What killed the dinosaurs? The asteroid and the evidence

What killed the dinosaurs was an asteroid roughly 10 kilometers wide that struck the Yucatán Peninsula, in what is now Mexico, about 66 million years ago. The impact threw up fire, dust and soot that darkened and cooled the planet, and about three-quarters of species died out. Most scientists see the impact as the main cause. Huge volcanic eruptions in India may have added stress, and how much is still argued.

In short

• The case rests on a chain of evidence: a worldwide layer rich in iridium, shocked minerals and debris, and a buried crater of the right age.
• The killing was done mostly by the aftermath: darkness and cold that collapsed food chains on land and in the sea.
• Not all dinosaurs died: birds are their direct descendants, and they survived.

How do scientists know an asteroid hit?

The answer was assembled in steps over about 30 years, each finding narrowing the possibilities.

Year	Finding
1980	Luis and Walter Alvarez, with Frank Asaro and Helen Michel, report that a thin clay layer at the end of the Cretaceous holds far more iridium than the rocks around it, in Italy and Denmark, and propose an asteroid impact
1980s	The same iridium layer turns up at dozens of sites worldwide, along with shocked quartz, grains that only extreme pressure creates
1991	Alan Hildebrand and colleagues identify the buried Chicxulub crater in Yucatán as the impact site
2010	An international panel of 41 scientists reviews the evidence and concludes the impact caused the extinction
2016	A drilling expedition recovers rock from the crater's inner ring, showing how it formed
2019–2020	Competing studies date the Indian eruptions and model their gases, reopening and then narrowing the volcano question

Iridium is a metal that is rare in Earth's crust but common in many asteroids, so a sudden global spike pointed to something arriving from space. As the paleontologist Michael Benton explains in The Conversation, the debris layer is about 1 centimeter thick in Italy and Denmark but about 1 meter thick in Texas and Haiti, which pointed toward an impact somewhere around the Gulf of Mexico. Hildebrand found the crater in data from oil-exploration boreholes drilled decades earlier. It is about 180 to 200 kilometers across and 20 kilometers deep, half under the Yucatán's forest and half under the sea, and its age matches the extinction layer.

How big was the impact?

A rough calculation shows why one rock could change the planet. Assume an asteroid 10 kilometers across, made of stony material with a density of about 2,600 kilograms per cubic meter, hitting at 20 kilometers a second, a typical speed for an asteroid.

1. Volume: a sphere 10 km across has a volume of about 524 cubic kilometers, or 5.2 × 10¹¹ cubic meters.
2. Mass: 5.2 × 10¹¹ × 2,600 ≈ 1.4 × 10¹⁵ kilograms, about 1.4 trillion tonnes.
3. Energy: half the mass times the speed squared, ½ × 1.4 × 10¹⁵ × (20,000)², comes to about 2.7 × 10²³ joules.
4. Comparison: one megaton of TNT is 4.2 × 10¹⁵ joules, so the impact released about 65 million megatons, more than a million times the largest of all nuclear weapons ever tested, the Soviet Union's 50-megaton bomb of 1961.

Change the assumptions, a faster or larger asteroid, and the answer moves by a few times either way. Published estimates vary for that reason, but all land in the same range.

What did the impact do to the planet?

The blast itself devastated the region: an earthquake far beyond any recorded, tsunamis across the Gulf, and a shock wave and heat pulse. The worldwide killing came from what followed. Molten and vaporized rock was flung above the atmosphere and fell back around the globe, heating the air, and the Natural History Museum in London notes evidence of substantial fires from that time. The asteroid also hit rock rich in sulfur and carbon, releasing gases that formed a haze of sulfate droplets.

Soot, dust and sulfate blocked so much sunlight that photosynthesis largely stopped for months, and the surface cooled sharply in what scientists call an impact winter. Plants and plankton died, then the animals that ate them, then the animals that ate those. Earth's climate has swung many times over its history, as during ice ages, but over thousands of years. This happened within months, too fast for most species to adapt or move. A 2022 study of fossil fish bones at Tanis, a site in North Dakota, suggests the impact came in the Northern Hemisphere's spring.

Which animals died, and which survived?

All the large land animals vanished, including every non-bird dinosaur, along with pterosaurs, the great marine reptiles such as mosasaurs and plesiosaurs, and the ammonites, shelled relatives of squid. In the oceans, many groups of plankton collapsed. About 75% of species disappeared.

Survivors tended to be small, to eat a varied diet, or to live in water, burrows or leaf litter, where food built on dead material lasted longer than food built on living plants. Many mammals, lizards, turtles, crocodiles, frogs and fish came through. Birds are dinosaurs: they descend from small feathered theropods, the same group as Tyrannosaurus, and some of their lineages survived. Scientists now say "non-avian dinosaurs" died out, because the avian ones are still here.

With the large reptiles gone, mammals spread into the empty niches over the next 10 million years, giving rise eventually to whales, horses, bats and primates. It is one of the clearest examples of how evolution responds to sudden change: not by planning, but by whichever survivors happen to fit the new world.

What about the volcanoes in India?

At around the same time, the Deccan Traps in western India were erupting, among the largest volcanic outpourings in Earth's history. Over hundreds of thousands of years they released carbon dioxide and sulfur gases that warmed and stressed the climate. Some researchers have argued that the volcanoes were a major cause of the extinction, or that the ecosystems they weakened were primed to collapse when the asteroid hit.

Still argued

The impact's role is settled; the volcanoes' share is not. In February 2019, two teams dated the Deccan lavas in the same issue of Science and reached different emphases. A Berkeley-led team, dating crystals in the lava itself, found that about three-quarters of it erupted after the impact; a Princeton-led team, dating crystals in sediment between the flows, put more of the eruptions earlier, as ScienceDaily reported. In January 2020, a Yale-led team modeled the gases and concluded that most were released well before the impact, so the asteroid was the main driver of the extinction (Yale News). Some geologists still argue the volcanoes did much of the damage. What the evidence shows is that the extinction lines up sharply with the impact layer; what it doesn't settle is how much the volcanoes had weakened life beforehand.

Why does this matter now?

The dinosaur extinction is a worked case of how science settles a big question: a strange measurement, a bold hypothesis, predictions that could have failed, and a crater found a decade later. It also showed that rare impacts shape life's history, which is why space agencies now track near-Earth asteroids. In 2022, NASA's DART spacecraft struck a small asteroid and changed its orbit, the first test of deflecting one.

Check yourself

• Without looking back, list the evidence that links the extinction to an asteroid, in the order it was found.
• How did an impact in Mexico kill animals on the other side of the world?
• Why do scientists say non-avian dinosaurs went extinct, rather than all dinosaurs?

Sources

• How an asteroid ended the age of the dinosaurs — Natural History Museum, London
• How does an invisible underwater crater prove an asteroid killed the dinosaurs? — Michael J. Benton, The Conversation, 2016
• Do volcanoes or an asteroid deserve blame for dinosaur extinction? — University of California, Berkeley, via ScienceDaily, 2019
• In death of dinosaurs, it was all about the asteroid, not volcanoes — Yale News, 2020
• Cretaceous–Paleogene extinction event — Wikipedia