Alvarez Iridium Layer Discovery

In 1980, the physicist Luis Alvarez and his son, the geologist Walter Alvarez, along with colleagues, reported an unusually high concentration of the element iridium in a thin layer of clay marking the boundary between the Cretaceous and Paleogene periods, roughly 66 million years ago, found in rock sections in Italy and confirmed at other sites worldwide. Because iridium is rare in the Earth's crust but relatively common in asteroids and comets, the team proposed that a large asteroid impact had struck the Earth at that boundary, and the hypothesis explained the abrupt mass extinction that ended the age of the dinosaurs. The subsequent identification of the buried Chicxulub crater off the coast of Mexico, dated to the same boundary, provided strong corroborating evidence and made the asteroid impact explanation for the Cretaceous-Paleogene extinction the scientific consensus.