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* The revolution began with an unassuming element known as iridium, a rare and hard silvery-white metal related to platinum and gold. In the spring of 1977, Geologist Walter Alvarez of the University of California, Berkeley, was carefully chiseling through the rocks outside Gubbio, a medieval Italian town halfway between Florence and Rome, seeking clues to continental drift. Gubbio has long been an appealing site to geologists and paleontologists because its rocks provide a complete geological record of the critical boundary line between the end of the Cretaceous period, when the dinosaurs disappeared, and the Tertiary period, which followed it.
As he chiseled, Alvarez was struck by a configuration of sediment layers, which resembled a sandwich in stone. The bottom or older layer consisted of Cretaceous limestone, which was full of tiny fossils. On top was a second slice of limestone, from the Tertiary period, almost devoid of these fossils. Like other samples of rock from that era, it showed that the creatures alive during the late Cretaceous period had, by geological time scales, suddenly disappeared. In between the limestone layers was a dull red layer of clay about half an inch thick, first discovered by an Italian paleontologist around 1960.
Alvarez, his curiosity aroused, shipped samples of the sediment back to the U.S. and showed them to his father Luis, a Nobel-prizewinning physicist also at the University of California, who had the clay analyzed. To everybody's surprise, it turned out to be 30 times as rich in iridium as normal rocks. The Berkeley team knew of only a few places where such high concentrations of the rare element might occur: in the earth's core, perhaps 2,000 miles belowground; in extraterrestrial objects like asteroids (or their fragments, meteors) and comets; or in the cosmic dust drifting to earth from a nearby supernova (exploding star). The core material seemed too deep to come to the surface, and further analysis ruled out a supernova as the source, so father and son concluded that the iridium had been left by a giant asteroid hitting the earth at the end of the Cretaceous period.
The Alvarezes did not stop there. Basing their calculations on the atmospheric consequences of the explosion of Krakatoa, they roughly estimated how much dust the impact of the asteroid would have thrown into the atmosphere, how long sunlight would have been blocked from the earth's surface, and what kinds of life would have been the most gravely affected by the climatic changes. They decided that plants, totally dependent on the sun for photosynthesis, and a variety of marine organisms would have died first, followed by the land animals highest on the food chain, the dinosaurs. Eventually the iridium-enriched debris, circulating around the world in the atmosphere, would have settled like a sprinkling of black snow over much of the globe. (It was the Alvarezes' theory that helped to shape the now familiar concept of nuclear winter.)