What really killed the dinosaurs?

What the phenomenon is

About 66 million years ago, at the boundary between the Cretaceous and Paleogene periods, most of life on Earth vanished within a geologically short span. NASA describes the toll as more than half of the planet's animal and plant species, on land and in the ocean, disappearing along with every dinosaur lineage except the ancestors of today's birds. Geologists call the event the Cretaceous-Paleogene extinction, or K-Pg extinction, and the evidence built up over the following decades points to one main trigger: a large asteroid striking what is now Mexico.

The popular claim

The story most people know is short: a giant space rock hit Earth and the dinosaurs died, end of story. That shorthand is not wrong, but it skips why the case took geologists three decades to prove, where the crater was hiding all along, and a separate volcanic catastrophe erupting in India at nearly the same time. The full picture is more interesting than the one-line version.

The clue in the clay: Alvarez and the iridium layer

In 1977, geologist Walter Alvarez was collecting limestone samples near Gubbio, Italy, when he noticed a thin band of clay separating rock laid down in the Cretaceous period from rock laid down afterward. He brought a sample to his father, physicist Luis Alvarez, who worked with nuclear chemists Frank Asaro and Helen Michel to analyze it using a technique called neutron activation analysis.

The clay turned out to be about 600 times richer in iridium than the surrounding limestone, a metal that is rare in Earth's crust but common in asteroids. The team proposed that the iridium had been delivered by an asteroid impact violent enough to end the Cretaceous period, publishing the idea in Science in 1980 under the title "Extraterrestrial Cause for the Cretaceous-Tertiary Extinction," credited to Luis Alvarez, Walter Alvarez, Asaro, and Michel.

Finding the crater

An impact large enough to end an era should have left a crater, and one was already sitting in an oil company's files. In 1990, researchers Kevin Pope, Adriana Ocampo, and Charles Duller spotted a huge, nearly perfect semicircular structure in Landsat satellite images of Mexico's Yucatán Peninsula while studying surface water and Maya archaeology. Pope later recalled, "We were working on a project on surface water and Mayan archaeology when we saw this perfect semi-circular structure."

The team traced a ring of natural sinkholes along the structure's edge, then obtained magnetic and gravity survey data that the Mexican oil company Pemex had collected while drilling in the area decades earlier. That data showed a buried circular structure consistent with an impact crater. Further analysis of magnetic, gravity, and sediment data by other researchers, published in 1991, persuaded the scientific community that this crater, named Chicxulub, was the impact site tied to the Cretaceous-Paleogene extinction.

Confirmation kept accumulating. In the March 5, 2010, issue of Science, an international panel of 41 experts led by Peter Schulte reviewed three decades of evidence and concluded that the Chicxulub impact was "the ultimate cause" of the mass extinction, a statement the panel described as closing a long-running scientific controversy.

How the impact triggered a mass extinction

NASA describes an asteroid more than 10 kilometers across striking the Yucatán platform hard enough to excavate a crater roughly 200 kilometers wide, centered near the town of Puerto Chicxulub. The collision blasted huge volumes of dust and gas into the atmosphere. The suspended dust blocked sunlight and cut off or sharply reduced photosynthesis worldwide, and with plant life collapsing at the base of the food chain, the disruption cascaded upward through the rest of the food web.

The volcanism debate

The asteroid was not the only catastrophe of that era. At roughly the same time, an enormous stretch of what is now western India was erupting as the Deccan Traps, one of the largest volcanic provinces on Earth, and researchers have long argued about how much that separate event contributed to the extinction.

A study led by Alessandro Chiarenza, done as a PhD researcher in Imperial College London's Department of Earth Science and Engineering with colleagues at the University of Bristol and University College London and published in June 2020, modeled the climate effects of both events on their own. Chiarenza concluded that "the only plausible explanation for the extinction is the impact winter that eradicated dinosaur habitats worldwide," since Deccan volcanism modeled alone would have left some regions near the equator climatically viable for dinosaurs. The same research found that the volcanism's warming afterward likely helped surviving plants and animals recover.

A 2024 clue to the impactor's origin

In August 2024, a team led by Mario Fischer-Gödde at the University of Cologne in Germany measured seven isotopes of ruthenium, a metal chemically similar to iridium, in samples from three Cretaceous-Paleogene boundary sites plus meteorites and older impact craters, using a technique called multicollector inductively coupled plasma mass spectrometry. The isotope pattern matched carbonaceous, carbon-rich asteroids that form far out in the solar system, beyond Jupiter, rather than the rockier asteroids typical of the inner solar system. Fischer-Gödde reported that 100 percent of the ruthenium in the boundary layer came from the impactor itself, in a study published in the journal Science.

The dinosaurs that survived

Birds are dinosaurs, and a handful of bird lineages made it through the same catastrophe that killed Tyrannosaurus rex and Triceratops. Research by Daniel Field at the University of Cambridge and Derek Larson at the University of Toronto found that the birds which survived tended to live on the ground or near water rather than in forests, which burned and were stripped bare, and had beaks suited to eating seeds that could still be found in the soil after other food sources vanished. Toothed, clawed, forest-adapted birds died out alongside their non-avian relatives.

Questions people ask

Verdict

Real or Legend has filed the Chicxulub asteroid impact as the confirmed main cause of the extinction that ended the age of non-avian dinosaurs: the iridium layer, the crater itself, and the 2024 isotope work identifying the impactor all point to the same event, about 66 million years ago. What is not fully settled is how much the Deccan Traps volcanism added to the damage beforehand, or subtracted from it by helping life recover afterward. The stamp covers the asteroid's role as the trigger, not a final word on every contributing factor.