A massive asteroid struck what is now Mexico’s Yucatan Peninsula about 66 million years ago, wiping out roughly 75 percent of all animal and plant species on Earth, including every non-avian dinosaur. The impact released more energy than a billion nuclear weapons and set off a chain of catastrophic events that made the planet uninhabitable for large land animals. While the asteroid was the primary killer, it didn’t act alone. Enormous volcanic eruptions in India were already stressing ecosystems, and the combination proved unsurvivable for the dinosaurs.
The Asteroid That Changed Everything
The impactor, known as the Chicxulub asteroid, was somewhere between 10 and 12 kilometers wide, roughly the size of a mid-sized city. It slammed into a shallow sea at the edge of the Yucatan Peninsula, carving out a crater more than 150 kilometers across. The collision generated a core of superheated plasma exceeding 10,000 degrees, and the thermal pulse that radiated outward was lethal for nearby life within minutes.
Scientists first identified the impact in 1980 after discovering an unusual layer of iridium, a metal rare on Earth’s surface but common in asteroids, deposited in rocks dating to exactly 66 million years ago. At a site in Gubbio, Italy, the iridium concentration in this thin clay layer reaches 3,000 parts per trillion, roughly 230 times the normal background level. That same iridium-rich layer appears in rocks on every continent, confirming a global event.
One of the most dramatic pieces of evidence comes from Tanis, a fossil site in North Dakota’s Hell Creek Formation. Paleontologists from the University of Kansas found exquisitely preserved fish that died within minutes of the impact, over 3,000 kilometers from the crater. The fish had tiny glass-like spherules, bits of molten rock ejected from the impact site, lodged in their gills. They inhaled these particles while swimming, and a massive water surge triggered by seismic waves buried them so quickly that their three-dimensional bodies were preserved rather than crushed flat. The timing of the spherules in the sediment matches the calculated arrival of seismic waves from the impact, which would have reached North Dakota in tens of minutes.
How the Impact Killed Globally
The asteroid itself was devastating, but most dinosaurs didn’t die from the initial blast. They died from what happened to the atmosphere, the oceans, and the food chain in the weeks, months, and years that followed.
The collision launched billions of tons of soot, dust, and sulfur-containing rock into the upper atmosphere. This debris spread around the globe and blocked sunlight, plunging the planet into what scientists call an “impact winter.” Within months, photosynthesis slowed dramatically in the oceans and collapsed on land. Mid- and high-latitude regions experienced severe cooling, while tropical areas suffered extreme drought along with milder temperature drops. The darkness and cold persisted for roughly one to two years before conditions began to improve.
Without photosynthesis, plant life died back. Herbivorous dinosaurs lost their food supply, and the carnivores that ate them followed. The entire food web, from the bottom up, collapsed.
Acid Rain and Ocean Collapse
The impact also generated enormous quantities of sulfuric and nitric acid that rained down across the planet. At the high end of scientific estimates, enough acid was produced to overwhelm the ocean’s natural chemical buffering capacity in the upper 100 meters of water. Surface ocean pH may have temporarily dropped to levels as low as 3, comparable to vinegar.
Even more moderate acid levels were catastrophic for marine life. The oceans became so acidic that shells and skeletons made of calcium carbonate began dissolving. More than 90 percent of calcium carbonate-shelled plankton species, including the tiny organisms that formed the base of the marine food chain, went extinct. Ammonites, the coiled-shell relatives of modern squid, disappeared entirely. Research published in the Proceedings of the National Academy of Sciences used chemical signatures in ancient shells to document a rapid pH drop of 0.2 to 0.3 units within about a thousand years of the impact. That may sound small, but it was fast enough to selectively kill off organisms that depended on building calcium carbonate shells, while groups that didn’t build shells, like certain types of algae, survived.
The oceans took a long time to recover. Surface water pH initially rebounded and overshot normal levels within about 40,000 years, then took an additional 80,000 years to return to pre-impact conditions. For context, that recovery period is roughly 400 times longer than all of recorded human history.
Volcanoes Were Already Weakening Ecosystems
The asteroid didn’t strike a healthy planet. Massive volcanic eruptions in what is now western India, known as the Deccan Traps, had been pouring lava and toxic gases into the atmosphere for hundreds of thousands of years before the impact. These eruptions produced enough basalt to cover an area roughly the size of France in layers kilometers thick.
Mercury deposits in sediment layers from southeastern China reveal that the major phase of Deccan volcanism began around 66.4 million years ago, roughly 400,000 years before the asteroid hit. Dinosaur species in the region began disappearing as volcanic activity intensified, with more species vanishing as eruptions continued. Research in Geophysical Research Letters found that the beginning of dinosaur extinction in that region corresponds to the onset of the heaviest volcanic activity, not the asteroid impact itself.
The exact role of the volcanoes remains debated. Some scientists argue they weakened dinosaur populations and destabilized climate before the asteroid delivered the final blow. Others point out that although dinosaur diversity was declining in some regions, global populations were still viable until the impact. What’s clear is that Deccan volcanism continued even after the asteroid struck and after the last dinosaurs were gone, meaning the volcanoes alone weren’t sufficient to cause the extinction. The asteroid was the decisive event.
Why Some Animals Survived and Dinosaurs Didn’t
The extinction wasn’t random. Body size was one of the strongest predictors of survival. Smaller animals, particularly those under a few kilograms, were far more likely to make it through. Small mammals, lizards, snakes, turtles, crocodilians, and birds all survived, while virtually every large-bodied land animal perished.
Large animals need more food, and when plant life collapsed, they had no way to sustain themselves. Smaller animals could survive on seeds, insects, decaying matter, and other food sources that persisted through the darkness. Many survivors were burrowers or aquatic species, sheltered from the worst of the thermal pulse and temperature swings at the surface.
Birds were the one group of dinosaurs that made it. They belonged to a lineage called Avialae, which was the only dinosaur group that consistently evolved body sizes below the 1 to 3 kilogram threshold that other dinosaurs rarely broke. Their small size, combined with flight, warm-bloodedness, and likely more flexible diets, gave them just enough of an edge. Every bird alive today descends from the survivors of that catastrophe, making them the last living dinosaurs.