About 66 million years ago, a rock roughly 10 to 15 kilometers wide hit what is now the Yucatán Peninsula in Mexico. Most scientists today regard that impact as the main cause of the extinction that ended the age of the non-bird dinosaurs. This article is about the evidence: how it was found, what the crater itself reveals, and which questions are still open. For a size comparison with Apophis, see our Apophis vs. the dinosaur asteroid article.
The story starts with a rock layer. Geologists call the boundary between the Cretaceous and Paleogene periods the K-Pg boundary. In many places it appears as a thin band of clay. In 1980, a team led by physicist Luis Alvarez and geologist Walter Alvarez reported that this clay, studied in Italy and elsewhere, contained far more iridium than the rocks above and below it. Iridium is rare in Earth's crust but more common in asteroids and meteorites.
Their proposal was that a large asteroid had delivered the iridium and that the impact had caused the extinction. Since then, the same iridium-rich layer has been found at hundreds of sites on land and on the sea floor, which is why it is treated as a global marker of one event.
Iridium alone could in principle have other sources, so scientists looked for impact-specific clues. Two of them turned up in the same layer:
The layer is thickest near the Gulf of Mexico, which pointed investigators toward the Caribbean region as the place to look for the crater.
The crater had in fact been seen before it was understood. In the 1970s, geophysicists Glen Penfield and Antonio Camargo, working for the Mexican oil company Pemex, noticed a huge circular pattern in gravity and magnetic surveys of the Yucatán. Because the structure lies under younger limestone, it is not visible as a bowl on the surface. In 1991, a paper led by Alan Hildebrand, with Penfield and Camargo among the authors, connected the buried structure to the K-Pg boundary and presented evidence such as shocked quartz in its rocks.
Today the Chicxulub crater is estimated to be about 180 kilometers across. Dating of melted rock from the site gives an age of about 66 million years, matching the boundary layer. Some of the cenotes (natural sinkholes) of the Yucatán roughly trace the crater's rim, a surface hint of the structure below.
In spring 2016, an international expedition (IODP-ICDP Expedition 364) drilled into the crater's peak ring, a circle of hills inside the main basin, from a platform in the Gulf of Mexico. The team recovered a nearly continuous core from roughly 500 to 1,300 meters below the sea floor.
The cores showed that granite from deep in the crust had been pushed up and outward within minutes of the impact, which supported a long-debated model of how peak rings form. On top of that rock lay layers of impact melt and debris, and above them sediment that records the first hours, days and later years after the event. Researchers who studied these layers have described a sequence in which seawater rushed back into the crater and deposits built up quickly, all within about a day. Chicxulub is the only large crater on Earth with a well-preserved peak ring, which makes it unusually valuable for understanding impacts on other planets as well.
An asteroid impact does not kill everything directly. The widely discussed chain of effects looks like this:
This pattern fits the fossil record: about three quarters of plant and animal species disappeared, while small mammals, some birds (the living dinosaurs) and many other groups survived.
At about the same time, huge lava eruptions were building the Deccan Traps in India, and some scientists argue they played a major role. The dating is still debated. A 2020 study in Science led by Pincelli Hull compared deep-sea temperature records with models of both causes and concluded that the impact was the main driver, with the main volcanic warming happening earlier. Other researchers, using radiometric dates of the lava flows, have argued that volcanism overlapped the extinction and may have added stress. Most researchers see the impact as essential; how much the eruptions contributed is the open part of the question.
Chicxulub is the extreme end of the impact scale. Objects that large are extremely rare, and astronomers believe they have found nearly all near-Earth asteroids over 1 kilometer (see near-Earth objects explained). None is known to be on a collision course. The more realistic hazards are smaller bodies like the ones behind the Tunguska and Chelyabinsk events, and that is why surveys and tests such as NASA's DART mission concentrate on them. See also our overview of planetary defense.
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