iridium

Did You Know? In 1978 geologist Walter Alvarez found an unusually high concentration of the element iridium in a layer of clay. This layer formed at the time dinosaurs and many other organisms went extinct. The iridium deposits were a great surprise, since iridium is very rare at the Earth's surface. Most surface iridium is believed to come from outer space—from dust left over after meteors disintegrate in the atmosphere or smash into the Earth. Walter's father, the physicist Luis Alvarez, suggested that the iridium came from the impact of a meteor about 6 miles (10 kilometers) across. He argued that such an impact would have caused an enormous explosion, sending huge clouds of dust into the atmosphere. The dust, blocking out the sun and causing acid rain for years, would have caused a worldwide ecological disaster. Many scientists think that such a disaster caused the extinction of dinosaurs and at least 70 percent of all other species alive at the time, including most of the Earth's land plants. Geologists have since found iridium deposits in rocks of a similar date in over 100 places worldwide. In the early 1990s, a large impact crater of the same age as the iridium deposits was identified in the Yucatan peninsula of central Mexico. It is 125 miles (200 kilometers) wide and may well have been caused by the impact hypothesized by Alvarez.