What Killed the Dinosaurs? The Chicxulub Impact
The Chicxulub impact, about 66 million years ago off the Yucatán coast, triggered the mass extinction that ended the non-avian dinosaurs and most other species.
The Chicxulub impact was the collision of an asteroid or comet, roughly 10 kilometres across by most estimates, with Earth about 66 million years ago. The site is the northern Yucatán Peninsula in Mexico. The strike and its aftermath triggered the Cretaceous–Paleogene extinction, which ended the non-avian dinosaurs and about three-quarters of all species. Birds were among the survivors, and most scientists regard the impact as the main cause, though not the only proposed factor.
Where is the Chicxulub crater?
The crater lies mostly beneath the northern Yucatán coast and the Gulf of Mexico, buried under younger sediment. Geophysicists working in the oil industry identified it in the late 1970s and early 1980s, using gravity and magnetic surveys that revealed a large ring of rock with unusual properties. Its diameter is usually given as about 180 kilometres, though estimates differ depending on which ring is measured. Drill cores from the crater recovered melted and shattered rock, which confirmed an impact origin.
Part of the crater's rim is marked at the surface by a ring of cenotes, the water-filled sinkholes of the limestone bedrock described in caves and karst landscapes. Their alignment was one of the clues that linked the buried structure to the impact.
How did the impact cause the extinction?
Estimates of the impact energy range from about 10²³ to more than 10²⁴ joules, and even the lowest is hundreds of thousands of times the energy of the largest nuclear weapon ever tested. The effects came in stages. The first minutes brought an intense heat pulse, shock waves and a very large earthquake. Debris was thrown high into the atmosphere and fell back across the globe as tiny glass spheres, now found in a thin layer of rock worldwide. Waves from the impact probably reached distant coasts as tsunamis.
The longer damage came from the atmosphere. Dust and soot blocked much of the sunlight. Sulphur from the sulphate rocks at the impact site may have formed aerosols that cooled the climate for years. With light and warmth reduced, photosynthesis collapsed on land and in the surface ocean, and the food chains built on plants and plankton failed. Marine reptiles and ammonites, which lived in the sea, also disappeared at the boundary. Ecosystems took millions of years to recover fully.
The table lists four lines of evidence.
| Evidence | What it shows |
|---|---|
| Iridium-rich clay layer | A thin layer at the boundary, found worldwide, rich in an element that is rare in Earth's crust |
| Shocked quartz | Quartz grains with features that form only under extreme pressure |
| Impact glass spherules | Tiny glass beads formed from molten rock thrown into the air |
| Crater age | The rock of the crater dates to the same time as the boundary layer |
Why did birds survive when other dinosaurs did not?
Birds descend from small theropod dinosaurs, and they were among the survivors. Researchers have proposed that small body size, seed-eating diets and the ability to live on stored food helped some lineages get through the darkness. These explanations are plausible, but the question is not settled, and the survival pattern across many groups is complicated. Mammals, lizards, amphibians and many plants also survived, so the extinction was selective rather than total.
Was the impact the only cause?
Not everyone agrees that the impact alone was decisive. The Deccan Traps in India, a vast series of lava flows, erupted around the same time and released gases that may also have cooled the planet. Many researchers think the volcanism added stress before the impact, while others think the impact did most of the damage. How much each mattered is still debated. How large eruptions release gas is described in volcanoes.
Radiometric dating of the crater and the boundary layer, as with radioactive decay clocks in general, is what allowed scientists to link the two events in time. The evidence that the impact and extinction coincide is strong, and it is the reason the impact remains the leading explanation.
Tags
dinosaurs earth science geology impact craters paleontology
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