Yucatan Crater Linked To Mass Extinctions Of Dinosaurs

This report on Mexico's Chicxulub crater will be presented by Dr. Gail Christeson, a research associate at UT Austin's Institute for Geophysics (UTIG), at the fall meeting of the American Geophysical Union in San Francisco. UT Austin's involvement in the project was sponsored by the National Science Foundation.

The Chicxulub structure was formed 65 million years ago when a large celestial body -- a comet or an asteroid -- slammed into the Yucatan Peninsula with a force that makes a nuclear blast seem like a firecracker.

The impact produced fires, acid rain and tsunami-like destructive waves. The collision gouged a crater nearly eight miles deep and sent 12,000 cubic miles of rock, dirt and debris spinning into the earth's atmosphere. The material blocked the sun, causing extreme changes in the Earth's climate, which many scientists believe resulted in mass extinctions.

The collision marked the abrupt end of the Cretaceous period in geologic time and the start of the Tertiary period. And many scientists currently believe that the event wiped out 80 percent of all living species in the ocean. It also may have destroyed many terrestrial species, including the dinosaurs.

Christeson and UTIG senior research scientists, including Dr. Richard T. Buffler and Dr. Yosio Nakamura, worked with an international team of scientists to survey the Chicxulub crater, which remains as an unusual circular feature buried beneath 1,000 meters of sediments under the northern Yucatan Peninsula and the Gulf of Mexico. Co-authors on Christeson's presentation are Jo Morgan and Mike Warner from Imperial College in London, and Colin Zelt from Rice University in Houston.

The aim of the researchers was to determine the Chicxulub crater's actual size and to characterize its internal structure. Such details should make it easier for scientists to understand how the crash actually could have caused mass extinctions. It should also allow them to assess the present-day risk posed by the thousands of comets and asteroids that cross earth's orbit.

The team collected seismic reflection, refraction, gravity and magnetic data over the crater. This research has provided the first direct evidence of a crater with the multi-ring basin shape that is typical of the largest impact craters on the moon and Venus.

The impact was so enormous it changed the shape of the earth's crust -- 22 miles below the surface of the planet. The Chicxulub crater is the first location where deformation at the base of the crust has been found in a terrestrial impact crater.

The scientific team concluded that the Chicxulub crater is about 125 miles in diameter, and that 12,000 cubic miles of debris was blasted out of the earth by the impact. The impact carved out a cavity about 7.5 miles below sea level. Mount Everest, in comparison, is 5.5 miles high. Prior to this research, the size and morphology of the Chicxulub crater had been in dispute, with estimates of its diameter ranging between 180 and 300 kilometers.

Such a large discrepancy in size translates to a factor of ten differences in the energy of the impact with quite different consequences for the Earth's environment. The energy released by the impact that blew out the Chicxulub crater was equivalent to about 100 million megatons, many orders of magnitude greater than the nuclear explosion at Hiroshima, a 15-kiloton blast.

To collect the seismic data in the Gulf of Mexico, the scientists deployed an array of Ocean Bottom Seismograph (OBS), instruments which had been developed at UT Austin's Institute for Geophysics for undersea projects such as this one. The OBS instruments were deployed from the UT Austin Marine Science Institute vessel RV Longhorn based out of Port Aransas.

Additional analysis of the OBS data revealed that a region at the center of the crater about 22 miles in diameter has been uplifted by about 11 miles as a result of the impact and removal of overlying material.

During the AGU meeting, Christeson will be a panelist at a press conference organized by AGU on large impact events.

For more information, click on "research" and "OBS" at the Website: http://www.ig.utexas.edu