About 65 million years ago, the largest, most fearsome creatures ever to rule the land died off in vast quantities. 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 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.
Death By Asteroid?
In 1980, Alvarez--along with his physicist son, Walter--put forth a startling hypothesis about the K/T 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 (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).
These scientists discovered that the sediments laid down at the K/T boundary are 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 would have scattered all over the globe--and the massive amounts of dust raised by the impact would have blotted out the sun, and thus killed the vegetation eaten by herbivorous dinosaurs. When the herbivores starved to death, the carnivores would have been next to succumb. The result: a long period of scarcity, cold, and darkness, and eventually no dinosaurs.
Where is the K/T Impact Crater?
It's one thing to propose a massive impact as the cause of the dinosaurs' demise, but it's another thing to actually find proof. The next challenge was to identify the crater of the responsible astronomical object--not an easy matter, since the earth's surface is geologically active and tends to erase evidence of large meteorite impacts over the course of millions of years.
Amazingly, a few years after Alvarex announced his theory, investigators found the buried remains of a huge crater in Chicxulub, on Mexico's Mayan peninsula. Analysis of sediments showed that this gigantic (over 100 miles in diameter) crater had been created 65 million years ago--and was caused by an astronomical object sufficiently large to cause the extinction of the dinosaurs (along with lots of other land-dwelling creatures).
Other Factors that Hastened Dinosaur Extinction
Today, most paleontologists agree that the K/T meteorite (or comet) was the prime cause of the dinosaurs' extinction. However, this doesn't mean there couldn't have been aggravating circumstances: for instance, it's possible that the impact caused an extended period of volcanic activity, further polluting the atmosphere, or that new diseases may have picked off dinos already weakened by hunger and cold. In any case, we won't know for certain until more evidence is found.
