The Asteroid-impact Theory
The primary cause of the Cretaceous mass extinction was a mystery for decades, until geologists discovered a thin layer of rock that marks the boundary between the Cretaceous period and following Tertiary period; this layer of sediments is termed the K-T boundary, and gave rise to the asteroid-impact theory of the Cretaceous extinction.
The asteroid-impact theory was first proposed in detail in 1978, by a team led by American geologist Walter Alvarez (1940–) and physicist Luis Alvarez (1911–). The Alvarez team analyzed sediment collected in the 1970s from the K-T layer near the town of Gubbio, Italy. The samples showed a high concentration of the element iridium, a substance rare on Earth but relatively abundant in meteorites. Other samples of K-T boundary strata from around the world were also analyzed; excess iridium was found in these samples as well. Using the average thickness of the sediment as a guide, they calculated that a meteorite about 6 mi (10 km) in diameter would be required to spread that much iridium over the whole Earth.
If a meteorite that size had hit Earth, the dust lofted into the air would have produced an enormous cloud of dust that would have encircled the world and blocked out the sunlight for months, possibly years. This climactic change would have severely depressed photosynthesis, resulting in the death of many plants, subsequent deaths of herbivores, and finally the death of their predators as well. (This chain of events would have occurred so rapidly that there would have been no chance for evolutionary adaptation to the new environment, which requires thousands of years at minimum.) A major problem with the theory, however, was that a 6-mi (10-km) meteorite would leave a very large crater, 93–124 mi (150–200 km) in diameter—and while Earth has many impact craters on its surface, few are even close to this size, and none of the right age was known.
Because 65 million years had passed since the hypothetical impact, scientists shifted the search underground. A crater that old would almost certainly have been filled in by now. In 1992, an impact crater was discovered under the surface near the village of Chicxulub (pronounced CHIX-uh-loob) on Mexico's Yucatan Peninsula. When core samples raised by drilling were analyzed, they showed the crater to be about 112 mi (80 km) in diameter and 65 million years old—the smoking gun that validated the Alvarez asteroid-impact theory.
The asteroid impact theory is now widely accepted as the most probable explanation of the K-T iridium anomaly, but many geologists still debate whether the impact of this large meteorite was the sole cause of the mass extinction of the dinosaurs and other life forms at that time, as the fossil record seems to show an above-average rate of extinctions in the time leading up to the K-T boundary. A number of gradual causes can accelerate extinction: falling ocean levels, for example, expose continental shelves, shrinking shallow marine environments and causing drier continental interiors, both changes that encourage extinction. Further, very large volcanic eruptions may stress the global environment. The asteroid that caused the Chicxulub crater may have coincidentally amplified or punctuated an independent extinction process that had already begun. There is no reason why many different causes cannot have acted, independently or in concert, to produce extinction events.
The asteroid-impact theory has been applied to many mass extinctions since the discovery of Chicxulub. Most of the five major mass extinctions of the last 540 million years, and several of the smaller ones, have been shown to coincide in time with large impact craters or iridium spikes (layers of heightened iridium concentration) in the geological column.