The Cretaceous-Paleogene (K-Pg) extinction event, approximately 66 million years ago, marks a profound shift in Earth’s biological history. This catastrophic mass extinction ended the Mesozoic Era (Age of Reptiles) and ushered in the Cenozoic Era (Age of Mammals). It stands as one of the five largest extinction episodes, dramatically reshaping life. This event ended the long reign of dinosaurs, its impact extending far beyond these iconic creatures to affect a vast array of plant and animal species globally.
The Chicxulub Impactor
A massive asteroid impact is the primary trigger for the K-Pg extinction event. Approximately 66 million years ago, an asteroid 10 to 15 kilometers (6 to 9 miles) in diameter struck Earth at an estimated speed of 20 kilometers (12.5 miles) per second.
The impact site is off the northern coast of the Yucatán Peninsula, forming the Chicxulub crater. This immense buried structure measures about 180 to 200 kilometers (110 to 120 miles) in diameter and is approximately 20 to 30 kilometers (12 to 19 miles) deep. Evidence supporting this theory includes a distinct, globally distributed thin layer of sediment known as the K-Pg boundary layer.
This boundary layer is characterized by high concentrations of iridium, a metal rare in Earth’s crust but abundant in asteroids. It also contains shocked quartz, mineral grains exhibiting microscopic deformation patterns from extreme pressures of a high-velocity impact. Tektites, small glass beads from molten rock ejected during the impact, further corroborate the asteroid’s role.
Widespread Volcanic Activity
Immense volcanic activity from India’s Deccan Traps also occurred around the K-Pg event. This large igneous province, one of Earth’s most extensive volcanic features, consists of multiple layers of solidified flood basalt. These eruptions covered approximately 500,000 square kilometers (200,000 sq mi) and accumulated to over 2 kilometers (1.2 miles) thick.
The Deccan Traps erupted over hundreds of thousands of years, with major phases around the K-Pg boundary. These prolonged eruptions released massive quantities of gases, including carbon dioxide (CO2) and sulfur dioxide (SO2), into the atmosphere. Such emissions could have led to significant climate alterations, with some studies suggesting a gradual global warming of about 2°C prior to the impact.
The timing and precise climatic effects of these eruptions are still subjects of scientific investigation, but Deccan Traps activity indicates a substantial, long-term environmental stressor that affected global ecosystems. Some research suggests these volcanic emissions contributed to environmental instability rather than directly causing the mass extinction.
Immediate Planetary Consequences
The asteroid impact unleashed immediate, devastating planetary consequences, compounded by existing environmental stressors. The collision generated seismic waves of extraordinary power, potentially triggering earthquakes of magnitude 10 or 11, far exceeding anything recorded. These shockwaves also spawned enormous mega-tsunamis, some hundreds of meters high, which inundated coastlines thousands of kilometers from the impact site.
A global heat pulse followed as trillions of tons of ejected rock and molten material, heated by atmospheric friction, fell back to Earth. This intense thermal radiation likely ignited widespread wildfires across continents, sending vast amounts of smoke and soot into the atmosphere. This global shroud of dust and soot blocked sunlight, leading to an “impact winter.” Global temperatures plummeted, and photosynthesis, the foundation of most food webs, largely ceased, causing a collapse of marine and terrestrial food chains.
Furthermore, the impact site’s geology, rich in sulfate-containing minerals like anhydrite, injected large quantities of sulfur vapor into the stratosphere. This sulfur, combined with atmospheric gases and vaporized rock, formed nitric and sulfuric acid rain. This acid rain acidified oceans and leached soils, stressing surviving ecosystems for months to years after the initial impact.
A World Reshaped by Extinction
The K-Pg extinction event eliminated an estimated 75% or more of all plant and animal species on Earth. All non-avian dinosaurs vanished, ending their 165-million-year dominance. Other prominent groups that disappeared included pterosaurs (large flying reptiles) and major marine reptiles such as mosasaurs and plesiosaurs.
In the oceans, ammonites (a diverse group of shelled cephalopods) became extinct, as did rudist bivalves (significant reef-building organisms). Many plankton species, forming the base of marine food webs, also suffered catastrophic losses. The abruptness of these extinctions across marine and terrestrial environments points to a rapid, global environmental collapse.
Despite widespread devastation, some groups of organisms survived, providing the foundation for future biodiversity. Avian dinosaurs, or birds, represent the only dinosaur lineage that persisted beyond the K-Pg boundary. Survivors included mammals, amphibians, turtles, crocodilians, lizards, and snakes. Insects and flowering plants also weathered the event, though their communities underwent significant shifts and declines.
The Dawn of a New Era
The K-Pg extinction event created unprecedented ecological opportunity by vacating numerous environmental niches. With dominant non-avian dinosaurs removed, competitive and predatory pressures on smaller creatures drastically reduced. This dramatic shift set the stage for a remarkable period of adaptive radiation, particularly among mammals.
Mammals, largely small and nocturnal during the Mesozoic Era, diversified rapidly during the subsequent Paleogene Period. They evolved to fill vacant roles in ecosystems, expanding into a wide array of forms and sizes previously occupied by dinosaurs. This adaptive radiation led to the emergence of diverse mammalian lineages, including ancestors of modern horses, whales, bats, and primates.
Post-extinction diversification of mammals allowed them to become the dominant large animals on Earth. This expansion into new niches, coupled with increases in body size and brain development, laid the groundwork for the evolution of incredible mammalian diversity seen today, ultimately leading to the appearance of humans. The K-Pg event represents a profound evolutionary bottleneck that dramatically redirected the course of life on our planet.