What killed the dinosaurs?
Most scientists agree that the main cause was an asteroid about 10 km across that struck Mexico's Yucatán Peninsula 66 million years ago, creating the Chicxulub crater; massive volcanic eruptions in India may also have played a part.
About 66 million years ago, all dinosaurs except birds disappeared, along with the great marine reptiles, the ammonites and around three-quarters of all species. For a long time the cause was a mystery. The key clue came in 1980, when the physicist Luis Alvarez, his geologist son Walter and their colleagues reported that a thin clay layer marking the end of the Cretaceous period was unusually rich in iridium, a metal rare in Earth's crust but more common in meteorites. They proposed that a large asteroid had struck Earth.
The impact site was confirmed in the early 1990s: the Chicxulub crater, about 180 km across, buried beneath the Yucatán Peninsula and the Gulf of Mexico. Rocks around the world from the same moment contain shocked quartz grains, tiny glass spherules and debris from tsunamis, all signs of a colossal impact. Dating shows that the crater and the extinction coincide closely.
The asteroid, roughly 10 km wide, struck with energy far beyond any nuclear arsenal. It threw vast amounts of rock into the atmosphere, set off fires, and released dust, soot and sulfur compounds that blocked sunlight for years. Temperatures fell, plants struggled to photosynthesise, and food chains collapsed on land and in the oceans. Small animals that could shelter, eat seeds or scavenge, including early mammals and birds, were more likely to survive.
Many researchers also point to the Deccan Traps, enormous volcanic eruptions in what is now India that were active around the same time and may have stressed ecosystems beforehand. How much each contributed is still debated, but the impact is widely regarded as the decisive blow. Birds, which are living dinosaurs, came through and remind us that the story did not end completely.