Hypotheses abound concerning the environmental upheaval that caused the final Cretaceous extinction 65 million years ago. In 1980, Nobel prize winner Luis Alvarez and his colleagues published a paper in Science magazine that proposed the extinction of the dinosaurs and many other creatures was due to the impact of an asteroid or comet with Earth. The element iridium was used as evidence to support this proposal.

Iridium, the most corrosion-resistant metal known, is rarely found in Earth’s crust. But, it is abundant in Earth’s core and in meteorites and asteroids. Large amounts of iridium have been found in a clay layer that marks the end of Cretaceous-period rock and the beginning of Tertiary rock (the K-T boundary). Researchers have found up to 100 times the amount of iridium in this clay layer than is normally found in the soils around the world.