The Last Days of T.rex

Few scientific theories have been accepted as quickly as the idea that an asteroid killed the dinosaurs. Hollywood blockbusters. Some of my colleagues will even tell you ground zero is the Chicxulub impact structure located off the coast of the Yucatan peninsula in Mexico. And most people seem happy to accept their word. Because I’m a scientist, people will ask me if there is life on Mars, if global warming is for real, or even if the moon landings were a hoax, but not once has someone asked me if I thought the dinosaurs were really toasted by a giant asteroid. After all, how else would you suddenly kill off an animal as cool as a T-Rex?

Back in 1980, the principal piece of evidence suggesting an asteroid was to blame came from a little known element called iridium. When the Alvarez group analyzed sediments deposited right after the dinosaurs died 65 million years ago—a moment in geologic time known as the K/T boundary—they found concentrations of iridium, which is extremely rare on Earth but abundant in asteroids. At the time, no one had identified an impact crater that was 65 million years old, so scientists started looking. Eventually geologists found evidence for such a crater in drill samples taken while prospecting for oil in the Yucatan. To many the "smoking gun" had been found.

Scientists are a skeptical bunch by nature, however, and while the press and Hollywood took the idea and ran with it, some of us began shaking our heads. One of the problems with the Chicxulub impact crater is that it’s about 100 miles in diameter. An important tenet of science is that a theory only has validity if it works all the time. Basically that means that other craters 100 miles in diameter or larger should also have wiped out most life on Earth. While few craters rival the size of Chicxulub, there are the 60-mile wide craters Manicouagan in Canada and Popigai in Russia. There is also the 150-mile wide Sudbury crater in Canada and the 180-mile wide Vredefort crater in South Africa. No mass extinctions have been associated with these impact structures, and Earth has experienced plenty of mass extinctions.

A Gerta Keller of Princeton University and her colleagues also suggests that Chicxulub may have formed 300,000 years before the dinosaurs actually went extinct. And so far, only the K/T boundary appears to show an iridium "anomaly," suggesting that some other mechanism is responsible for other mass extinctions on Earth. While this evidence remains controversial, more and more scientists are giving up on Chicxulub as the smoking gun.

So how important was the giant impact in killing off the dinosaurs? Despite all the hype, it may have had nothing to do with it.

At the same time the dinosaurs were dying, an enormous volcanic eruption was occurring in what is now India. More than 12,000 cubic miles of lava poured out onto Earth’s surface within a very short period of time. The ash and gases associated with this eruption would have certainly affected climate in much the same way as a giant impact would.

These so-called flood basalts have occurred throughout Earth’s history, and each time they’ve been associated with large mass extinctions. American Geophysical Union, Courtillot showed a relation between the larger cycles of magnetic reversals and mass extinctions. Many different pieces of his theory seem to fit together nicely.

Of course, this explanation has some problems as well. At the same AGU Meeting, I had the opportunity to chat with geophysicist Silver, magma actually forms enormous puddles under the continents over time. The giant stresses and faulting that occur when crustal plates collide creates the pathways for this magma to reach the surface—usually over very short timescales. What I particularly like about Silver’s paper is that he provides ideas for testing his model, including seismic surveys that may show magma puddles forming under the continents.

There is still a lot of work to do. But the next time you see a picture of dinosaurs running from a giant asteroid falling from the sky, you might also want to think about the magma slowly collecting beneath your feet.

Posted January 16, 2007.

Bob Craddockis a geologist with the National Air and Space Museum's Center for Earth and Planetary Studies.