The evidence for a large asteroid impact causing mass extinction, particularly the one associated with the end of the Cretaceous period around 66 million years ago (which is famously linked to the extinction of the dinosaurs), includes several key lines of evidence:
-
Iridium Anomaly: One of the most compelling pieces of evidence is the discovery of a layer of sediment rich in iridium, a rare element that is more abundant in asteroids than in the Earth's crust. This layer, often referred to as the K-Pg (Cretaceous-Paleogene) boundary layer, can be found in various locations around the world, indicating a global event. The presence of this iridium layer supports the hypothesis that an asteroid impact occurred.
-
Fossil Evidence: The fossil record shows a sharp decline in biodiversity at the K-Pg boundary. This rapid extinction event is observed in the fossilized remains of many species, particularly large dinosaurs and various marine organisms. The abrupt nature of this extinction is consistent with a sudden and catastrophic event, such as an asteroid impact.
-
Impact Crater: The discovery of the Chicxulub crater in the Yucatán Peninsula of Mexico, which is approximately 150 kilometers (93 miles) in diameter, provides direct physical evidence of a massive impact event. Geological studies of the crater indicate that it matches the timing of the mass extinction.
-
Shock Metamorphosed Materials: The analysis of rocks from the impact site shows evidence of shock metamorphism, which occurs under extremely high pressures and temperatures typical of an asteroid impact. Materials like shocked quartz have been found in the boundary layers.
-
Carbon Dating: While carbon dating is not typically applied to rocks of this age (as it is most effective for dating organic materials up to about 50,000 years old), scientists use other radiometric dating methods (e.g., uranium-lead dating) to establish the age of the layers surrounding the K-Pg boundary and confirm the synchrony of the extinction event with the impact.
In summary, the evidence supporting the asteroid impact hypothesis for mass extinction is robust, combining chemical, geological, paleontological, and geophysical data.