Chicxulub Asteroid Impact: How the Dinosaurs Were Wiped Out

The Chicxulub asteroid impact was the collision of an asteroid roughly 10 kilometres wide with Earth about 66 million years ago, at what is now the Yucatán Peninsula in Mexico. It triggered one of the largest mass extinctions in the planet’s history, wiping out the non-avian dinosaurs and around three-quarters of all plant and animal species. The event marks the boundary between the Cretaceous and Paleogene periods and opened the way for the age of mammals.

Background: Earth at the end of the Cretaceous

At the end of the Cretaceous Period, dinosaurs had dominated life on land for well over 100 million years. Familiar animals such as Tyrannosaurus and Triceratops lived in North America, while marine reptiles such as mosasaurs hunted in the oceans and pterosaurs flew overhead.

The climate was generally warmer than today, and sea levels were high. Small mammals already existed, but most were modest in size. Flowering plants had spread widely, and the seas were full of plankton, ammonites and fish.

In what is now India, a long series of huge volcanic eruptions, called the Deccan Traps, was pouring out lava and gases during the same period. The eruptions continued for hundreds of thousands of years on either side of the impact, which is one reason their role in the extinction is still discussed.

Earth had suffered mass extinctions before. The largest, the Permian–Triassic extinction about 252 million years ago, is linked to massive volcanism rather than an impact. The end-Cretaceous extinction is the only one firmly tied to a collision from space.

What happened

The asteroid struck a shallow tropical sea on the edge of the Yucatán Peninsula, travelling at a speed usually estimated at around 20 kilometres per second. The rocks there were rich in carbonates and sulphur-bearing minerals, which turned out to matter a great deal. The energy released is commonly estimated to have been billions of times greater than that of the atomic bomb dropped on Hiroshima.

The impact excavated a crater about 180 kilometres (110 miles) wide. Today it lies buried under hundreds of metres of younger sediment, with its centre near the town of Chicxulub Puerto, which gave the crater its name. Molten rock was blasted into space and fell back around the world as tiny glassy spheres.

In the hours that followed, the impact generated enormous tsunamis across the Gulf of Mexico and beyond, along with powerful earthquakes and intense heat from falling debris. Some researchers argue that this heat ignited widespread wildfires, although the scale of the fires is still debated.

The impact winter

The deadliest effects came later. Soot, dust and sulphur compounds thrown into the upper atmosphere blocked sunlight for months to years. Photosynthesis on land and in the oceans collapsed, and global temperatures fell sharply in what scientists call an “impact winter”. Some climate models suggest that average surface temperatures dropped by well over 10°C for several years, although estimates differ.

Food chains broke down from the bottom up. Large animals that needed a lot of food were especially vulnerable. Species that were small, could burrow, ate seeds or detritus, or lived in fresh water were more likely to survive. Sulphur and carbon released from the target rocks may also have acidified the surface ocean, adding to the losses among shelled marine life.

How scientists found the evidence

In 1980 the physicist Luis Alvarez, his son the geologist Walter Alvarez, and the chemists Frank Asaro and Helen Michel published a landmark paper. They had found unusually high levels of iridium, an element rare in Earth’s crust but common in asteroids, in a thin clay layer at the Cretaceous–Paleogene boundary near Gubbio, Italy. Similar enrichment appeared at Stevns Klint in Denmark and, soon after, at sites around the world. They proposed that a giant impact caused the extinction.

The idea was controversial at first. Many palaeontologists believed the dinosaurs had declined gradually because of climate change or volcanism, and for a decade the main objection was simple: where was the crater?

The crater itself had already been noticed. In 1978 the geophysicist Glen Penfield, working with Antonio Camargo for the Mexican state oil company Pemex, identified a huge buried circular structure in survey data, and they presented it in 1981. In 1991 Alan Hildebrand and colleagues published evidence linking it to the boundary layer, including shocked quartz and impact glass.

Further studies dated the impact precisely to about 66 million years ago. In 2010 an international panel of 41 scientists reviewed the evidence in the journal Science and concluded that the impact was the cause of the mass extinction. In 2016 a joint drilling expedition recovered rock cores from the crater’s “peak ring”, a ring of mountains formed within minutes of the impact. A 2024 study of ruthenium isotopes in the boundary layer suggested that the impactor was a carbon-rich asteroid that formed in the outer Solar System.

By the numbers

Key people

Luis Alvarez (1911–1988) was an American physicist who had won the Nobel Prize in Physics in 1968 for his work on subatomic particles. His expertise in measurement helped turn an unusual geological sample into a testable theory about the history of life.

Walter Alvarez (born 1940), a geologist, was studying the limestone layers near Gubbio when he brought the boundary clay to his father’s attention. He later wrote the popular book T. rex and the Crater of Doom (1997) about the search for the impact site.

Frank Asaro and Helen Michel, nuclear chemists at the Lawrence Berkeley Laboratory, carried out the precise measurements that revealed the iridium anomaly. Their analysis made the 1980 paper possible.

Glen Penfield and Antonio Camargo recognised the buried structure while surveying for oil, but their findings, presented at a 1981 conference, attracted little attention at the time because much of the data belonged to Pemex.

Alan Hildebrand, then a graduate student at the University of Arizona, tracked down the crater by following clues in boundary deposits around the Caribbean. His 1991 paper with Penfield and others firmly connected Chicxulub to the extinction.

Timeline

• c. 66 million years ago: An asteroid strikes the Yucatán area, ending the Cretaceous Period.
• 1978: Glen Penfield and Antonio Camargo identify a large buried circular structure in Pemex survey data.
• 1980, June: The Alvarez team publishes its iridium evidence and impact hypothesis in Science.
• 1981: Penfield and Camargo present the Yucatán structure as a possible impact crater.
• 1991: Alan Hildebrand and colleagues link the Chicxulub crater to the extinction boundary.
• 2010, March: A review by 41 scientists in Science concludes the impact caused the extinction.
• 2016, April–May: A scientific drilling expedition samples the crater’s peak ring.

Aftermath and recovery

With the large dinosaurs gone, mammals diversified rapidly over the following millions of years, eventually giving rise to whales, horses, bats and primates. Birds, the only surviving dinosaur lineage, also spread into many new niches. Feathered fossils such as Sinosauropteryx helped confirm the close link between birds and other dinosaurs.

The extinction was uneven. Ammonites, the coiled-shell relatives of squid, disappeared entirely, as did most marine reptiles and the pterosaurs. Crocodilians, turtles, many fish, amphibians and some groups of insects came through with lighter losses. Fossil spores show that ferns briefly dominated many landscapes after the impact, a “fern spike” typical of ground recovering from disaster.

Scientists continue to debate the role of the Deccan Traps volcanism. Most now see the impact as the decisive blow, with volcanic activity possibly adding stress to ecosystems before and after the event. Like later volcanic disasters such as the eruption of Krakatoa in 1883, the impact showed how material injected into the atmosphere can alter the global climate, though on a vastly greater scale.

Myths and facts

Myth: all the dinosaurs died out. Birds are living dinosaurs, descended from small feathered theropods. Only the non-avian dinosaurs became extinct.

Myth: the dinosaurs died in a single day. Animals near the impact died within minutes or hours, but most species were killed by the collapse of food chains during the months and years of darkness and cold that followed.

Myth: pterosaurs, plesiosaurs and mosasaurs were dinosaurs. They were reptiles that lived alongside dinosaurs and died out at the same time, but scientists classify them as separate groups.

Myth: early humans lived alongside dinosaurs. The non-avian dinosaurs vanished about 66 million years before modern humans appeared, roughly 300,000 years ago.

Why it matters

The Chicxulub impact reshaped life on Earth and made the later rise of mammals, including humans, possible. It also changed science itself, showing that sudden events from space can drive major turning points in the history of life. Today it informs research into planetary defence and the monitoring of near-Earth asteroids. In 2022 NASA’s DART spacecraft deliberately struck a small asteroid and changed its orbit, the first test of a technique that could one day deflect a threatening object.