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What evidence supports the asteroid-impact explanation for the Cretaceous-Paleogene extinction?

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Illustration showing various dinosaur species drawn to scale and grouped by geologic era: Late Cretaceous, Early Cretaceous, Jurassic, and Triassic. Dinosaurs range from large species like Brachiosaurus and Tyrannosaurus to smaller ones like Velociraptor and Compsognathus, with a human silhouette included for size comparison. Measurement bars at the bottom indicate 1 meter and 3 feet for scale reference. — dinosaur A selection of dinosaurs grouped by the geologic interval in which they lived.

The asteroid-impact hypothesis, often called the Alvarez theory, posits that a massive collision at the end of the Cretaceous Period triggered a global mass extinction event. Significant geological and physical evidence supports this explanation:12

Iridium Anomaly: A worldwide layer of clay at the Cretaceous-Paleogene (K–T) boundary contains abnormally high concentrations of iridium. Because iridium is rare in Earth’s crust but abundant in meteorites, this spike indicates an extraterrestrial impact.34
Chicxulub Crater: Scientists identified a massive, 180-km-wide impact crater buried beneath the Yucatán Peninsula. Geological samples from the site contain shocked quartz and tektites, which are characteristic of high-energy meteorite impacts.12
Environmental Consequences: The impact is believed to have ejected vast amounts of dust and sulfur into the atmosphere, blocking sunlight and halting photosynthesis. This led to a collapse of the food chain, global cooling, and acid rain, alongside evidence of widespread wildfires and tsunamis.123

While this theory is widely accepted, some paleontologists note that the fossil record for land-dwelling dinosaurs suggests a more gradual decline, potentially influenced by additional factors like volcanic activity and continental drift.13