Word: comets
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Dates: during 1990-1999
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Hurtling through the atmosphere at nearly 70 km per sec. (150,000 m.p.h.), the giant comet struck with catastrophic force, punching a hole some 40 km (25 miles) deep through the earth's crust and into the mantle. The violence of the collision 65 million years ago completely vaporized the 8-km-wide (5 miles) comet and blasted out a tremendous crater. Huge rocks, hurled high into the + air, rained down for hundreds of kilometers. A great fireball rose above the atmosphere, carrying with it vast amounts of pulverized debris...
Some 130 terrestrial impact craters had been identified, but none of them near the age of 65 million years was large enough to qualify as the Crater. Yet if a comet or asteroid massive enough to cause the extinction had struck the earth, it would have left a crater hundreds of kilometers wide. Some traces would still exist, despite the intervening millenniums of erosion, sedimentation and tectonic-plate movement...
...town of Chicxulub, on the northern tip of Mexico's Yucatan Peninsula, and extending out under the Gulf of Mexico. The nature of the basin, its location and a preliminary estimate of its age suggest that it is the Crater, the one gouged into the earth by the comet or asteroid that killed the dinosaurs...
...Alvarezes analyzed this clay in the late 1970s and showed it had a far higher content of the rare element iridium than ordinarily found in the earth's crust. It was this discovery that led Luis Alvarez to his momentous - insight. Comets and asteroids have high iridium content, he reasoned, and the clay layer could have been formed by the worldwide fallout of the material vaporized when an errant asteroid or, as most scientists now suspect, a giant comet smacked into the earth...
...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...