Word: crater
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Dates: during 1990-1999
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EVERY AUGUST THE EARTH PASSES THROUGH THE orbital path of Comet Swift-Tuttle. If the comet ever happened to be there, the 10-km-wide (6-mile) chunk of ice and rock could slam into the planet, carving an enormous crater, generating tidal waves and throwing up a worldwide pall of dust that could block sunlight for months. Plants would be largely wiped out, and so would many species that ultimately depend on plants for food -- including, perhaps, the human race. Just such a disaster, many scientists believe, killed off the dinosaurs 65 million years ago. A smaller strike...
...vertically, punching a hole 10 km wide through the atmosphere, generating temperatures so high that the air itself started to burn. When it hit the ground near the Gulf of Mexico, rock turned to liquid and spread outward in mountainous waves, not freezing until it had formed a crater 200 km across...
...search for the Crater, the first clues were sifted out of clumps of gray clay. At dozens of sites around the world, that clay has been found in a thin boundary layer between the rock of the Tertiary period and the formations of the late Cretaceous period, which ended 65 million years ago. In the Cretaceous rock lie the fossil remains of giant dinosaurs and a profusion of other species. But in the Tertiary formations, just above the clay, no trace exists of the dinosaurs or many of the other Cretaceous species...
...evidence mounts, more researchers are convinced that the Chicxulub crater marks the impact point of the killer comet. Says Boynton: "This is nearly as close to a certainty as one can get in science." Some scientists disagree. David Archibald, a biologist at San Diego State University, believes the extinctions took place more gradually and in a complex pattern. "There is zero evidence that dinosaurs became extinct virtually overnight...
...with the largest chunk, causing the Great Extinction. Perhaps only a year or two later, as the earth again entered the trail of cometary debris, it met a second, smaller chunk. Where did the second impact occur? This time no search is necessary. Shoemaker points to a well-known crater, 35 km (22 miles) across, that lies partly buried near Manson, Iowa. Its age, established by radioactive dating: 65 million years. / Shoemaker believes the new findings will help persuade more scientists to "get off the fence" and side with the Alvarez theory. "Chicxulub is the smoking cannon," he says...