Walter Alvarez

Born on 3 October 1940 in Berkeley, California, Walter Alvarez is a geologist who developed the theory that the dinosaurs were killed by a meteor impact. Alvarez (right in the photo) earned his PhD in geology at Princeton University and then worked for an oil company in Holland and Libya—he was forced to leave the latter country after Colonel Gaddafi’s 1969 revolution. In 1977 Alvarez joined the faculty at the University of California, Berkeley and began investigating layers of exposed limestone in the Umbria region of Italy. He noticed that the diversity of the fossilized remains of single-celled creatures called forams seemed to decrease significantly about 65 million years ago, between the Cretaceous and Tertiary periods. In between the limestone layers corresponding to those periods was a thin strip of clay with no forams at all. He and other geologists soon found the clay layer in other parts of the world, suggesting it was linked to a global event. To determine whether the catastrophe that deposited the clay was gradual or sudden, Alvarez turned to his father, Luis Alvarez (left), a particle physicist and Nobel laureate. At Luis’s suggestion, the scientists looked in the clay for the element iridium, which falls onto Earth from space at a low but measurable rate. The scientists found abnormally high levels, indicating that something extraterrestrial had deposited the iridium extremely quickly. The Alvarezes put forth a meteor impact as the most likely culprit, and in the late 1980s scientists identified the crater at Mexico’s Yucatan Peninsula. Most scientists now accept that an asteroid measuring about 10 km across slammed into Earth 65 million years ago, triggering the extinction of many organisms, including the dinosaurs. (Photo credit: Lawrence Berkeley National Laboratory)

Date in History: 3 October 1940