps have identified and organized the many layers of lava into three phases: first was the Kalsubai, then the Lonvala, and finally the Wai. It has been very difficult to precisely place the extinction event within that stack of lava, but the guess was that it probably occurred around the time of the Lonvala eruptions in the middle. The Wai eruptions that followed are markedly different from the others—they include something like 70 percent of the total volume of lava, and it was released in less frequent but much larger flows.

So the researchers collected samples from a number of Kalsubai lavas and one of the Wai lavas. (It’s hard to find good samples of the “middle” lavas, Renne and Richards told Ars, but they will be looking to do so next year given these results.) Combined with a couple other existing dates, they were able to firm up the timing. Interpolating between their data points, they estimated the age of the first Wai lavas to be within 50,000 years of the asteroid impact. They were also able to estimate the eruption rates before and after, finding a doubling in the average volume of lava flying out of the ground per year.

So it looks like the Deccan eruptions went through a major change right around the time of the asteroid impact—a coincidence within a coincidence. The researchers suggested ways the impact could potentially have directly triggered this change. The impact could have released as much as 1,000 times the seismic energy of the 2011 magnitude 9 earthquake in Japan, and those seismic waves would have shaken up the massive volcanic guts beneath the Deccan Traps.

That shaking could have jostled the mix of molten and solid rock in the magma, increasing connections between the molten pockets. Or it might have shaken out some of the gases dissolved in the melt. Either way, the result could be an increase in the supply of magma that kicked the eruptive action up a notch.

Maybe. The researchers aren’t claiming to know that this actually happened at this point. In the paper published earlier this year, they wrote, “If future geochronologic data confirm that the huge Wai Subgroup eruptions coincided closely in time with the Chicxulub impact, additional investigation of possible causal links and environmental consequences (including the possible consequences of triggered volcanic eruptions worldwide) would be warranted.”

In other words, the hypothesis is looking reasonable enough that it’s worth the effort to look for more ways to test it. Luckily, we’re here to comfortably find all this fascinating because our very distant ancestors dodged an asteroid apocalypse—and a volcanic cataclysm.

Science, 2015. DOI: 10.1126/science.aac7549 (About DOIs).