The K/T Extinction Event

The Asteroid Impact that Doomed the Dinosaurs

K/T meteor
An artist's impression of the K/T meteor impact (NASA).

About 65 and a half million years ago, at the end of the Cretaceous period, dinosaurs, the largest, most fearsome creatures ever to rule the planet, died off in vast quantities, along with their cousins, the pterosaurs, and marine reptiles. Although this mass extinction didn't happen literally overnight, in evolutionary terms, it may as well have--within a few thousand years of whatever catastrophe caused their demise, the dinosaurs had been wiped off the face of the earth.

 

The Cretaceous-Tertiary Extinction Event--or K/T Extinction Event, as it's known in scientific shorthand--has spawned a variety of less-than-convincing theories. Up until a few decades ago, paleontologists, climatologists, and assorted cranks blamed everything from epidemic disease to lemming-like suicides to intervention by aliens. That all changed, though, when the Cuban-born physicist Luis Alvarez had an inspired hunch.

Did a Meteor Impact Cause the Extinction of the Dinosaurs?

In 1980, Alvarez--along with his physicist son, Walter--put forth a startling hypothesis about the K/T Extinction Event. Along with other researchers, the Alvarezes had been investigating sediments laid down all over the world around the time of the K/T boundary 65 million years ago (it's generally a straightforward matter to match geologic strata--layers of sediment in rock formations, river beds, etc.--with specific epochs in geologic history, especially in areas of the world where these sediments accumulate in roughly linear fashion).

These scientists discovered that the sediments laid down at the K/T boundary were unusually rich in the element iridium. In normal conditions, iridium is extremely rare, leading the Alvarezes to conclude that the earth was struck 65 million years ago by an iridium-rich meteorite or comet. The iridium residue from the impact object, along with millions of tons of debris from the impact crater, would have quickly spread all over the globe; the massive amounts of dust blotted out the sun, and thus killed the vegetation eaten by herbivorous dinosaurs, the disappearance of which caused the starvation of carnivorous dinosaurs.

(Presumably, a similar chain of events led to the extinction of ocean-dwelling mosasaurs and giant pterosaurs like Quetzalcoatlus.)

Where is the K/T Impact Crater?

It's one thing to propose a massive meteor impact as the cause of the K/T Extinction, but it's quite another to adduce the necessary proof for such a bold hypothesis. The next challenge the Alvarezes faced was to identify the responsible astronomical object, as well as its signature impact crater--not as easy a matter as you might think since the earth's surface is geologically active and tends to erase evidence of even large meteorite impacts over the course of millions of years.

Amazingly, a few years after the Alvarezes published their theory, investigators found the buried remains of a huge crater in the region of Chicxulub, on Mexico's Mayan peninsula. Analysis of its sediments demonstrated that this gigantic (over 100 miles in diameter) crater had been created 65 million years ago--and was clearly caused by an astronomical object, either a comet or a meteor, sufficiently large (anywhere from six to nine miles wide) to occasion the extinction of the dinosaurs. In fact, the size of the crater closely matched the rough estimate proposed by the Alvarezes in their original paper!

Was the K/T Impact the Only Factor in Dinosaur Extinction?

Today, most paleontologists agree that the K/T meteorite (or comet) was the prime cause of the extinction of the dinosaurs--and in 2010, an international panel of experts endorsed this conclusion after re-examining massive amounts of evidence  However, this doesn't mean there couldn't have been aggravating circumstances: for instance, it's possible that the impact was roughly concurrent with an extended period of volcanic activity on the Indian subcontinent, which would have further polluted the atmosphere, or that dinosaurs were dwindling in diversity and ripe for extinction (by the end of the Cretaceous period, there was less variety among dinosaurs than at earlier times in the Mesozoic Era).

It's also important to remember that the K/T Extinction Event wasn't the only such catastrophe in the history of life on earth--or even the worst, statistically speaking (see Earth's 10 Biggest Mass Extinctions).

For example, the end of the Permian period, 250 million years ago, witnessed the Permian-Triassic Extinction Event, a still-mysterious global catastrophe in which over 70 percent of land-dwelling animals and a whopping 95 percent of marine animals went kaput. Ironically, it was this extinction that cleared the field for the rise of the dinosaurs toward the end of the Triassic period--after which they managed to hold the world stage for a whopping 150 million years, until that unfortunate visit from the Chicxulub comet.