Moment of impact: A journey into the Chicxulub Crater

When the Chicxulub asteroid slammed into Earth about 66 million years ago, it obliterated 80 percent of Earth's species, blasted out a crater 200 kilometers across, and signaled an abrupt end to the Cretaceous Period. The impact, its catastrophic effects, and its aftermath have engrossed scientists and the public alike since it was first discovered.

Artist’s impression of what the Chicxulub crater might have looked like soon after an asteroid struck the 
Yucatán Peninsula in Mexico. Researchers studied the peak rings, or circular hills, inside the crater
[Credit Detlev van Ravenswaay/Science Source]

In spring 2016, the International Ocean Discovery Program (IODP) and International Continental Scientific Drilling (ICDP) drilled into the Chicxulub crater off the coast of Mexico. The expedition targeted Chicxulub crater's peak ring and overlying rock sequences. Peak rings, as seen on the moon, form when rocks rebound into a peak inside the crater. The peak then collapses, leaving a center ring of rock within the larger crater. The expedition is helping answer important questions about the Chicxulub impact event and peak-ring crater formation on planetary bodies.

Highlights of IODP-ICDP Expedition 364 are featured in the October 2017 issue of GSA Today and discussed at the GSA Annual Meeting.

Each talk features findings from the 800-plus meters of core samples retrieved by the expedition. Topics include how the impact affected terrestrial and atmospheric chemistry, timing of events, and the Cretaceous-Paleogene mass extinction and subsequent recovery of life.

Melt rock from ~745 mbsf containing igneous clasts, dominated by granite 
[Credit: D.A. Kring et al. 2017]

Some highlights:

* David Kring explores the hydrothermal system triggered by the impact, including metasomatism (lithologic chemistry altered by hot fluids), and how the system could have incubated new life as it cooled over thousands of years post-impact.

* Joanna Morgan will share results of modeling to determine more accurately the amount of sulfur and CO2 shot into the atmosphere, including a second phase of gas release when ejecta was re-vaporized. Release of gas from the impact is connected to cooling of over 20 degrees C, with sub-freezing temperatures for three years.

* In large impacts, rocks from deep in the crust are brought upward into the crater. Drilling confirmed that the peak ring at Chicxulub consists of granite, brought up from mid-crustal depths. Sean Gulick takes us back to the first day of the Cenozoic, with a look at the granite and the overlying sequence of melt glass, breccia, and deposits from a seiche (an oscillation wave within a semi-enclosed or enclosed body of water) or tsunami.

* Finally, Christopher Lowery follows the recovery of life at ground zero, focusing on an 80-centimeter transitional layer and the limestone just above. These units record the devastation of the impact, trace fossils from surviving species, and fossils within the limestone revealing that within 30,000 years of impact, life inside the crater was back in full swing.

Source: Geological Society of America [October 23, 2017]