About 66 million years ago, a major extinction event ended the age of nonavian dinosaurs and reshaped life on Earth. This event is called the K-Pg extinction because it marks the boundary between the Cretaceous and Paleogene periods. Evidence from rocks, fossils, and geochemistry points to a large asteroid impact near present-day Chicxulub on the Yucatán Peninsula.
Understanding this event helps scientists connect Earth science, biology, chemistry, and astronomy in one major historical case study.
The impact released enormous energy, blasted rock into the atmosphere, triggered fires and tsunamis, and spread dust and sulfate aerosols around the globe. These particles blocked sunlight, cooled the surface, and disrupted photosynthesis, causing food webs to collapse. A thin clay layer rich in iridium appears worldwide at the K-Pg boundary and acts like a global timestamp of the catastrophe.
Paleontologists study fossils above and below this layer to measure extinction, survival, and recovery.
Key Facts
- The K-Pg extinction occurred about 66 million years ago and eliminated roughly 75% of known species.
- The Chicxulub impact crater is about 180 km in diameter and lies partly under the Yucatán Peninsula and the Gulf of Mexico.
- Impact energy can be estimated with KE = 1/2 mv^2, where m is asteroid mass and v is impact speed.
- The K-Pg boundary layer contains unusually high iridium, an element rare in Earth’s crust but more common in many asteroids.
- Dust, soot, and sulfate aerosols reduced sunlight, lowering photosynthesis and weakening food chains on land and in the oceans.
- Birds are surviving theropod dinosaurs, so the extinction ended nonavian dinosaurs but not the entire dinosaur lineage.
Vocabulary
- K-Pg boundary
- The rock layer marking the transition from the Cretaceous Period to the Paleogene Period about 66 million years ago.
- Chicxulub crater
- The large buried impact crater linked to the asteroid strike that helped cause the K-Pg extinction.
- Iridium anomaly
- An unusually high concentration of iridium in the K-Pg boundary layer that supports an extraterrestrial impact origin.
- Ejecta
- Rock, dust, glass droplets, and other material thrown out from an impact crater during a collision.
- Mass extinction
- A rapid global loss of many species across different environments in a relatively short geologic time.
Common Mistakes to Avoid
- Saying all dinosaurs went extinct is wrong because birds are living descendants of theropod dinosaurs.
- Treating the asteroid impact as only a local disaster is wrong because ejecta, aerosols, climate cooling, and food web collapse affected the entire planet.
- Assuming one fossil layer proves the full extinction pattern is wrong because paleontologists compare many sites and fossil groups to see global trends.
- Confusing the K-Pg extinction with the end-Permian extinction is wrong because they happened at different times and involved different causes and affected groups.
Practice Questions
- 1 An asteroid has a mass of 1.0 x 10^15 kg and strikes Earth at 2.0 x 10^4 m/s. Use KE = 1/2 mv^2 to calculate its kinetic energy in joules.
- 2 A sediment core contains the K-Pg boundary at a depth of 48 m. If sediment above it accumulated at an average rate of 0.75 mm per 1,000 years, estimate the time represented by the 48 m of sediment above the boundary.
- 3 Explain why a thin iridium-rich clay layer found on several continents is stronger evidence for a global event than the same layer found at only one fossil site.