PSR Discoveries

Mass extinctions

Dinosaurs and more than half of all life on Earth vanished approximately 65 million years ago. Geologists call this the Cretaceous-Tertiary (K-T) boundary. What was the radical change in Earth's environment that led to the extinctions? Some scientists suggest that volcanic eruptions of lava, dust, ash, and gases brought on global darkness, toxic rains, and collapse of the food chain. The more compelling hypothesis is that these calamities resulted from a giant asteroid or comet impact on Earth about 65 million years ago.

The impact hypothesis

In a 1980 Science article, Luis Alvarez and his son, Walter, gave geologic evidence for a centimeter-thick clay layer between rock layers of Cretaceous age (between 135 and 65 million years ago) and Tertiary age (between 65 and 2 million years ago.) Most important, this clay contained high concentrations of the element iridium, which is extremely rare in rocks of the Earth's crust, but is abundant in stony meteorites. The Alvarez team proposed that the clay was the altered remains of a global dust shroud from an impact event that occurred about 65 million years ago. The task of finding the source crater of this momentous event began.

Putting the pieces together at Chicxulub

Beginning as early as the 1950s, geologists in Mexico collected gravity data and deep-drilling samples of rocks off the Yucatan peninsula and found unusual results. A 200-300-kilometer-wide circular structure or crater seemed to be buried there, beneath more than a kilometer of limestone. A volcanic origin for the structure prevailed, yet by 1980, thoughts began to turn to a catastrophic or explosive impact origin. It wasn't until the early 1990s that scientists from Mexico and the United States (including Dave Kring and PSRD's own editorial advisor, Buck Sharpton) presented the geological data for an impact origin as well as the age of the structure -- uppermost Cretaceous rocks, consistent with the K-T boundary deposits. The name Chicxulub comes from the Mayan village of the same name where the first exploratory well was drilled.

References are listed in the Additional Resources section at the end of the "Damage by Impact" article.
RETURN to "Damage by Impact", in PSRD December 1997 issue.

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