Cretaceous–Paleogene (K–Pg) Extinction

Mass extinction 66 million years ago triggered by the Chicxulub asteroid impact, ending the non-avian dinosaurs.

Specifications & Metrics

type: Standard Stratigraphic Chronology
timespan: 66.043 ± 0.011 Ma (Mesozoic–Cenozoic boundary)
defining boundary: Global iridium-enriched clay boundary layer at El Kef GSSP, Tunisia
proponents or Body: Luis & Walter Alvarez, Frank Asaro & Helen Michel (1980); Alan Hildebrand (Chicxulub, 1991)
key evidence: Boundary iridium anomaly, shocked quartz, spherules, 180-km Chicxulub crater in Yucatán

In 1980, Luis and Walter Alvarez, Frank Asaro, and Helen Michel discovered that the thin clay layer at the Cretaceous–Paleogene boundary (66 Ma) in Gubbio, Italy, and worldwide is enriched 30- to 160-fold in iridium, an element rare in Earth's crust but abundant in chondritic asteroids.
In 1991, the impact site was identified as the 180-kilometer-wide Chicxulub crater buried beneath carbonate platform rocks and a semicircular ring of karst cenotes on Mexico's Yucatán Peninsula, excavated by a 10- to 15-km asteroid.
Vaporized sulfate evaporites and ejecta plunged Earth into an impact winter that halted photosynthesis and eliminated ~75% of all species — including all non-avian dinosaurs, pterosaurs, mosasaurs, plesiosaurs, and ammonites — clearing ecological niches for Cenozoic mammal radiation.