The term iridium anomaly commonly refers to an unusual abundance of the chemical element iridium in a layer of rock strata, often taken as evidence of an extraterrestrial impact event because of the case of such an anomaly at the Cretaceous–Tertiary boundary (often abbreviated K-T boundary). Iridium is a very rare element in the Earth's crust, but is found in anomalously high concentrations (around 100 times greater than normal) in a thin world-wide layer of clay marking the boundary between the Cretaceous and Tertiary periods, 65 million years ago. This boundary is marked by a major extinction event, including that of the dinosaurs along with about 70% of all other species. The physicist Luis Alvarez and his coworkers were the first to link the extinction to an impact event based on the observation that iridium is much more abundant in meteorites than it is on Earth. This theory was later substantiated by other evidence, including the eventual discovery of the impact crater, known as Chicxulub, on the Yucatan Peninsula in Mexico.
