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Planet Earth

Surf's Up, Dinos Are Down


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Sixty-five million years ago, a giant meteorite struck what is now the northern edge of the Yucatan Peninsula in Mexico, leaving a crater 120 miles wide and wiping out nearly half the world's species, including the dinosaurs. Now geologists have found that the impact shattered the edge of the continent, which collapsed in a giant submarine landslide.

Marine geologist Tim Bralower of the University of North Carolina was studying deep-sea sediment cores drilled from the middle of the Caribbean when he noticed a curious combination of materials in his sample. In sediment layers from the time of the impact, he found tiny glassy beads of melted rock, called spherules, mixed in with rock fragments and fossils that clearly predated the impact. When he looked at the same layer in cores from the Gulf of Mexico due north of the impact site, he found the same cocktail of particles. Spherules are a telltale marker of an impact, created from the heat of collision. The other rock fragments and fossils were very similar to rocks from the edge of the continent.

Bralower realized that the impact must have fractured the continental margin more than 100 miles away. The massive amount of energy released from the impact--about a million times that of the largest earthquake ever recorded--would have launched a submarine avalanche moving at hundreds of miles per hour.

"The impact caused an incredible modification of the seafloor," Bralower says. The billions of gallons of water displaced by the landslide rose up in huge tsunamis that crashed into what is now Texas and Mexico. Until now researchers didn't know precisely what caused these tidal waves.

Bralower hopes his particle cocktail will help geologists recognize other previously unknown impacts. "Probably the most simple thing to recognize in the rock record are extinctions," he says. "It's very difficult to determine exactly what caused those extinctions. And it's even more difficult, if you're looking for an impact, to find a crater, because these things are often buried. So I think our study might help geologists recognize other ancient impact deposits. I think people are going to find more and more of them."

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