A former senior lecturer at the University of California, Davis, Richard Cowen, writes of the sudden extinction of all dinosaurs, about 65 million years ago. This is referred to as the K--T extinction event, which refers to the Cretaceous and Tertiary periods of geologic time. At around the same time, a meteorite or small asteroid struck the Earth. University of California geologist Walter Alvarez and colleagues found evidence of the impact after examining K-T boundary rocks and discovering unusual quantities of iridium. Iridium is rarer than gold on Earth, yet a common constituent of meteorites.
A crater was discovered near Chicxulub, on the Yucatan Peninsula in southeastern Mexico, in the late 1970s. In time, the theory was advanced that the Chicxulub Crater was the probable the site of an asteroid impact. Many scientists agree that the K--T Extinction Event was caused by this impact. But Missouri State University's Kevin R. Evans is less sure. Evans thinks the sedimentary record indicates that the two events could have been 300,000 years apart.
If the Chicxulub impact occurred 300,000 years before dinosaurs became extinct, its correlation with the disappearance of dinosaurs becomes much less persuasive. Gerta Keller of Princeton University contends the Chicxulub asteroid impact and the K-T mass extinction are two distinct and unrelated events. Keller says that, "Not a single species went extinct as a result of the Chicxulub impact."
If the Chicxulub impact was not the cause of dinosaur extinction, what was? Keller concedes that another large impact, from an asteroid or meteor, may have been responsible. But large volcanic eruptions, perhaps from the Deccan Traps in India, one of the Earth's most significant volcanic areas, might be to blame. If huge amounts of volcanic ash and gases blocked out the sun's rays, dinosaurs might have froze to death. Alternatively, a lack of sun might have had a devastating effect on plant life and the whole food chain. Dinosaurs might have starved to death.