Asteroid Rained Glass Over Entire Earth, Scientists Say

Hillary Mayell
for National Geographic News
April 15, 2005

Scientists studying the fallout from a huge asteroid that crashed into Earth 65 million years ago have gained better understanding of the event that most likely took out the dinosaurs and much other life on the planet.

The asteroid that created the Chicxulub (pronounced CHEEK-shoo-loob) crater, located on the Yucatán Peninsula of Mexico, was probably more than 6 miles (10 kilometers) wide, researchers estimate. The resulting crater was 110 to 125 miles (180 to 200 kilometers) wide and very deep. Today it is buried under several miles of limestone and is mostly underwater.

"When this thing came in, it cleaved right through the atmosphere, and you had a huge amount of meteoritic material with a lot of iridium in it smack into the Earth and vaporize totally," said Lawrence Grossman, a geochemist at the University of Chicago.

All that iridium—an element that is abundant in space but rare on Earth—was thrown up in a huge plume of superheated gas, ash, dust, and pulverized rock.

"This high-temperature, high-pressure plume punches through the atmosphere, way up into the stratosphere. Calculations based on the fallout material left behind indicate that this cloud would envelop the globe," said Denton Ebel, a chemist and assistant curator of meteorites at New York City's American Museum of Natural History.

The scientists concluded that minute droplets of superhot liquid rock condensed in the vapor cloud as it expanded and cooled, and then rained down all over Earth.

"Everything at ground zero of course gets wiped out," Grossman said. "But why do species all over the Earth die? The idea is this cloud of dust blocked sunlight. And in order for all these species to die off, it had to take weeks to months for that dust to settle back to Earth and the sun to shine again."

Grossman and Ebel's study is published in the April issue of the journal Geology.

Examining the Iridium Spike

Today the impact debris exists as a thin layer of reddish clay that can be found in rock strata across continents and beneath the ocean floor. The amount of iridium found in this layer is several hundred times higher than normal in some places.

The iridium-rich layer forms what geologists call the Cretaceous-Tertiary boundary (or Cretaceous-Paleogene boundary)—"K-T boundary," for short. In soil below the boundary, fossils of dinosaurs and many other plants and animals that existed during the Cretaceous period (144 million to 65 million years ago) are abundant. In soil layers above the iridium spike, the fossil record tells the story of a world without dinosaurs.

The obvious conclusion is that the layer of debris ejected by the impact draws the line of extinction for the dinosaurs. The K-T boundary ends the age of reptiles and marks the dawn of the age of mammals.

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