Sixty Six million years ago a 14 kilometer long, Mount Everest sized asteroid blasted a hole in the ground, the Chicxulub Impact, releasing the equivalent of 100 million megatons of TNT creating a 20-mile deep, 110-mile hole and sterilizing the remaining 170 million square miles of the ancient continent of Pangaea, killing virtually every species on Earth and, oddly, paving the way for the emergence of the human species.
“There’s no real way to internalize that number,” says Jay Melosh, University Distinguished Professor at Purdue. “It’s certainly enough to lift a mountain back into space at escape velocity.”
“It would have felt like the ground beneath your feet had become a ship in the middle of the ocean,” says earth and space science professor Mark Richards at the University of Washington. “Then rocks would have bombarded you from a boiling sky that was beginning to take on a hazy glow. Massive wildfires would have sprouted up as the ground burst into flames. It would have seemed like the end of the world.”
The asteroid itself was so large that, even at the moment of impact, the top of it might have still towered more than a mile above the cruising altitude of a 747, writes author Peter Brannen in Ends of the World. “In its nearly instantaneous descent, it compressed the air below it so violently that it briefly became several times hotter than the surface of the sun.”
“The pressure of the atmosphere in front of the asteroid started excavating the crater before it even got there,” geophysicist Mario Rebolledo at the Centro de Investigación Científica de Yucatán, told Brannen. “Then, when the meteorite touched ground zero, it was totally intact. It was so massive that the atmosphere didn’t even make a scratch on it.”
In the Yucatán, Rebolledo continues, “it would have been a pleasant day one second and the world was already over by the next. As the asteroid collided with the earth, in the sky above it where there should have been air, the rock had punched a hole of outer space vacuum in the atmosphere. As the heavens rushed in to close this hole, enormous volumes of earth were expelled into orbit and beyond—all within a second or two of impact.”
“So there’s probably little bits of dinosaur bone up on the moon?” Brannen mused.
“Then you’d get the seismic shaking,” observed Melosh. “It would be comparable to a magnitude 12 earthquake. Which . . . well, there’s no such thing as a magnitude 12 seismic earthquake because the elastic strain [of the earth’s crust] can’t contain that much energy, but it is certainly possible for a big impact.”
The earthquakes would have been quite noticeable even on the opposite side of the planet. As a geophysicist later put it to Brannen, “a magnitude 11 to 12 earthquake at any one location would feel like a magnitude 9 earthquake everywhere else on the planet.”
At impact, the Earth probably rang like a bell, triggering volcanic eruptions around the globe that may have contributed to the devastation, according to UC Berkeley geophysicists.
The researchers argue that the impact likely triggered most of the immense eruptions of lava in India known as the Deccan Traps, explaining the “uncomfortably close” coincidence between the Deccan Traps eruptions and the impact, which has always cast doubt on the theory that the asteroid was the sole cause of the end-Cretaceous mass extinction.
“If you try to explain why the largest impact we know of in the last billion years happened within 100,000 years of these massive lava flows at Deccan … the chances of that occurring at random are minuscule,” said team leader Richards, at the time, UC Berkeley professor of earth and planetary science.
Richards and his colleagues marshal evidence for their theory that the impact reignited the Deccan flood lavas in a paper published in The Geological Society of America Bulletin.
While the Deccan lava flows, which started before the impact but erupted for several hundred thousand years after re-ignition, probably spewed immense amounts of carbon dioxide and other noxious, climate-modifying gases into the atmosphere, it’s still unclear if this contributed to the demise of most of life on Earth at the end of the Age of Dinosaurs, Richards said.
“This connection between the impact and the Deccan lava flows is a great story and might even be true, but it doesn’t yet take us closer to understanding what actually killed the dinosaurs and the ‘forams,’” he said, referring to tiny sea creatures called foraminifera, many of which disappeared from the fossil record virtually overnight at the boundary between the Cretaceous and Tertiary periods, called the KT boundary. The disappearance of the landscape-dominating dinosaurs is widely credited with ushering in the age of mammals, eventually including humans.
He stresses that his proposal differs from an earlier hypothesis that the energy of the impact was focused around Earth to a spot directly opposite, or antipodal, to the impact, triggering the eruption of the Deccan Traps. The “antipodal focusing” theory died when the impact crater, called Chicxulub, was found off the Yucatán coast of Mexico, which is about 5,000 kilometers from the antipode of the Deccan traps.
Richards proposed in 1989 that plumes of hot rock, called “plume heads,” rise through Earth’s mantle every 20-30 million years and generate huge lava flows, called flood basalts, like the Deccan Traps. It struck him as more than coincidence that the last four of the six known mass extinctions of life occurred at the same time as one of these massive eruptions.
Illustration of a hot mantle plume “head” pancaked beneath the Indian Plate. The theory by Richards and his colleagues suggests that existing magma within this plume head was mobilized by strong seismic shaking from the Chicxulub asteroid impact, resulting in the largest of the Deccan Traps flood basalt eruptions.
“Paul Renne’s group at Berkeley showed years ago that the Central Atlantic Magmatic Province is associated with the mass extinction at the Triassic/Jurassic boundary 200 million years ago, and the Siberian Traps are associated with the end Permian extinction 250 million years ago, and now we also know that a big volcanic eruption in China called the Emeishan Traps is associated with the end-Guadalupian extinction 260 million years ago,” Richards said. “Then you have the Deccan eruptions – including the largest mapped lava flows on Earth – occurring 66 million years ago coincident with the KT mass extinction. So what really happened at the KT boundary?”
Richards teamed up with experts in many areas to try to discover faults with his radical idea that the impact triggered the Deccan eruptions, but instead came up with supporting evidence. Paul Renne, a professor in residence in the UC Berkeley Department of Earth and Planetary Science and director of the Berkeley Geochronology Center, re-dated the asteroid impact and mass extinction two years ago and found them essentially simultaneous, but also within approximately 100,000 years of the largest Deccan eruptions, referred to as the Wai subgroup flows, which produced about 70 percent of the lavas that now stretch across the Indian subcontinent from Mumbai to Kolkata.
Michael Manga, a professor in the same department, has shown over the past decade that large earthquakes – equivalent to Japan’s 9.0 Tohoku quake in 2011 – can trigger nearby volcanic eruptions. Richards calculates that the asteroid that created the Chicxulub crater might have generated the equivalent of a magnitude 9 or larger earthquake everywhere on Earth, sufficient to ignite the Deccan flood basalts and perhaps eruptions many places around the globe, including at mid-ocean ridges.
“It’s inconceivable that the impact could have melted a whole lot of rock away from the impact site itself, but if you had a system that already had magma and you gave it a little extra kick, it could produce a big eruption,” Manga said.
Similarly, Deccan lava from before the impact is chemically different from that after the impact, indicating a faster rise to the surface after the impact, while the pattern of dikes from which the supercharged lava flowed – “like cracks in a soufflé,” Renne said – are more randomly oriented post-impact.
“There is a profound break in the style of eruptions and the volume and composition of the eruptions,” said Renne. “The whole question is, ‘Is that discontinuity synchronous with the impact?’”
Richards, Renne and colleagues visited India in April 2014 to obtain lava samples for dating, and noticed that there are pronounced weathering surfaces, or terraces, marking the onset of the huge Wai subgroup flows. Geological evidence suggests that these terraces may signal a period of quiescence in Deccan volcanism prior to the Chicxulub impact. Apparently never before noticed, these terraces are part of the western Ghats, a mountain chain named after the Hindu word for steps.
Photograph of part of the main stack of 66 million year old Deccan Traps lava flows near the city of Mahabaleshwar, India. The entire volume of the Deccan Traps could have covered an area as large as the state of California in a mile deep pile of lava flows.(Mark Richards photo)
“This was an existing massive volcanic system that had been there probably several million years, and the impact gave this thing a shake and it mobilized a huge amount of magma over a short amount of time,” Richards said. “The beauty of this theory is that it is very testable, because it predicts that you should have the impact and the beginning of the extinction, and within 100,000 years or so you should have these massive eruptions coming out, which is about how long it might take for the magma to reach the surface.”
At the edge of the Western Ghats, Buddhists carved some two dozen monasteries and five temples into the cliffs of this Deccan basalt, writes Brannen. “These are the stunning Ajanta Caves, which, like the Ring of Cenotes, are a UNESCO World Heritage Site. Thousands of years ago, monks sat in silent meditation deep inside the rock that might have destroyed the world, a fitting place to contemplate that foundational Buddhist concept of anicca—the impermanence of all things.”
Since the team’s paper was accepted for publication, a group from Princeton University published new radioisotopic dates for the Deccan Traps lavas that are consistent with these predictions. Renne and Sprain at UC Berkeley also have preliminary, unpublished dates for the Deccan lavas that could help solidify Richards’ theory, Renne said
Image credit; With thanks to Taylor Hammes