November 30. It’s the end of the world as we know it….

The end of the Cretaceous – and the end of the Mesozoic Era – were marked by a mass extinction, one of the largest in earth history. Unlike some of the earlier extinctions, we pretty much have a smoking gun for this one: the impact of an asteroid in what is now northwestern Yucatan, Mexico. 

Not that it was an easy sell. Since I was a grown-up geologist in the early 1980s when the team led by Luis Alvarez and his son Walter began to suggest the impact as the cause of the extinction, I remember some of the controversy. It was a radical idea; any kind of catastrophe flew in the face of the long-accepted idea of uniformitarianism, that changes on earth resulted from the application of slow, steady changes over long, long times. I’m really not sure why the idea that catastrophes DID happen was so hard to accept, but it was. The Alvarezes were ridiculed in some professional circles. But accumulating evidence really pretty rapidly, in just a few years, convinced I would say all but a few geoscientists of the validity of the impact as at least a major factor in the extinction. 

Gravity map of Chixulub crater (source: NASA)

The evidence includes a layer around the world right at the top of the Cretaceous, coinciding with the extinction, of iridium, an element that’s abundant in asteroids. Shocked quartz grains are present as well, and result from impacts. Rocks of the right age in Texas and elsewhere in the Gulf of Mexico and Caribbean region look like gigantic tsunami deposits. And finally, magnetic data being analyzed by geophysicist Glen Penfield for oil exploration in Yucatan defined a likely crater on the northwestern coast of the peninsula. It took a while for the age of the Chicxulub impact crater to be determined accurately, but now it is considered to have happened at 66.04 million years ago. It is within 33,000 years of the best estimate for the extinction event, so practically, the two are simultaneous. The Chixulub crater shows up nicely in gravity data too, and even today the cenotes, natural sinkholes scattered across Yucatan, follow the crater margin even though it is deep in the subsurface.

The potential effects of such an impact can be estimated from the size of the crater and the impact angle. Vast quantities of dust, ash, and aerosols likely went into the atmosphere, creating toxicity as well as probably a nuclear winter; it’s been speculated that most of North America burned in an incendiary forest fire across the continent.

The effects resulted in the destruction of about 75% of all species on earth. The most famous victims are the non-avian dinosaurs, all of which perished. All the ammonites died. The giant marine reptiles and the pterosaurs were gone. Pretty much every group, including things like mammals and birds that survived as a class, lost many members. Everything from plankton to plants to snakes and turtles suffered. Surviving birds and mammals did undergo a dramatic diversification and radiation soon after the extinction – a typical reaction, as the survivors adapt and expand into niches vacated by other animals.

While it’s pretty much agreed that the impact was a major factor in the extinction that ended the Mesozoic, there’s still debate about whether it was the only factor. Other things were going on that might have had roles. We’ve talked several times in this series about massive volcanism as a possible factor in extinctions, especially the Siberian flood basalts at the end of the Permian. From about 68 to 66 million years ago in India, similar flood basalts erupted to create the Deccan Traps. Before the asteroid impact was known, the Deccan was usually seen as the gradual, 2-million-year cause of the extinction. There are some declines in diversity among several groups, including some dinosaurs, that may be related to an ongoing effect of the Deccan. The asteroid might have simply been an exclamation point on an already-in-progress extinction. Besides the Deccan volcanics, there were other ongoing climate changes that could have had a role in the general decline and ultimate extinction.

There was also a sharp regression or sea-level drop during the Maastrictian, the last epoch of the Cretaceous. It’s not clear what might have caused that fall in sea level, but other extinctions have been tied pretty clearly to such activity.

So the impact is pretty well accepted as the killer. It’s possible that there were actually multiple impacts. Craters in various parts of the world, from the Ukraine to the North Sea to India are dated to within a few million years of the Cretaceous extinction. There’s a well-known crater in Iowa, the Manson Crater, but it is now dated to about 74 million years ago, too early to have much effect 8 million years later.      

—Richard I. Gibson

The K-T extinction (Richard Cowan) 
Long-term climate stress caused extinction 
Single impact killed dinosaurs

Chixulub gravity map from NASA (public domain)