The Sixth Extinction Chapter 4 - Elyonni Tordesillas

      Elizabeth Kolbert introduces chapter 4 by explaining how the town of Gubbio, which is about a hundred miles north of Rome, might be described as a municipal fossil. Gubbio is notable for its beautiful limestone, including a massive limestone gorge with steep, smooth walls. In the 1970's it was here that geologist, Walter Alvarez, discovered traces of a huge asteroid—the asteroid, which, scientists later decided, hit the Earth during the Cretaceous period, causing the mass-extinction of the dinosaurs. The extinction of dinosaurs millions of years ago is arguably the most famous extinction in history. Walter Alvarez had come to Italy to study plate tectonics. Beneath the surface of the earth, he found layers of marine fossils, and he also noticed that there were thick layers of clay, containing no fossils, imbedded in the limestone of Gubbio. Walter found that clay contained huge amounts of iridium, suggesting that the clay may have originated in an asteroid. Further tests showed that there were thick layers of iridium dating back to the end of the Cretaceous period. In 1980, Luis and Walter Alvarez co-wrote an influential paper arguing that an asteroid containing iridium struck the Earth at the end of the Cretaceous period, killing the dinosaurs. Many biologists and paleontologist rejected Alvarez's idea of the mass extinction of dinosaurs. They pointed out that extinction is a slow, gradual process, not the result of a sudden catastrophe like an asteroid collision. The Alvarez theory of dinosaur extinction was “falsifiable,” meaning that further evidence could be offered to support or disprove the theory. In the decades following the appearance of the theory, new evidence surfaced that seemed to support the Alvarez. As a result, the scientific community came to accept the asteroid theory.