Around 66 million years ago, an asteroid roughly 10 kilometers wide slammed into what is now Mexico’s Yucatan Peninsula, triggering the most famous mass extinction in Earth’s history. About 75% of all animal species vanished, including every dinosaur that wasn’t a bird. The event, known as the Cretaceous-Paleogene (K-Pg) extinction, reshaped life on Earth in a matter of years and opened the door for mammals to eventually dominate the planet.
The Asteroid Strike
The impactor hit at an estimated speed of 15 to 22 kilometers per second, striking at a steep angle between 45 and 60 degrees from the horizontal. To put that velocity in perspective, it was traveling roughly 50 times faster than a rifle bullet. The collision created the Chicxulub crater, a structure about 180 kilometers across that now lies buried beneath the seafloor and the Yucatan coastline. The energy released was billions of times greater than any nuclear weapon ever detonated.
The First Minutes and Hours
The impact sent a shockwave rippling through the Earth’s crust and launched enormous volumes of molten rock and debris into the atmosphere. A fossil site in North Dakota called Tanis has given scientists an extraordinarily detailed snapshot of what happened in the minutes that followed. Seismic waves from the impact reached the site, over 3,000 kilometers away, within tens of minutes. The shaking triggered massive inland surges of water, similar to the way the 2011 Japanese earthquake caused water surges nearly 5,000 miles away in Norway.
At Tanis, researchers found ancient sturgeon with tiny glass-like spherules (droplets of molten rock ejected from the impact) lodged in their gills. The fish had inhaled the falling debris while swimming. A tangled mass of freshwater fish, land animals, trees, marine creatures, and impact ejecta was packed together by the surge and buried so rapidly that fossils were preserved in three dimensions rather than being crushed flat.
The impact also generated an intense thermal pulse. Close to the strike zone, the heat was extreme but lasted less than a minute. At distances thousands of kilometers away, the thermal radiation was less intense but lasted around six minutes, potentially long enough to ignite dry vegetation and leaf litter in some regions.
Impact Winter and Climate Collapse
The longer-term devastation came from what the asteroid put into the atmosphere. Massive quantities of dust, soot, and sulfur-rich particles from vaporized rock blocked sunlight across the globe. Photosynthesis slowed or stopped. Temperatures plunged. Sea surface temperatures dropped by an average of 2°C below pre-impact levels, with two sharp dips reaching as much as 7°C below normal. This cooling phase, often called an “impact winter,” likely persisted for several decades. Even after most of the dust settled out of the atmosphere within months, ocean-atmosphere models suggest the cooling lingered because the oceans themselves had absorbed the temperature change and were slow to recover.
The darkness alone was catastrophic. Without sunlight, plant life collapsed on land and in the sea. Food chains that depended on photosynthesis unraveled from the bottom up.
Acid Rain and Ocean Devastation
The asteroid struck a region rich in sulfur-bearing rock. The impact vaporized this material and sent sulfur compounds into the atmosphere, where they combined with water to form sulfuric acid. When this rained back down into the oceans, it drove a rapid drop in pH. Under the worst-case estimates, enough acid reached the ocean to overwhelm the natural buffering capacity of the upper 100 meters of seawater, potentially pushing pH as low as 3, far more acidic than normal seawater’s pH of just over 8.
Even a pH drop of less than one unit would have made seawater corrosive to the calcium carbonate shells that plankton depend on. The result was devastating: more than 90% of species of calcium carbonate-shelled plankton, including coccolithophores and foraminifera, went extinct. These microscopic organisms formed the base of the marine food web, so their collapse cascaded upward through the entire ocean ecosystem.
Volcanism Was Already Underway
The asteroid didn’t act entirely alone. In what is now India, one of the largest volcanic eruption sequences in Earth’s history was already in progress. The Deccan Traps, a vast region of layered lava flows, saw at least 15 major eruptions clustered between 66.3 and 66.2 million years ago, right around the time of the impact. Earlier pulses of volcanism had already begun disrupting the global carbon cycle as far back as 66.49 million years ago. Some of these eruptions released enormous amounts of sulfur, while later phases pumped carbon dioxide into the atmosphere, likely by intruding into carbon-rich rock.
The relationship between the volcanism and the extinction is still debated. Some researchers argue the Deccan eruptions had already stressed global ecosystems before the asteroid delivered the killing blow. Others see the impact as the primary cause, with the volcanism playing a secondary role. What’s clear is that Earth’s biosphere was being hit from two directions at once.
Who Died and Who Survived
The extinction wiped out roughly 75% of all animal species. All non-avian dinosaurs disappeared, along with the flying pterosaurs and most marine reptiles. Giant marine predators, ammonites (the coiled-shell relatives of squid), and the reef-building rudist clams all vanished permanently.
On land, the collapse of forests was a critical filter. Arboreal birds, the ones living in trees, were driven to extinction when global forests were knocked down or destroyed. Ground-dwelling bird lineages fared better and gave rise to all modern birds. A distinctive “fern spike” appears in the fossil record just above the extinction boundary at many sites across North America: fern spores suddenly dominate the pollen record, indicating that ferns were the first plants to recolonize devastated landscapes where forests once stood. Some researchers attribute this to forests being flattened by the blast wave rather than burned.
Among mammals, survival favored specific traits. Burrowing and semi-aquatic species had a clear advantage because living underground or in water shielded them from the initial heat pulse and wildfires. Small body size helped, since smaller animals need less food and can survive on insects, seeds, and detritus when plant productivity collapses. Dietary generalists, animals that could eat a range of foods, outlasted specialists locked into narrow food sources. Large-bodied mammals and picky eaters were heavily selected against.
The Evidence in the Rock
The extinction boundary is visible in rock outcrops around the world as a thin layer of clay. In 1980, researchers discovered that this layer contains unusually high concentrations of iridium, a metal rare on Earth’s surface but common in asteroids. At sites on Seymour Island near Antarctica, iridium levels at the boundary are 40 times higher than background concentrations. This global iridium anomaly was the first strong evidence that an extraterrestrial impact had caused the extinction, and it led directly to the discovery of the Chicxulub crater a decade later.
The boundary layer also contains shocked quartz grains, tiny glass spherules from molten rock, and soot consistent with widespread fires. Together, these markers create an unmistakable geological signature found on every continent, recording the single worst day in the history of complex life on Earth.