Did an asteroid or a volcano end the dinosaurs?

The asteroid is the main suspect, but the volcano has not been fully cleared. The K–Pg extinction wiped out three-quarters of plant and animal species around 66 million years ago. Climate modelling in 2020 favoured the asteroid impact over volcanism, and it is now generally accepted that the volcanic eruptions of the Deccan Traps in India played a minor role at most, or may even have partially cancelled the impact's effects.

The asteroid idea began with a chemical clue. In 1980 Luis and Walter Alvarez and colleagues found that rock layers at the Cretaceous-Paleogene boundary all over the world contain iridium many times above normal. Iridium is extremely rare in Earth's crust, because it mostly sank into the core with iron, but it is more common in comets and asteroids.

Then came the objections and the crater. Many paleontologists rejected the idea because few fossils were found close to the boundary, which looked like a gradual die-off. Yet the fossil record is so incomplete that most extinct species probably died out long after their most recent known fossil. And the crater had already been spotted: oil-company geophysicist Glen Penfield saw a 180 km 'bullseye' in magnetic data off Yucatán, but his employer Pemex rejected the crater theory, preferring a volcanic explanation. The crater was identified in 1990 on the strength of his 1978 work, and its diameter, roughly 180 km, matched the size the Alvarez team had calculated.

The volcanoes keep their defenders. The Deccan Traps are layers of flood basalt with a volume of about 1 million cubic kilometres, erupted over about 800,000 years spanning the boundary. Gerta Keller is a leading proponent of Deccan causation, and at Seymour Island in Antarctica extinction came in two pulses that match both a volcanic and an impact warming.