Cretaceous boundary



Four years after it was first proposed, the theory that a colossal meteorite struck the earth 65 million years ago continues to build toward a consensus—this despite recent findings that volcanic eruptions might also have caused the “iridium anomaly” that is the impact theory's best evidence (Eos, February 7, 1984, p. 41).

High concentrations of iridium at the Cretaceous-tertiary boundary were first noticed in sediments from Italy, Denmark, Spain, and other locations around the world by Walter Alvarez, his father Luis, and their Berkeley colleagues. They concluded that the source of the extra iridium must have been extra-terrestrial because the element shows up in crustal rocks only in very small amounts and no one could think of a mechanism that would distribute iridium from the mantle so widely around the surface of the globe. The Alvarez group postulated that a meteorite about 10-km wide had collided with the earth, throwing up a planet-encircling cloud that blocked out sunlight, ended photosynthesis, and snuffed out many of the land and sea creatures of the Mesozoic, including the dinosaurs.