Media Advisory 5: Fall Meeting - Shoemaker Lecture: Issues Surrounding The Chicxulub Impact

December 01, 1998

WASHINGTON, D.C. -- The Shoemaker Lecture at the American Geophysical Union's Fall Meeting honors Eugene M. Shoemaker. Dr. Susan W. Kieffer of Kieffer & Woo in Palgrave, Ontario, will deliver the 1998 Shoemaker Lecture. She will discuss the impact 65 million years ago that created a large crater at Chicxulub in Mexico's Yucatan Peninsula.

The Chicxulub impact crater was enormous. Material was ejected across a diameter of about 100 kilometers [60 miles], and the walls of that cavity collapsed to produce a final crater nearly 200 kilometers [120 miles] in diameter.

At the time of the impact, Kieffer notes, the conditions at Chicxulub were among the most complicated possible on the earth. A shallow layer of water covered the Yucatan platform, which was composed of carbonates and sulfates that were several kilometers thick. Beneath this carbonate-sulfate pile lay the silicate rock of the Earth's crust.

Not only were the conditions at Chicxulub chemically and mechanically complicated, Kieffer says, but they were also geometrically complicated. The center of the impact was only a few hundred kilometers from steep escarpments that drop 3,000 meters [1.8 miles] from the carbonate platform into the depths of the Gulf of Mexico and the northern Caribbean.

And, she adds, complicating all of this is the problem that we do not know the nature of the impacting body: was it a comet or an asteroid? Was it rocky or icy, dense or porous?

During her lecture on December 9, Kieffer will discuss an intriguing array of impact phenomena possible in this complicated setting. These include the hot fireball of vaporized meteorite and target rock that spread around the globe--the unusual amount of iridium in a layer that we now know came from the impact. There was also a cooler fireball of vaporized carbonate, sulfate, and water that carried shocked quartz far aloft. As it drifted back down, the rotation of the Earth caused the quartz to be deposited primarily to the west of Chicxulub.

Debris that landed closer to the crater scoured the carbonate platform--analogous to the way the lateral blast at Mount St. Helens scraped the forest and soils from the devastated area. It is possible that a layer of this debris as thick as 10-100 meters [30-300 feet] reached the Campeche Escarpment to the northwest of Chicxulub, pouring over it like a giant landslide into the Gulf of Mexico. Evidence for tsunamis generated during this complicated impact has been found around the Gulf of Mexico.

Eugene Shoemaker inspired many scientists into full-time careers of research in impact cratering mechanics, Kieffer says. He trained astronauts to explore the lunar surface, and for many decades he conveyed his knowledge and enthusiasm about meteorites to the general public.

Kieffer recalls that when she was a graduate student, frequently discouraged by the complexity of geological problems, Shoemaker would rub his hands in glee and cheerfully say "THAT is the beauty of geology--it's so wonderfully complicated!" A discussion of the complexity of the Chicxulub impact is, she feels, a fitting tribute to this remarkable geologist.
The Shoemaker Lecture, "Physical and Chemical Processes During the Impact at Chicxulub," will be delivered by Dr. Susan W. Kieffer on Wednesday, December 9, at 4:00 PM, during AGU's Fall Meeting. The venue is Room 103 of the Moscone Convention Center in San Francisco, Calif. The session is designated P32D. The Shoemaker Lecture is open to media representatives who have registered for Fall Meeting.

A list of all special lectures at Fall Meeting will be found in Media Advisory 2 of October 27 (

American Geophysical Union

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