The evidence supporting the idea that a large asteroid impact caused a mass extinction, particularly the one that occurred around 66 million years ago marking the end of the Cretaceous period, includes the following key points:
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Iridium Anomaly: One of the strongest pieces of evidence is the discovery of iridium, an element that is relatively rare on Earth’s surface but more common in asteroids and comets. A layer of clay rich in iridium has been found in various locations around the world, precisely at the geological boundary marking the end of the Cretaceous. This layer is known as the Cretaceous-Paleogene (K-Pg) boundary, and the global distribution of this layer supports the idea of a massive extraterrestrial impact.
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Fossil Evidence: The fossil record shows a significant and abrupt loss of species at the K-Pg boundary, with many taxa, particularly dinosaurs, going extinct. The suddenness of this extinction event aligns with the idea of a catastrophic event, such as an asteroid impact, rather than a gradual process.
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Impact Crater: The Chicxulub crater on the Yucatán Peninsula in Mexico is believed to be the impact site of the asteroid that caused the K-Pg extinction event. This impact structure is about 150 kilometers (93 miles) in diameter and dates to the same time as the mass extinction. Geological studies of the crater provide additional evidence of the impact's size and potential consequences, such as fires, tsunamis, and a "nuclear winter" scenario due to debris blocking sunlight.
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Shock Metamorphism: In the vicinity of the Chicxulub crater, scientists have found minerals that have been altered by the high pressures and temperatures associated with asteroid impacts. This includes evidence of shock diamond formations and other geological changes that indicate an explosive event.
In summary, the combination of the iridium layer, fossil evidence showing mass extinction patterns, the Chicxulub impact crater, and findings of shock-metamorphosed minerals collectively supports the hypothesis that a large asteroid impact played a critical role in causing mass extinction on Earth.