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Sixty-five million years ago, about 70 percent of all species then living on Earth disappeared within a very short period. The disappearances included the last of the great dinosaurs. Paleontologists speculated and theorized for many years about what could have caused this mass extinction, known as the K-T event (Cretaceous-Tertiary Mass Extinction event). Then in 1980, Alvarez, Alvarez, Asaro, and Michel reported their discovery that the peculiar sedimentary clay layer that was laid down at the time of the extinction showed an enormous amount of the rare element iridium. First seen in the layer near Gubbio, Italy, the same enhancement was soon discovered to be worldwide in that one particular 1-centimeter (0.4-inch) layer, both on land and at sea. The Alvarez team suggested that the enhancement was the product of a huge asteroid impact.
On Earth, most of the iridium and a number of other rare elements such as platinum, osmium, ruthenium, rhodium, and palladium are believed to have been carried down into Earth's core, along with much of the iron, when Earth was largely molten. Primitive chondritic meteorites (and presumably their asteroidal parents) still have the primordial solar system abundances of these elements. A chondritic asteroid 10 kilometers (6 miles) in diameter would contain enough iridium to account for the worldwide clay layer enhancement. This enhancement appears to hold for the other elements mentioned as well.
Since the original discovery, many other pieces of evidence have come to light that strongly support the impact theory. The high temperatures generated by the impact would have caused enormous fires, and indeed soot is found in the boundary clays. A physically altered form of the mineral quartz that can only be formed by the very high pressures associated with impacts has been found in the K-T layer.
Geologists who preferred other explanations for the K-T event said, 'Show us the crater.' In 1990, a cosmochemist named Alan Hildebrand became aware of geophysical data taken 10 years earlier by geophysicists looking for oil in the Yucatan region of Mexico. There, a 180-kilometer (112-mile) diameter ring structure called Chicxulub seemed to fit what would be expected from a 65-million-year-old impact, and further studies have largely served to confirm its impact origin. The Chicxulub crater has been age dated (by the 40Ar/39Ar method) at 65 million years! Such an impact would cause enormous tidal waves, and evidence of just such waves at about that time has been found all around the Gulf of Mexico. Similarly, glassy debris of appropriate age called tektites (and their decomposition products), which are produced by large impacts, have been found all around the Gulf.
One can never prove that an asteroid impact killed the dinosaurs. Many species of dinosaurs (and smaller flora and fauna) had in fact died out over the millions of years preceding the K-T event. The impact of a 10-kilometer (6 mile) asteroid would most certainly have been an enormous insult to life on Earth. Locally, there would have been enormous shock wave heating and fires, a tremendous earthquake, hurricane winds, and trillions of tons of debris thrown everywhere. It would have created months of darkness and cooler temperatures globally. There would have been concentrated nitric acid rains worldwide. Sulfuric acid aerosols may have cooled Earth for years. Life certainly could not have been easy for those species which did survive. Fortunately, such impacts occur only about once every hundred million years.
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Appendix C