The leading theory: asteroid impact
The most widely accepted explanation for the extinction of the dinosaurs is that an asteroid roughly 10 kilometers across struck what is now Mexico's YucatΓ‘n Peninsula about 66 million years ago. In 1980, father-and-son researchers Luis and Walter Alvarez discovered an unusually thick layer of iridium β an element extremely rare on Earth but common in asteroids β in rock strata from locations around the world. When a massive impact crater (the Chicxulub crater) was later confirmed beneath the YucatΓ‘n Peninsula, the asteroid-impact theory became the widely accepted explanation among paleontologists for the trigger behind the end-Cretaceous mass extinction.
What happened on Earth right after impact
Dust and sulfate particles thrown up by the impact are believed to have blanketed the atmosphere, blocking sunlight, causing temperatures to plummet, and halting photosynthesis β collapsing the food chain from the bottom up. The massive collision is thought to have unleashed instantaneous fires, earthquakes, and tsunamis, while also injecting enormous quantities of dust, soot, and sulfate aerosols into the atmosphere. As these particles blocked sunlight for an extended period β an "impact winter" β plant photosynthesis dropped sharply, and the resulting scenario, currently favored by scientists, is that the food chain collapsed in a chain reaction from herbivores up through predators.
Another factor: the Deccan Traps volcanism
Around the same time, a massive series of lava eruptions known as the Deccan Traps was occurring in what is now India, and research continues into how much the carbon dioxide and sulfur dioxide it released may have already destabilized the climate. Separately from the asteroid impact, the Deccan Traps volcanism released enormous quantities of lava over hundreds of thousands of years in present-day India. There is an active line of research into whether the greenhouse gases and sulfates it released had already been stressing Earth's climate and ecosystems even before the impact, and exactly how large its contribution was remains an actively studied question. That said, most researchers regard the asteroid impact as the decisive trigger of the mass extinction, treating the volcanism as a contributing or ecosystem-weakening factor rather than the primary cause.
Why the K-Pg boundary layer is used as global evidence
The iridium-rich layer found at the boundary between the Cretaceous and Paleogene periods (the K-Pg boundary) serves as a global marker pointing to the same moment in time, found virtually anywhere on Earth. This thin layer of clay has been confirmed across rock strata on multiple continents, from Antarctica to Europe to the Americas, serving as key evidence that the asteroid impact was not a localized event but one that affected the entire planet. Diverse dinosaur fossils are found in the strata below this boundary, while directly above it, they disappear abruptly and dramatically.
Not all dinosaurs disappeared
The non-avian dinosaurs we typically picture, like Tyrannosaurus, went extinct, but birds β which evolved from a branch of theropod dinosaurs β survived to the present day, and are effectively considered "living dinosaurs." In paleontology, birds are classified not as descendants of dinosaurs but as a branch belonging to the dinosaur group itself. That means some bird lineages that survived the end-Cretaceous extinction continue on today, so strictly speaking, it is only the non-avian dinosaurs that went extinct, not the larger dinosaur group as a whole β an important distinction often raised when discussing this extinction event.
Why some creatures survived
One hypothesis is that small-bodied, omnivorous animals, or those able to hide underground or in water, were relatively better positioned to survive the abrupt environmental changes. In an environment where food was suddenly scarce, larger animals requiring more food are thought to have been at a disadvantage. By contrast, small animals that could get by on limited food, omnivores able to eat a variety of things like seeds or dead organic matter, and animals that could burrow or hide in water are widely believed to have had comparatively higher odds of survival. Mammals, some birds, crocodiles, and turtles are frequently cited as survivor groups sharing these characteristics.
Questions still under active research
Whether the extinction happened almost instantly after the impact, or unfolded gradually over tens of thousands of years overlapping with the volcanism, remains an actively studied question without a fully settled answer. While there is broad agreement that the asteroid impact was the key trigger for the mass extinction, exactly how the impact and the volcanism interacted, the precise timescale over which the extinction unfolded, and how the pace of ecosystem collapse varied by region all remain areas that geology and paleontology continue to test using new strata data and precise dating.