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Single Impact Killed the Dinosaurs, International Team Confirms

University of Missouri geologist part of 41-member scientific collaboration

March 5th, 2010

Story Contact: Christian Basi, 573-882-4430,
Ken Macleod, associate professor of geological sciences in the College of Arts and Science.


COLUMBIA, Mo. ­— For decades, scientists have argued about the cause of dinosaur extinction. Theories have included a single impact from a meteor, which then had devastating effects on the global environment, to long-lasting volcanic activity. Now, a group of 41 international scientists, including one University of Missouri geologist, have completed a comprehensive review of all the data and confirmed that a single impact of a meteor near Chicxulub, Mexico, was the fatal blow to numerous plant and animal species, including the dinosaurs, 65.5 million years ago. The study is being published in this week’s edition of Science.

“The debate on this subject was quite heated until the early 1990s, but the discovery of the crater was a key turning point for many scientists,” said Ken MacLeod, associate professor of geological sciences in the College of Arts and Science. “Most geologic changes are too slow to be easily detected and even very large events, like the recent earthquakes, have only regional consequences. This event created the boundary between the age of dinosaurs and the age of mammals, but it played out in a biological instant. The event was an experiment done in the past that we wouldn’t want to have happen in the present. It might be similar to what would happen should we ever engage in nuclear war.”

In the study, the scientific team studied the thickness and abundance of materials in a thin layer of clay that formed 65.5 million years ago and marks the end of the Cretaceous Period. The researchers found rare elements, special minerals and “melt-glass,” materials that are typically generated by large impacts. The scientists also found that the thickness of the clay layer and the abundance of the impact debris increased the closer they were to the crater near Chicxulub (pronounced CHICK – sha – loob).

The clay layer is found around the globe at more than 350 sites. One key observation supporting the impact theory is that the clay layer contains high levels of iridium, an element that is extremely rare at the Earth’s surface but that is relatively common in asteroids. Because iridium and other impact indicators are found across the globe at the level of the extinction, the two are likely related.

The research team concludes that the extinction event resulted from the collision of a space rock that was roughly 6 miles in diameter. The collision created a crater that is more than 110 miles in diameter and caused  tsunamis, earthquakes greater than magnitude 11, fires, extended darkness, cooling temperatures and acid rain.

“While there were big volcanic eruptions around this time, they started before and kept happening after this event,” MacLeod said. “They were so big that they disrupted the environment, but those changes were separate from the extinctions. The volcanic activity happened over hundreds of thousands of years and the extinction event was much quicker and more catastrophic. “